Indoor air quality and health in schools: A critical review for developing the roadmap for the future school environment

Abstract

Several research studies have ranked indoor pollution among the top environmental risks to public health in recent years. Good indoor air quality is an essential component of a healthy indoor environment and significantly affects human health and well-being. Poor air quality in such environments may cause respiratory disease for millions of pupils around the globe and, in the current pandemic-dominated era, require ever more urgent actions to tackle the burden of its impacts.
The poor indoor quality in such environments could result from poor management, operation, maintenance, and cleaning. Pupils are a different segment of the population from adults in many ways, and they are more exposed to the poor indoor environment: They breathe in more air per unit weight and are more sensitive to heat/cold and moisture. Thus, their vulnerability is higher than adults, and poor conditions may affect proper development.
However, a healthy learning environment can reduce the absence rate, improves test scores, and enhances pupil/teacher learning/teaching productivity. In this article, we analyzed recent literature on indoor air quality and health in schools, with the primary focus on ventilation, thermal comfort, productivity, and exposure risk. This study conducts a comprehensive review to summarizes the existing knowledge to highlight the latest research and solutions and proposes a roadmap for the future school environment. In conclusion, we summarize the critical limitations of the existing studies, reveal insights for future research directions, and propose a roadmap for further improvements in school air quality. More parameters and specific data should be obtained from in-site measurements to get a more in-depth understanding at contaminant characteristics. Meanwhile, site-specific strategies for different school locations, such as proximity to transportation routes and industrial areas, should be developed to suit the characteristics of schools in different regions. The socio-economic consequences of health and performance effects on children in classrooms should be considered. There is a great need for more comprehensive studies with larger sample sizes to study on environmental health exposure, student performance, and indoor satisfaction. More complex mitigation measures should be evaluated by considering energy efficiency, IAQ and health effects.

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Indoor air quality and health in schools: A critical review for developing the roadmap
for the future school environment
Sasan Sadrizadeh, Runming Yao, Feng Yuan, Hazim Awbi, William Bahnfleth, Yang
Bi, Guangyu Cao, Cristiana Croitoru, Richard de Dear, Fariborz Haghighat, Prashant
Kumar, Mojtaba Malayeri, Fuzhan Nasiri, Mathilde Ruud, Parastoo Sadeghian, Pawel
Wargocki, Jing Xiong, Wei Yu, Baizhan Li
PII: S2352-7102(22)00920-2
DOI: https://doi.org/10.1016/j.jobe.2022.104908
Reference: JOBE 104908
To appear in: Journal of Building Engineering
Received Date: 4 March 2022
Revised Date: 6 June 2022
Accepted Date: 30 June 2022
Please cite this article as: S. Sadrizadeh, R. Yao, F. Yuan, H. Awbi, W. Bahnfleth, Y. Bi, G. Cao,
C. Croitoru, R. de Dear, F. Haghighat, P. Kumar, M. Malayeri, F. Nasiri, M. Ruud, P. Sadeghian, P.
Wargocki, J. Xiong, W. Yu, B. Li, Indoor air quality and health in schools: A critical review for developing
the roadmap for the future school environment, Journal of Building Engineering (2022), doi: https://
doi.org/10.1016/j.jobe.2022.104908.
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1
Indoor Air Quality and Health in Schools: A critical review for
developing the roadmap for the future school environment
Sasan Sadrizadeh1*, Runming Yao2,3*, Feng Yuan2, Hazim Awbi2,3, William Bahnfleth4, Yang
Bi5, Guangyu Cao5, Cristiana Croitoru6, Richard de Dear7, Fariborz Haghighat8, Prashant
Kumar9, Mojtaba Malayeri8, Fuzhan Nasiri8, Mathilde Ruud5, Parastoo Sadeghian1, Pawel
Wargocki10, Jing Xiong7, Wei Yu2, Baizhan Li2
1 Department of Civil and Architectural Engineering, KTH Royal Institute of Technology, Stockholm, Sweden
2 Joint International Research Laboratory of Green Buildings and Built Environments, School of the Civil
Engineering, Chongqing University, China
3 School of the Built Environment, University of Reading, UK
4 Department of Architectural Engineering, The Pennsylvania State University, University Park, PA, USA
5 Department of Energy and Process Engineering, Norwegian University of Science and Technology, Norway
6 Technical University of Civil Engineering Bucharest, CAMBI Research Centre, Romania
7 School of Architecture, Design, and Planning, The University of Sydney, NSW, Australia
8 Department of Building, Civil and Environmental Engineering, Concordia University, Montreal, Canada
9 Department of Civil and Environmental Engineering, University of Surrey, UK
10 Department of Civil Engineering, Technical University of Denmark, Kongens Lyngby, Denmark
Corresponding authors: ssad@kth.se; r.yao@cqu.edu.cn
Keywords: Classroom air quality, Ventilation, Exposure risk, Energy use in schools,
Particle matter, Volatile organic compounds
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Abstract
Several research studies have ranked indoor pollution among the top environmental risks to public health
in recent years. Good indoor air quality is an essential component of a healthy indoor environment and
significantly affects human health and well-being. Poor air quality in such environments may cause
respiratory disease for millions of pupils around the globe and, in the current pandemic-dominated era,
require ever more urgent actions to tackle the burden of its impacts.
The poor indoor quality in such environments could result from poor management, operation, maintenance,
and cleaning. Pupils are a different segment of the population from adults in many ways, and they are more
exposed to the poor indoor environment: They breathe in more air per unit weight and are more sensitive
to heat/cold and moisture. Thus, their vulnerability is higher than adults, and poor conditions may affect
proper development.
However, a healthy learning environment can reduce the absence rate, improves test scores, and enhances
pupil/teacher learning/teaching productivity. In this article, we analyzed recent literature on indoor air
quality and health in schools, with the primary focus on ventilation, thermal comfort, productivity, and
exposure risk. This study conducts a comprehensive review to summarizes the existing knowledge to
highlight the latest research and solutions and proposes a roadmap for the future school environment. In
conclusion, we summarize the critical limitations of the existing studies, reveal insights for future research
directions, and propose a roadmap for further improvements in school air quality. More parameters and
specific data should be obtained from in-site measurements to get a more in-depth understanding at
contaminant characteristics. Meanwhile, site-specific strategies for different school locations, such as
proximity to transportation routes and industrial areas, should be developed to suit the characteristics of
schools in different regions. The socio-economic consequences of health and performance effects on
children in classrooms should be considered. There is a great need for more comprehensive studies with
larger sample sizes to study on environmental health exposure, student performance, and indoor
satisfaction. More complex mitigation measures should be evaluated by considering energy efficiency, IAQ
and health effects.
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Abbreviation list
AI
Artificial intelligence
CAQ
Classroom air quality
CAV
Constant air volume
CFD
Computational fluid dynamics
CFU
Colony-forming units
CR
Cancer risk
DCV
Demand controlled ventilation
DREAM
Danish Rational Economic Agency Model
DV
Displacement Ventilation
EPA
Environmental Protection Agency
EUI
Energy Use Intensity
GDP
Gross domestic product
HI
Hazard index
HQ
Hazard quotient
HVAC
Heating, ventilation, and air conditioning
I/O
Indoor/Outdoor (Time spend inside versus the outside)
IAQ
Indoor air quality
ICT
Information and Communications Technology
IoT
Internet of things
MV
Mixing ventilation
PM
Particle matter
NATA
National Air Toxics Assessment
PV
Personalized ventilation
TVOC
Total volatile organic compounds
VAV
Variable air volume
VOC
Volatile organic compounds
WHO
World health organization
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1. Introduction
The primary purpose of a school is to provide children with the optimal environment for their learning and
development. Schools have always been a second home for the pupils, and they spend most of their time
indoors while at school (almost 12% of their time inside classrooms) [1–4]. Schools are among the critical
social infrastructures in society and are often the focus for children's social activity. Classrooms are more
congested than other workplaces, with an occupancy density of approximately four times that of office
buildings [5]. Good indoor air quality (IAQ) in classrooms is essential because it may affect the health,
performance, alertness, ability to concentrate, and comfort of pupils and teachers. Classrooms have
typically been justified as an important built environment type by reference to the adverse effects of
unfavorable indoor conditions on pupils' health, comfort, and academic performance [2]. Children are
sensitive to various environmental exposures during this developmental stage of their life, which can have
long-term negative consequences such as respiratory disease and low cognitive function [6]. In addition,
the risk of cross-contamination in classrooms is usually higher than in other indoor environments and poses
logistical challenges and/or risks of transmission.
Studies have shown that the conditions in schools are inadequate and often significantly worse than in
offices and dwellings [7–9]. These conditions are known to reduce comfort and can also cause health
problems [8,10–12]. This is particularly unfortunate as children of school age are vulnerable, and their
bodies are still growing [13–15]. Poor conditions in schools also impact learning progression [16,17]. This
is particularly important as it may affect the children's future quality of life with economic implications for
society [18,19]. All conditions that shape indoor environmental quality in classrooms influence children's
learning progression and cognitive performance. Pupil's performance are affected by many parameters,
such as classroom temperature, air quality, lighting, and acoustics [20,21].
The school ventilation system is a primary tool for ensuring a safe, comfortable, and healthy indoor
environment. Thermal comfort levels and acceptable IAQ are crucial in producing an environment that
promotes optimal educational and health outcomes [22–24]. Previous research studies in school
environments have revealed inadequate and often poor classroom air quality (CAQ), causing an increased
risk for respiratory illnesses and other health-related symptoms [25–27]. Researchers reported diverse CAQ
levels in school buildings in different parts of the world depending on climate conditions, outdoor pollution
levels, occupancy rates, activity levels, ventilation types and their corresponding flow rate, and also
building practices [28,29].
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The CAQ depends on several factors, including the sources of indoor and outdoor pollutions, dilution, and
removal of pollutants by ventilation [30–32]. The type of ventilation system and air distribution within the
classroom will also affect air quality.
Research to date that examined the effects of CAQ on children's cognitive performance and learning has
addressed the factors that impact indoor air quality and emphasized outdoor air ventilation rates as the CAQ
indicator [17]. The reason is that there are no agreed indexes of indoor air quality and ventilation rate is
associated with contaminant exposure levels [20,33]. Often, carbon dioxide (CO2) concentration is used
[34] as the marker of ventilation adequacy in the presence of occupants [35] because, treated as a tracer
gas, it is related to the ventilation rate per person. Research has shown that the level of CO2 in classrooms
can increase to very high levels due to inadequate ventilation rates [36,37]. It is generally assumed that the
higher the CO2 concentration, the poorer the air quality (less dilution). Although CO2 has frequently been
used to characterize air quality in classrooms, some research has focused on specific pollutants such as
particulate matter or contaminants with outdoor sources [38–41].
In this article, we summarize and explore the most relevant and recent research studies that have been
conducted on school IAQ and related social and health impacts on pupils and staff. We also critically reflect
on the existing knowledge and literature whilst highlighting the areas with the highest uncertainties. Our
focus is on identifying how different factors affect CAQ and comfort in schools, and hence pupils’ health
and wellbeing. Based on this review of the literature, we have also proposed a roadmap to improve indoor
air quality in schools.
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2. Methodology
2.1. Data inclusion, extraction, and analysis
This section presents the research methdology and brief statistical analysis on the reviewed articles to
understand the current research trends. This review is formulated based on peer-reviewed journal articles
from several renowned academic databases, such as Web of Science, Scopus, Science Direct, and SAGE
journals. The fundamental purpose of critically review the most recent links between CAQ and the cognitive
skills and abilities of pupils along with the consequences for progressive learning, to highlight research
gaps, and to propose recommendations for further research. In this review, peer-reviewed journals across
the world were considered. A few conference papers, thesis, standards, and technical guidelines were also
analyzed to enhance the quality of the review.
Students’ perceptions of the indoor environmental quality are affected by multiple parameters [42]. Among
all these parameters, thermal comfort and IAQ are the key factors that significantly affect students’ feelings
of the indoor environments [43,44]. Moreover, the IAQ also interacts with thermal comfort through varied
occupant sensations [44]. The sources of indoor and outdoor pollutions, dilution, and removal of pollutants
by ventilation are the key factors in determining IAQ [30–32]. The type of ventilation system and air
distribution within the classroom will also affect air quality.
Therefore, relevant keywords were selected, e.g., indoor air quality, primary school, health impact,
exposure risk, thermal comfort, pupils’ performance, energy use in schools, and school ventilation. These
keywords were searched in the journal title, abstract, and keywords for primary selection of peer-reviewed
papers. To search conference papers, thesises, standards, and technical guidelines, these keywords were
searched only in the title. The selected studies were classified into specific categories according to the aim
of the review. Figure 1 shows the literature search overview with the selection criteria.
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Figure 1: Flow chart of the procedure followed for the inclusion of research articles.
2.2. Year of Publication
The publication year for the distribution of the collected bibliographic records on CAQ was studied. After
eliminating duplication, all records were examined by their titles, keywords, and abstracts. The yearly
distribution of published articles is shown in Figure 2. The number of papers on CAQ showed an overall
increasing trend, suggesting that the rate of research work in this area was growing over time. It can be
concluded that people have started paying more attention to indoor air quality in schools with increasing
demands of providing a healthy environment for children.
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Figure 2: The yearly distribution of the bibliographic records in the current study
2.3. Country and Region of Publication
Figure 2 represents the geographical distribution of articles summarized in the current study. Exposure risk
and its impact on health and learning performance are attractive research problems. Energy consumption
for improving thermal comfort and CAQ are commonly discussed research problems. Also, the effect of
the outdoor environment on indoor ones has inevitably attracted researchers’ attention around the world.
Figure 3: The geographical distribution of research articles summarized in this study
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3. Analysis and discussion
3.1. Exposure risk in school classrooms
Pupils' exposure to indoor air pollutants in school buildings is a leading public concern and may cause
severe damage to the pupils’ health since they inhale a larger volume of air corresponding to their body
weights than do adults [45–48]. The respiratory, immunological, reproductive, central nervous, and
digestive system of childrens are not fully matured. The route of breathing, nasal versus oral, as well as the
efficacy of the nose with aerosols, may also vary between children and adults, exposing children's lungs to
higher quantities of air pollutants [49,50]. Some research studies have also confirmed the presence of animal
dander allergens in school that might pose serious health issues in Pupils' with mild asthma and animal
dander allergy [51,52].
Several research studies found that pollutant concentrations in schools were higher than concentrations in
households and commercial buildings [53,54]. Children and adults bring chalk dust, fungi, bacteria, and
viruses into the school environment, and vapors and odors from laboratories and art courses are also
common sources of pollutants in schools [28].
Inhalation exposure to air pollution has increased children's mortality rate, acute respiratory disease, and
asthma [45]. Due to different responses of the children's immune systems to indoor air exposures, various
chronic diseases and symptoms have been reported and characterized as "sick building syndrome" [55].
Indoor pollution such as CO2, PM, VOCs, NOx, and ozone are recognized as indoor contaminants causing
severe health problems for adults and children [56–58]. In general, the CAQ is characterized by a complex
of contaminants, including VOCs, PM, aldehydes, bacteria, and molds [59,60]. Several studies have
produced health risk assessments for the inhalation of indoor pollutants by considering various standards
and recommendations, including the United States Environmental Protection Agency (EPA), WHO,
ASHRAE, and GB/T [61–64].
Risk assessment
The U.S. EPA standards compute both the non-carcinogenic and carcinogenic effects of indoor air
pollutants. The cumulative hazard index (HI) can be computed according to The National Air Toxics
Assessment (NATA) U.S EPA, 2014 [65]. NATA air quality monitoring suggests the long-term risks to
human health if air toxics emissions are steady over time. In this regard, summation of the hazard quotient
(HQ) for the ith pollutant is considered as follows:
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==

i
i
ii
i
ADI
HI HQ RfD
(1)
where ADIi is the daily average intake and RfDi is the reference dose that has no negative impact on the
human body. An HQi below one for the ith pollutant means zero increase in the occurrence of health
problems.
The cancer risk (CR) is defined by the U.S EPA 2009 [3] standard to calculate the probability of cancer
occurring during 70 years in a person exposed to carcinogenic materials. Although this method is not an
accurate estimation for predicting the CR for exposed persons over time, it has been a common approach
to evaluate the toxicity of various indoor environments [66–68].
The total CR due to exposure to air pollution is computed by Equation 2 below:
( )
.==

i i i
ii
CR CR LDI CSF
(2)
where the LDIi is the lifetime daily intake defined as a dose of ith contaminant an individual is exposed to
in 70 years. CSFi is the cancer slope factor that calculates the carcinogenicity of the ith chemical substance
that can cause cancer.
Schibuola et al. [64] evaluated the health risk of indoor air pollutants in school environments by adopting
the HI and CR equations. They calculated the health risk of children's daily exposure to PM10 and CO2.
Applying the HQ equations for calculating the health risk has been validated by various research studies
[69–71]. However, the health risk assessment results in Madureira et al. [72] showed the limitations in
calculating HQ. The main weakness of HQ is its failure to consider the deposition area for particles in the
respiratory system. Thus, it is challenging to define the health risk of various respiratory parts, including
trachea, bronchia, etc., exposed to indoor air pollutants. It is important to notice that linking school
environmental exposures to specific health symptoms is challenging because it is difficult to distinguish
between school-based and non-school-based exposures, such as those caused by the home environment,
regarding an observable health consequence [73].
VOC exposure
The VOC pollutants are among the leading indoor air pollutants causing severe health issues for children
and adults. Construction materials, furnishings such as desks and shelves, resins of wood products,
adhesives, glues, paints, cleaning chemicals, and carpets are primary VOC emission sources in schools
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[67,74–77]. The VOC concentrations in newly built or recently renovated school buildings may be
significantly higher than ordinary ambient levels.
There has been a growing interest in evaluating the impact of exposure to VOCs on children's educational
performance and health risk [78]. Kim et al. [79] studied the effect of microbial VOCs on asthma and atopy
in 1,482 pupils in eight schools in Sweden by using a questionnaire. Their results revealed a direct
relationship between the concentration of the microbial VOCs and the presence of asthmatic symptoms in
pupils. Johnson et al. [63] showed that lack of adequate air change and ventilation rates increased the
concentration of the indoor contaminations, including VOCs, in the classrooms of twelve Oklahoma City
schools.
The concentration of various VOCs, including formaldehyde, benzene, toluene, naphthalene, and xylene,
has been monitored in different seasons during the year to evaluate the exposure risk level [9]. Another
VOC found in schools is formaldehyde, which is frequently utilized to produce construction materials and
a variety of other products [80,81].
Specific VOCs, such as benzene and formaldehyde, recognized carcinogens, have been strongly connected
to health effects [82,83]. Sofuoglu et al. [84] showed that the formaldehyde concentration was the highest
among the detected VOCs in three primary schools in Turkey. They characterized formaldehyde as a
concerning pollutant with multiple carcinogenic risk levels in Turkish schools. Their results revealed that,
besides formaldehyde, naphthalene, benzene, and toluene were indoor air pollutants with high
concentrations. The measurement of fifteen typical VOCs concentrations in Minnesota (USA) schools
showed that the exposure level of children to VOCs was higher in winter than spring [85].
High levels of VOCs in schools are suspected of causing irritation, throat dryness, allergies, and respiratory
health problems [86,87]. Current asthma risk is raised by 1.3 when VOC concentrations are increased by
10μg/m3 [88]. Furthermore, TVOC levels are associated with chronic airway, general, and eye symptoms
[89]. Daisey et al. [27] indicated that exposure to formaldehyde emitted by the polyurethane foams and
adhesives causes eye, skin, and respiratory problems, which in severe cases can lead to asthma in children.
However, exposure to persistent compounds (such as polycyclic aromatic hydrocarbons) can lead to
specific types of cancer in individuals [27].
CO2 exposure
The CO2 concentrations are high in most school environments since a natural ventilation system is the
primary approach to improving indoor air quality [30,90,91]. The indoor CO2 level is not considered a
pollutant by the WHO. While indoor CO2 concentration is used as an indicator to evaluate IAQ [64], this
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meaning is commonly misinterpreted within the HVAC industry, despite efforts to address this confusion
in standards, technical reports, conferences, and workshops [92].
Pupils' physical activity, window and door opening patterns in the classrooms, and ventilation performance
can control the CO2 levels in classrooms [61,93,94]. Awadi et al. [95] investigated the impact of CO2 levels
on the health risk of pupils in three schools in Kuwait. Their results showed a high concentration of CO2 in
classrooms, which indicated poor indoor quality, consequently increasing the health risk of pupils and
reducing their educational performance. Madureira et al. [96] studied the relation between the indoor air
pollution level and health issues, such as allergy and asthma, in primary schools in Portugal. Their
measurements indicated that the concentration of CO2 exceeded 1,000ppm in highly occupied classrooms,
thus decreasing the indoor air quality. CO2 concentration data was used to evaluate airborne infectious
diseases in 45 classrooms in 11 UK schools [97]. In this research, the variation in CO2 concentration and
ventilation rate affected the infection risk in different seasons with the greatest risk being in January.
Kalimeri et al. [47] measured various parameters in school environments in Greece. The parameters
measured were, amongst others, CO2 concentration, relative humidity, temperature, and formaldehyde, and
it was reported that inadequate ventilation was a major indicator of bad indoor air quality. Turunen et al.
[98] investigated IAQ and pupils’ health for 6th-grade pupils in schools in Finland, and found a significant
statistical correlation between temperature and self-reported bad indoor air quality. Another finding was
that the lower the ventilation rate and the higher the temperature, the higher were pupil reports that the CAQ
was poor. Smedje et al. [88] found no significant relationship between asthma symptoms and normal
measured IAQ parameters, such as CO2 concentration and humidity in Sweden. Simoni et al. [99]
researched respiratory health for pupils in Norway and reported that children exposed to CO2 levels above
1,000ppm had a higher risk of having a dry cough. PM10 values above recommended levels also showed
that nasal patency was lower than for children less exposed.
CO exposure
CO exposure is an acute hazard because it is odorless, colorless, and lethal. CO has been detected
infrequently in schools with its primary source being automobile emissions [100]. When permitted, CO is
mainly produced in school buildings by combustion sources such as heaters, gas and wood stoves, and
smoking [101]. CO was found to be substantially linked with asthma and eczema [102].
NO2 exposure
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In the indoor environment, NO2 emissions are produced by gas appliances, heaters, and cigarette smoking.
These sources are rare in the majority of schools. Without interior pollution sources, NO2 levels in
classrooms are often associated with outdoor levels [103]. NO2 concentrations in schools increased
throughout the warmer season, enhancing greater NO to NO2 conversion and resulting in O3 production in
the presence of VOCs and sunlight [28]. NO2 exposure is associated with increased respiratory symptoms,
allergy exacerbations (particularly to indoor allergens), conjunctivitis, wheezing, and itchy skin rash
[104,105]. Exposure to higher indoor NO2 concentrations in schools (higher than the 40µg/m3 limit
recommended by WHO) was strongly associated with the prevalence of asthma and respiratory morbidity
[29,104,106].
Ozone [O3] exposure
Overall, outdoor O3 concentrations are greater than those found inside schools [76,107]. In addition to
filtration of the ventilation air as it enters the building, deposition on different solid surfaces, and chemical
reactions in the indoor air result in a decreased indoor/outdoor ratio for O3 in the school [107,108]. Lower
indoor O3 concentrations may also be caused by the absence of large sources in classrooms, such as
photocopying machines or ozone generators [103].
WHO [82] recommended ozone values of less than 100mg/m3 for eight hours. However, the total evidence
revealed that an increase in the range of 30–50mg/m3 could result in a minimum 6% rise in the relative risks
of illness-related absence among pupils. Specific health effects accounting for absenteeism at elevated
ozone levels are primarily related to respiratory illness, with the relative risk of respiratory diseases, wet
cough, and nocturnal attacks of breathlessness [106,109,110]. Is it worth mentioning that ASHRAE
Standard 62.1 requires mitigation of ventilation air if outdoor ozone exceeds 0.100 ppm (195μg/m3) [111].
PM exposure
Many schools have identified particulate matter (PM) pollution as a major source of indoor air pollution.
Particulate pollutants come from various sources, including chalk dust, soil dust, new furniture, cleaning
operations, particle resuspension due to pupil movements, combustion sources (including heaters, gas and
wood stoves), smoking where permitted, and also outdoor sources (traffic, industrial emissions, and wild
fires). However, Raysoni et al. [112] showed that the primary source of PM contamination in schools is
outdoor air. Particles also enter schools via ventilation and infiltration from the outside environment,
especially in metropolitan areas where automobile exhausts are the primary source [107,113,114].
Fine and ultrafine particulate matter may pose a serious health concern due to their origin in combustion
processes [115]. Such pollutants can cause health issues, including asthma and respiratory system problems
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in children [116]. Various research results showed that PM could carry heavy metals and polycyclic
aromatic hydrocarbons [117–119]. It is proven that inhaling PM causes greater risk to children than to
healthy adults, owing to their lower fractional deposition efficiency and greater breaths/minute resulting
from their lung size [120].
It was also reported that the concentration of PM10 particles increases in highly occupied classrooms.
Moreover, high levels of pupil activity increase PM levels in the air due to the resuspension of particles
already present on surfaces [121].
Exposure to heavy metals and the contaminants carried by PM10 particles increases the risk of respiratory
sickness, and lung cancer among pupils [72]. PM1 exposure at school had toxicological consequences,
mostly on baseline lung function in children with chronic respiratory illness [122]. Exposure to mean PM2.5
concentrations in the range of 20.5±2.2mg/m3 was linked to conjunctivitis, hay fever, an itchy rash, and
sensitization to outdoor allergens [104]. Fonseca et al. [71] investigated the impact of particle
contamination exposure doses in preschool children in Portugal. Their results revealed that children
attending schools in urban areas were exposed to a higher level of PM contamination due to higher traffic
density.
Fungi and Bacteria exposure
Penicillium, Cladosporium, Aspergillus, and Alternaria are the most common fungi found in indoor school
environments, and their prevalence varies depending on climate and location, whether rural or urban.
According to several studies [123–125], the mean total indoor fungi concentrations (CFU/m3) in school
classrooms ranged from 92 to 505 colony-forming units (CFU). Numerous studies have found positive
relationships between exposure to fungi particles at mean concentrations of 260 to 1297CFU/m3 with
general and respiratory symptoms among pupils [106,126]. Incidence of wheezing, asthmatic attacks,
headaches, sore throat, weariness, and coughing were also reported in schools as severe general symptoms
of fungi in the school buildings [106,126].
Bacteria concentrations ranged from 250 to 17,000CFU/m3 in schools [79], and Staphylococcus,
Corynebacterium, and Bacillus are the most commonly found types [127]. Although exposure to damp
buildings has been shown to increase the risk of developing health problems, there is no explicit minimum
threshold for microbiological concentrations and microbial by-products. Bacteria have been associated with
the current risk of asthma and nocturnal breathlessness [79]. According to the "hygiene hypothesis",
exposure to low microbial concentrations and endotoxins may protect pupils from school respiratory
symptoms and asthma [79,127,128].
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3.2. Ventilation in classrooms
ASHRAE Standard 62.1 recommends a minimum ventilation rate of 6.7L/s-person for classrooms (5L/s-
person + 0.6 L/s-m2, asuming the default occupant density of 35 person (age 9+) in 100m2) [111]. Increases
in ventilation rates of up to 20L/s per individual have been found to reduce the prevalence of sick building
syndrome symptoms and enhance IAQ [129]. Poor ventilation rates in classrooms influence not just the
comfort and health of pupils but also their learning performance [130]. According to Shaughnessy et al.
[131], there is a negative relationship between pupils' math standardized exam results and ventilation rates.
In general, specific air pollutants may cause significant and persistent immunosuppressive reactions,
leading to increased infectious diseases and neoplasia (abnormal benign or malignant cell growth) [132].
School absenteeism can be correlated with low air quality and pollution problems [133]. The sources are
multiple, and standard do not always ensure the acceptable pollution levels. A range of significant pollutants
(CO2, Particulate matter (PM2.5, PM10), Total volatile organic compounds (TVOC), and a set of specific
volatile organic compounds (VOC) including aldehydes) was identified as critical for educational building
environments in nine Mediterranean schools [134]. Also, PM2.5 and nitrogen dioxide (NO2) were found to
have similar dynamics in 109 French schools due to their outdoor sources whilst certain classes of
pollutants, such as the VOCs, are less easily treated since they are more likely to vary in concentrations
within the indoor premises and may not be controlled at all [135]. Moreover, it was demonstrated that most
poor indoor air quality is highly correlated with outdoor pollution levels [136–138]. Thus, the presence of
pollutants specific to the outdoor environment, like NO2, equivalent black carbon, PM2.5, the number and
concentration of ultrafine particles, road-traffic-related trace metals, and particularly the particulate matter,
underscores the need to consider mandating proper filtration of the fresh air intake.
Moreover, the pandemic period has highlighted the importance of proper ventilation, air distribution, and
effective air change in schools [139–141] since in crowded indoor settings, infectious diseases can
propagate faster, and children are usually a transmission vector towards families, even when the symptoms
are milder in a younger population [142]. The long period of time spent in classrooms significantly increases
the risk of infection, risk also observed in the case of other types of high density occupancies (e.g.
restaurants, public events, or public transportation), especially when inadequate ventilation and air
distribution systems are in place [143]. Additionally, one of the most encountered pollutants in schools,
PM2.5 [144], has a positive association with the spread of COVID-19, as it can act as a nucleation site to
transport viruses directly into the respiratory system [145]. This could also be due to an inadequate
ventilation rate.
Current norms and guidelines for the ventilation of educational buildings differ among countries and
regions. In general, a minimum airflow rate per person and/or per floor area unit is required to dilute the air
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pollutant concentrations to a specific level of air quality. Usually, the ventilation rate is expressed in L/s
(m3/h) per person or L/s (m3/h) per m2 floor area. However, these minimal requirements may not address
specific occupancy types, levels of activity, or types of pollutants, leading to ventilation rates in classrooms
that are often lower than the minimum ventilation rates specified in building codes and standards [146].
Furthermore, the maximum concentration of CO2 in classrooms might vary by different standards, however,
the upper threshold is about 1,000ppm [147–149]. However, it remains the primary indicator for IAQ level,
even if other pollutants or respiratory airborne transmission contaminants pose higher risks to the occupants.
So far, there is a clear knowledge gap related to ventilation constraints necessary to provide acceptable
safety concerning airborne transmissible diseases in classroom environments but to consider at the same
time other types of concerns like air pollutants (chemical gaseous and particulates) or energy efficiency.
Comprehensive ventilation strategies are needed to tackle infectious respiratory risks and provide pupils
with healthy CAQ conditions [150].
Due to high occupancy rates, it is mandatory to provide classrooms with ventilation systems that can deliver
outdoor air to the breathing zone, prevent indoor cross-infection, and dilute pollutants. Ventilation strategies
can be classified as natural, mechanical (unidirectional and bidirectional flow), or hybrid ventilation, which
represents a combination of systems designed to supply interior spaces with (filtered) outdoor air and to
extract polluted indoor air. An adequately controlled hybrid ventilation system operating in mechanical
supply mode can provide adequate ventilation and effectively decrease the concentrations of some indoor-
generated pollutants [151]. The mechanical supply should function in heat recovery mode in colder periods
and when avoiding overheating is necessary. Usually, old schools are not equipped with mechanical
ventilation, relying on natural ventilation (natural driving forces), which require careful management of
opening windows to be effective [152]. However, the COVID-19 pandemic has generated an increased
awareness of the need for proper ventilation, and national and international guidelines have been released
to promote rigorous natural ventilation plans [153].
Natural ventilation is the most common type of ventilation system used in educational buildings, being
predominant in the US, Southern, and South-Eastern Europe, China, India, Australia, etc. [16], while the
Nordic countries have similar percentages of mechanical or hybrid ventilation versus natural ventilation.
The UK has a significant percentage of schools naturally ventilated, the mechanical ventilation systems
being present in approx. 12% of the buildings [150], while Canada is intensely investing in equipping all
educational buildings with heating, ventilation, and air conditioning (HVAC) systems, given the recent
pandemic concerns.
Ensuring good CAQ depends on several factors. The air distribution system has an important function in
introdusing the outdoor air into the classroom. When a mechanical ventilation system is in place, mixing
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ventilation (MV) systems are used for fresh air provision in classrooms and the dilution principle is applied.
However, studies indicate that other air distribution systems (displacement or personalized ventilation) can
provide efficient ventilation in classrooms, considering the constraints of infectious respiratory diseases.
Several studies indicate that existing ventilation methods are not appropriate for preventing short-range
airborne transmission of respiratory droplets between indoor occupants (even more so if the occupancy rate
indoors is high), and new intervention methods, for example, personalized ventilation, which delivers fresh
air in the breathing zone, are recommended [154,155]. However, personalized ventilation should
complement other HVAC systems, and complex installation is needed if this was not considered at the
concept phase of the building. Additionally, local discomfort can be felt by the users [156].
Displacement Ventilation (DV) has a higher ventilation efficiency in the occupant zone [25,26] and is
characterized by a low momentum flow. Compared with mixing ventilation, DV provides better air quality
in the occupatied zone. The type of supply air diffuser does not seem to be of significant importance as long
as the principle of displacement is respected. However, the system is efficient when the supply operates in
isothermal conditions or with cooler airflow, while for heating mode, complementary or adjustable systems
are necessary [157]. Moreover, the children can feel thermal discomfort at specific air velocities. When
increasing the momentum, a hybrid system replaces the DV with a confluent jet system, which performs
slightly better for higher heat loads [158,159]. The stratum ventilation system also brings fresh air into the
breathing zone, allowing the inlet air to flow out horizontally [160]. Though, in the case of schools with
high occupancy, the crossflow infection risk for highly contagious diseases could be substantial.
Another low momentum system is represented by an underfloor air distribution system which can perform
better in the case of airborne infection risk due to vertical flow. For such reasons, it is required that the
exhaust outlets should be far away from the breathing zone of the occupants, and special attention should
be given to teachers who are usually standing [161]. Nevertheless, in such cases, the dust and particles on
the ground can be driven into the flow.
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3.3. Thermal comfort in school classrooms
Indoor thermal conditions in classrooms are particularly significant as school children are more vulnerable
to adverse environmental stimuli than adults [2,162–164]. Research literature reports physical and
physiological differences between children and adults, including different surface-area to mass ratios,
sweating rates, metabolism, body temperature, and cardiac output [165–167]. Havenith [165] collected
data on the metabolic rate of Dutch school children in classrooms and found their metabolic rates (watts
per square meter body surface area) were lower than an adult for a similar level of activity [164]. For
instance, the metabolic rate of senior primary school children (i.e. 10-11 year olds) for passive activities in
the classroom ranging from 62 to 64W/m² is 10% lower than the values for office sedentary activity (i.e.
70W/m²) stipulated in ISO 7730 [168]. Children have a larger ratio of body surface area to mass compared
to adults; the surface-area to mass ratio of an 8-9 year old child (e.g. 130cm, 20kg, 0.87m2) can be 40%
greater than that of an adult (e.g. 175cm, 67kg, 1.81m2) [169]. Also, children have a lower sweating rate
(which is proportional to metabolic rate) [166,169] and lower cardiac output [167].
Aside from the physical and physiological differences between children and adults that may influence their
thermal regulation and perception, distinctive contextual factors should also be considered [170]. Adjusting
clothing based on indoor and outdoor temperatures is an important method to help occupants adapt to the
surrounding thermal environment. However, Kim and de Dear [171] found that Australian pupils' clothing
insulation remained almost unchanged across the entire range of indoor and outdoor temperatures during
the whole survey period in subtropical Sydney, perhaps because of the presence of a school uniform dress
code or peer group norms. In addition to the reduced degrees of freedom for clothing adjustment, the
possibilities of modifying activity level (metabolic rate) or adjusting environmental variables (e.g. opening
windows or doors, using fans, etc.) are limited for pupils during lessons. Pupils in classrooms are not active
users of the environment but rather passive recipients of the conditions. Teachers are active users, but they
are more likely to adjust classroom temperatures based on their thermal preferences rather than those of
pupils.
Haddad et al. [172] discussed Iranian pupils' thermal comfort and confirmed that children's thermal
neutrality was a few degrees lower than adults under identical thermal conditions, which could be due to a
difference in their metabolic rate level. Similarly, Kim and de Dear [171] collected 4,866 responses from
school classrooms in Australia across two summer seasons. They found that the pupils generally preferred
"cooler-than-neutral" sensations. The preferred temperature was estimated to be 2–3°C below the neutrality
predicted for adults under the same thermal environmental exposures. Studies in Chile [173] and the
Netherlands [174] also indicated lower comfort temperatures of pupils compared to adaptive models.
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Dorizas et al. [175] investigated CAQ in schools and found that a temperature of 22.31°C made the pupils
feel satisfied, while temperatures above 25°C made them feel dissatisfied.
On the other hand, Liang et al. [176] found the neutral temperature for the pupils in the hottest month in
Taiwan to be up to 29.2°C, which is higher than the corresponding value stipulated in the ASHRAE
Standard 55 [177]. According to recent reviews [178,179], the general consensus is that school pupils tend
to feel comfortable in indoor climates that are "slightly cooler" than the adult thermal neutralities observed
in office settings.
A high-quality classroom thermal environment should benefit pupils' academic performance. It is suggested
that the magnitude of the negative effect of classroom temperature on performance was, for some tasks, as
high as 30% [180]. Still, there are few studies on direct associations between indoor classroom thermal
conditions and performance [2]. Romieu et al. [181] found a connection between temperature and
absenteeism for respiratory illness. The probability of absenteeism was 1.28-fold higher in high-exposure
compared to low-exposure pupils. There are two competing schools of thought on the relationship between
temperature and performance.
Five decades ago, Wyon conducted an experiment with Swedish children under three classroom
temperatures, concluding that children's performance of school exercises was significantly lower at 27°C
and 30°C in comparison with 20°C [180]. Haverinen-Shaughnessy and Shaughnessy correlated state-wide
assessment of learning with measured classroom temperatures, finding a 13% increase in math scores for
every 1°C decrease of classroom temperature [182]; however, the state-wide assessment was not always on
the same day that the temperature measurement was carried out, making their conclusions about
temperature-performance relationships tenuous. Porras-Salazar et al. [183] found the neutral temperature
for pupils in a tropical climate to be 27°C and a slightly cool environment most conducive to the
performance of schoolwork to be 25°C. Wargocki et al. [17] examined all studies on the effect of thermal
environment on pupils’ performance and found that temperatures below 22°C would be optimal. However,
it is hard to reach a consensus with limited studies reflecting the direct associations between indoor
classroom thermal conditions and performance so more research is needed to confirm the most appropriate
model to guide the design and operation of the classroom environment.
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3.4. Pupils' performance and classroom air quality
Classroom air quality and cognitive performance
Most of the studies investigating the effects of CAQ on cognitive performance are summarized in Table 1.
These studies confirm that poor air quality affects both typical schoolwork of pupils, i.e. performance in
simple learning tasks such as math and language exercises and pupils' examination grades and end-of-the-
year results. Some studies observed that poor CAQ also increased absenteeism, a marker of health effects
and their impact on proper learning.
Low classroom ventilation rates can impair pupils' attention and vigilance, lowering memory and
concentration [184]. This study showed that in poorly ventilated classrooms, pupils are likely to be less
attentive. The magnitude of the adverse effects of inadequate ventilation was even higher for tasks that
require more complex skills such as spatial working memory and verbal ability to recognize words and non-
word data.
Table 1: The effects of indoor air quality in classrooms on cognitive performance and learning by children [38]
Study
Classroom air
quality (CAQ)
Measurements of
cognitive performance
or learning or absence
rate
Major results
Myhrvold et al. [185]
CO2: 1500-4000 vs.
<1000ppm
Simple reaction time.
Reduced CO2 levels improved
performance.
Ribic [186]
CO2: 3800 to 870ppm
Concentration and attention
(d2-test).
Reduced CO2 improved
performance.
Sarbu and Parcurar
[187]
CO2: 2,000 to
500ppm
Concentration and cue-
utilization (Kraepelin and
Prague tests).
Reduced CO2 improved
performance.
Coley et al. [188]
CO2: 2900 to 690ppm
Reaction time.
Improved performance at lower
CO2 levels.
Bakó-Biró et al. [184]
1 L/sp to 8 L/sp
(1,500-5000 to
<1,000ppm)
Reaction time, concentration
and attention, recognition
and memory.
Improved performance at a higher
ventilation rate.
Mattsson and Hygge
[189]
Reduced particle
levels and cat
allergen.
Five performance tests.
Finding synonyms improved but
most likely due to chance.
Hutter et al. [190]
Reduced levels of
tris(2-chlorethyl)-
phosphate (TCEP) in
PM10, PM2.5, and
dust; and CO2.
Reasoning component of
general intelligence
(Standard Progressive
Matrices).
Cognitive performance improved
with reduced levels of pollutants.
Wargocki and Wyon
[191]
Ventilation rate
between 3 and 10
L/sp.
Arithmetical calculations
and language-based tasks.
The speed at which tasks were
performed improved with no effects
on errors.
Bakó-Biró et al. [184]
Ventilation rates
changed from 0.3-0.5
to 13-16L/sp.
Arithmetical calculations
and language-based tasks.
Task performance improved with
increased ventilation.
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Petersen et al. [192]
Ventilation rates
changed between 1.7
and 6.6L/sp.
Arithmetical calculations
and language-based tasks.
Performance of addition, number
comparison, grammatical
reasoning, and reading and
comprehension improved at a
higher ventilation rate.
Hviid et al. [193]
The ventilation rate
changed from 3.9 to
10.6L/sp.
Arithmetical calculations
and language-based tasks.
Processing speed, concentration,
and mathematical processing
improved.
Wargocki et al. [194]
Concentrations of
airborne particles
reduced in all size
ranges and reduced
settled dust on
horizontal surfaces.
Arithmetical calculations
and language-based tasks.
No effects on cognitive
performance.
Haverinen-
Shaughnessy et al.
[195]
Different CO2 levels
corresponding to
ventilation rates up to
7L/sp.
Language and mathematical
examinations.
3% more pupils passed the tests for
every 1 L/sp increase in ventilation.
Haverinen-
Shaughnessy and
Shaughnessy [182]
Different CO2 levels
corresponding to
ventilation rates up to
7L/sp.
Mathematical scores.
Math scores improved by 0.5% for
every 1 L/sp increase in ventilation.
Mendell et al. [196]
Ventilation rates and
CO2 levels.
Scores in mathematics and
English.
A 10% increase in ventilation
resulted in a 0.6 point increase in
the score obtained in the English
test.
Toftum et al. [197]
Classrooms with
natural ventilation,
exhaust ventilation,
and mechanical
ventilation systems.
Academic achievements
(Standardized Danish test
scheme) – mainly language-
based and math tests.
The lowest scores were observed in
naturally ventilated classrooms with
the highest CO2 levels.
Gaihre et al. [198]
CO2: 600 to
2,100ppm
Educational attainment
measured as the % of class
attaining the average level
expected for the group.
No relationship was observed
between CO2 levels and educational
attainment.
Kabirikopaei et al.
[199]
Ventilation, particle
levels, ozone.
Reading and mathematical
scores.
Scores higher with increased
ventilation rates. Fine particles
were associated with math scores
and ozone with reading scores.
Lau et al. [200]
Presence of unit
ventilators.
Reading and mathematical
scores.
Presence of unit ventilators
associated with higher coarse
particles, lower ventilation rates,
higher noise, and lower
mathematics scores.
Murakami et al. [201]
Ventilation rate
changed between 0.4
and 3.5h-1.
Learning by college pupils.
Learning improved with increased
ventilation.
Ito et al. [202]
Ventilation rate
changed between 0.4
and 3.5h-1.
Learning by college pupils.
Learning improved with increased
ventilation.
Pilotto et al. [203]
Pollutants from gas
heaters.
Attendance in schools.
The presence of pollutants reduced
attendance.
Berner [204]
School maintenance.
Academic achievements.
Poor maintenance reduces
academic achievement.
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Ervasti et al. [205]
Perceived air quality.
Short-term sick leave.
Sick leave (of teachers) increased
with poor perceived air quality.
Shendell et al. [206]
CO2 concentration.
Sick absence.
Pupil absence decreased by 10-20%
when the CO2 concentration
decreased by 1,000ppm.
Gaihre et al. [198]
CO2: 600 to
2,100ppm
Absence rates.
An increase of 100ppm of CO2
corresponds to a 0.2% increase in
absence rates.
Simons et al. [207]
Poor ventilation.
Sick absence.
Higher sick absence linked with
poor ventilation.
Kolarik et al. [208]
CO2 below 1000ppm
(average 640ppm).
Sick absence (day-care
centers).
Increasing the air change rate by
1 h-1 would reduce the number of
sick days by 12%.
Mendell et al. [209]
CO2 levels.
Illness absence.
Illness absence decreased by as
much as 1.6% for each additional 1
L/s per person of the ventilation
rate.
Deng and Lau [210]
Different parameters
characterizing CAQ.
Illness-related absenteeism.
Presence of fine particles during the
cooling season increased absence
rates, while the increased
absenteeism during the heating
season was caused by reduced
ventilation (indicated by the
increased CO2 levels).
Wargocki et al. [17] analyzed all published evidence on the effects of CAQ where the measurements of
CO2 (a proxy for classroom ventilation) were reported along with the cognitive performance of pupils. They
aimed to establish the impact of the indoor environment parameters on pupils’ performance and attempted
to identify the minimum air quality levels needed to avoid the risk of reduced performance. They separately
analyzed the results from the studies examining schoolwork, grades and exams, and absence rates. In the
absence of an air quality metric, they used CO2 as an indicator of IAQ (ventilation).
Figure 3 shows the relationships established by Wargocki et al. [17]. They concluded that increasing the
ventilation rate in classrooms to 10L/s per person would bring significant benefits and improve learning
and reduce absenteeism. It was found that the CO2 concentration should be kept at or below 900ppm. No
data could be found on whether CO2 levels lower than 900ppm or ventilation rates higher than 10L/s per
person would bring additional benefits. However, considering that the relationship between the
performance of office work and ventilation is log-linear [211], it is likely that additional ventilation
improvements would bring further benefits, as also suggested by the relationships presented in Figure 3.
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Figure 4: Performance of schoolwork (speed), national and aptitude tests and exams, and pupils' daily attendance as a
function of classroom ventilation rates [17]
Social impact of classroom air quality
There are very few assessment studies on the impact of improving CAQ on socio-economic benefits.
Wargocki et al. [212] estimated the benefits of improved ventilation in Danish classrooms. Assuming that
all Danish classrooms are ventilated at a rate of 6L/s per person, which is the case for about 50% of
classrooms [197], an assessment was made of the benefits that might be obtained if the ventilation rate is
increased to 8.4L/s per person, which is the requirement in Sweden. Using the Danish Rational Economic
Agency Model (DREAM) and data from Chetty et al. [18], it was estimated that improvements in
ventilation would yield an average annual increase in the gross domestic product (GDP) of €173 million
and an average annual increase in the public budget of €37 million in the following 20 years. The impact is
generally due to more pupils completing their education under favorable learning conditions. These
estimates were based on increased productivity in adult life due to better exam grades in school, fewer
pupils staying longer in elementary schools (which is a non-compulsory 10th grade in Denmark), resulting
in overall shorter education periods, reducing the period for joining the job market, and reduced teacher
sick leave.
It is well established that indoor air quality improves by increasing the outdoor air supply rate. There are
also studies showing the benefits of using mechanical ventilation systems in schools [213]. However, very
limited data exist on the effects of using other approaches to control sources of pollution in classrooms or
the use of air purifiers [214].
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3.5. Energy and classroom environment quality
Worldwide energy consumption is continually growing, and a large proportion of this growth is associated
with non-domestic buildings which consume 11% of European and 18% of the USA's total energy [215].
The most influential factors in building energy consumption have been reported by Yu et al. [216]: 1)
Climate; 2) Occupant behavior; 3) Building-related; 4) User-related; 5) Service and operation of the
building; 6) Social and economic factors; and 7) The required indoor air quality. All of these parameters
can contribute to higher or lower energy consumption. Building-related parameters can be age, orientation,
window-to-wall ratio, area, leakage of the building envelope, and U-value. The service and operation of the
building also influence energy consumption. This can be the operation time of the HVAC system,
maintenance procedures, and the age of the system [216].
Breakdown of energy consumption
Although some benchmarks provide category energy breakdowns (e.g. lights, cooling, heating, ventilation),
such data still fail to show the yearly energy consumption associated with each category. Norwegian
Standard NS 3031:2014 [217] presents a breakdown of energy consumption into six categories,
distinguishing between thermal energy needs (Categories 1, 2, and 3) and electricity-specific energy needs
(Categories 4, 5, and 6). These energy posts can generally be simulated in software with standard values
from NS 3031:2014. On the other hand, field measurements of energy per purpose are often impossible
because some buildings are not equipped with detailed sensors [218]. Sometimes buildings are equipped
with sensors to measure energy consumption, but there can be problems with data communication. Ouf and
Issa [219] state that different metrics serve various purposes and presented results on the same data set by
analyzing energy consumption per occupant and per floor area at schools in Manitoba, Canada. This study
shows that middle-aged schools consumed the most energy when using energy consumption per floor area.
Analyzing energy consumption per occupant, the oldest schools consumed the most energy.
School buildings' energy consumption
There have been many reports summarizing the energy consumption of school buildings in various
countries and regions worldwide. According to the National Center for Education Statistics, almost 100,000
public K-12 schools representing 5% of commercial building energy consumption expend $8billion in
utility bills and serve 50 million students plus three million teachers. A report from Texas found that 71%
of school units use $70-200 of energy per pupil [220]. The annual energy consumption of primary and
secondary schools in the eight US climate zones are 173kWh/m2 and 257kWh/m2 respectively [221]. In
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China, a survey shows that the total annual energy consumption of school buildings in the cold region is
about 103kWh/m2, and the average annual power consumption per unit area of the surveyed public
buildings is 24.31kWh/m2 [222]. However, school buildings' average annual power consumption in
temperate regions is about 30.61kWh/m2, which is slightly higher than the national average of 29.6kWh/m2
[223]. In Hong Kong, the reported comprehensive annual energy consumption of school buildings is
105.61kWh/m2 [224].
Most Norwegian schools have high energy needs for ventilation, heating, and a minimum of cooling
demand. The report highlights the increasing cooling need because of increasing heat-generating ICT
(Information and Communications Technology) infrastructure in schools [218]. Kilpatrick and Banfill
[225] have collected energy data from 48 schools, including 32 Secondary schools, 11 primary schools, and
five specialized schools, and showed when and how much energy is used in a wide range of schools.
Table 2: School details investigated by Kilpatrick and Banfill [225]
Year
FA
(m²)
TEU
(kWh)
Year
FA
(m²)
TEU
(kWh)
Year
FA (m²)
TEU
(kWh)
Year
FA
(m²)
TEU
(kWh)
1960
2535
195,221
1960
15368
695,154
1960
9561
888,443
1960
11852
605,890
1980
9835
342,507
1970
11535
643,994
1930*
14909
687,511
1979
10156
492,587
1989
11430
512,819
1893*
11742
565,302
1940*
13559
607,708
1975
11927
945,627
1991
12349
863,421
1978
11436
1,433,075
1940*
11052
730,518
1960
1225
235,543
1954
13145
441,056
1965
11918
584,281
1950
14265
602,720
1980
7871
354,727
*School built at this date, but renovated after 2000; FA (Floor Area); TEU (Total Energy use)
The average specific energy consumption for schools in Norway is reported as 170kWh/m2 per year. The
total energy consumption includes space heating, ventilation, hot water, ventilation aggregates, lighting,
and other electricity use. Ding et al. [226] investigated 40 schools connected to the district heating grid in
Trondheim. The main finding was that the predicted annual demand for district heating and electricity was
respectively 72kWh/m2 and 57kWh/m2. This gives a total annual energy consumption of 129kWh/m2.
In a recent publication [227], the annual energy consumption value presented for Cyprus schools, based on
billed energy, is 62.75kWh/m2 and 116.22kWh/m2, when expressed in primary energy.
The topic of energy relating to Hellenic schools has been abundantly published [228–234]. Greek climatic
zone definitions have been changed. There were three climate zones within the previous regulation (TIR)
(A–C). KENAK introduced an additional climate zone (D) within the northern regions of the country (zone
C) [234]. In 2011, Dascalaki and Sermpetzoglou [229] undertook a comprehensive study aiming at
assessing the energy performance of schools on a national level, embracing the three climatic zones (A–C).
The collected data was used to define “typical” values, in other words, energy performance benchmarks.
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From a total selection of 500 schools, the average thermal, electrical, and total annual energy consumption
was found equal to 57, 12, and 69kWh/m2, respectively.
In March 2012, in a press release reported by the Paris mayor, the energy profile of schools in this city was
revealed as 224kWh/m2 [221]. The value presented is expressed in primary energy comprising all the energy
consumption in the Parisian schools (half of which were constructed between 1880 and 1948).
In North America, a reference table for Canada has been designed for different buildings to help balance
their energy use to the national median. Herein, the recommended benchmark metric is the national median
source – Energy Use Intensity (EUI), expressed in GJ/m2. The median EUI value is 197kWh/m2. Since site
EUI results in a mixture of energy (primary and secondary energy, depending on the type of energy provided
to the building, e.g. raw fuel like natural gas vs. a converted product like electricity), the use of source EUI
is recommended (the median of 283kWh/m2) [235].
The values presented by Kim et al. [236], relating the average energy consumption of the elementary
schools in South Korea, are expressed in MJ/m2 in terms of annual energy use (electricity, oil, and gas) and
per capita, ranging between 2,951MJ/pupil to 3,889MJ/pupil. The sum of the three fuel types (energy
consumption per unit area) was determined as 101.4kWh/m2, 72% of which corresponds to electric energy
use.
Indoor environment and energy consumption
The Energy Efficiency–Thermal Comfort–Indoor Air Quality dilemma is a relationship discussed in the
research, amongst others [237]. It is essential to investigate and establish this relationship because energy
efficiency measures in a building cannot be at the expense of the indoor environment. Zhang and Bluyssen
[238] studied the indoor environment and energy consumption at nine primary schools in the Netherlands.
Energy consumption was analyzed and categorized based on total energy consumption per category: year
of construction, area, number of occupants, and ventilation system. The low-consumption buildings were
the newest with fewer occupants, while the high-consumption schools were older, with more occupants.
The low energy consumption schools had lower measured relative humidity than the high-consumption
schools.
Pearson’s correlation coefficient was used to assess potential correlations between energy consumption,
measured indoor environmental parameters, and perceived indoor climate based on user satisfaction
surveys. They concluded that the higher the electricity consumption, the more pupils complained about the
IAQ. In general, they uncovered more complaints in the high-consumption schools. None of the correlations
between measured indoor environment and energy consumption was found to be significant. The
researchers recommended a higher resolution analysis.
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A study from Gothenburg investigated 30 schools regarding yearly energy consumption per unit floor area
and indoor environmental parameters [239]. Ventilation categories of the investigated schools were: (A)
natural ventilation, (B) balanced ventilation with constant air volume (CAV), and (C) balanced ventilation
with variable air volume (VAV) or demand-control ventilation (DCV). Based on field measurements of
CO2 concentration, temperature, and added humidity, this study reported a negative correlation between the
year of construction and yearly energy use [kWh/m2] in the whole sample. The weekly average temperature
and energy performance for category A was positive and weak. For categories B and C, it was negatively
solid and significant. The weekly average CO2-concentration and energy performance found weak and
insignificant relationships. This showed that the correlations were sporadic and differed over the categories.
Other studies also showed that DCV could reduce energy consumption [240]. The measurements show that
in all the case studies, the DCV system delivered and maintained good IAQ, even at reduced airflow rates
[241]. Results of the case studies show that significant reductions in energy consumption are achieved for
both the fans (50–55%) and ventilation heat losses (34–47%) [242].
Diffuse ceiling ventilation works through the low-impulse supply of air through the perforated panels
installed as the suspended ceiling and was also subjected to investigation in many studies [243–247]. This
ventilation system is proven to provide a good IAQ while lowering ventilation energy consumption.
Allab et al. [248], Ghita and Catalina [249], Dascalaki and Sermpetzoglou [229], and Pereira et al. [250]
also investigated energy consumption and IAQ. A recent review article [44] suggests that control based on
the internet of things (IoT) or artificial intelligence (AI) could be an effective method of providing
optimized solutions for mixed ventilation strategies to balance natural and mechanical ventilation types in
school buildings.
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3.6. The impact of the outdoor environment on classroom air quality
The WHO indicates that all non-communicable diseases together accounted for 74% of the total deaths
globally in 2019 [251]. Comparing different global risk factors shows that ambient air pollution is a leading
cause of excess mortality and decreased life expectancy [252]. Air pollution was even more of a serious
health problem than COVID-19 in 2020 [253].
Outdoor air pollution impacts IAQ using air change rates, including natural ventilation, mechanical
ventilation, and infiltration [12,254–257]. Meanwhile, some pollutants are brought indoors through people's
activities; for example, environmental bacteria and particles are transferred from shoes onto floors and
carpets [254,258]. The main influencing factors can be grouped into outdoor contaminant concentrations
and meteorological conditions [259,260].
Outdoor contaminant concentrations
For schools, outdoor air pollutant sources, such as high-density traffic areas or industrial and construction
activities, play an important role in final IAQ performance [134]. Studies have shown that numerous schools
are located in areas with high levels of air pollutants [136,261,262]. The school's geographical location
plays a significant role in formulating its indoor and direct outdoor air quality.
Many field tests have measured indoor and outdoor air quality in schools near particular locations such as
industrial areas [263–265], transportation zones [266], and port areas [267]. The indoor air quality in a
primary school located near a high-impact industrial site in Italy was assessed. The VOC concentrations
were in line with or above those of other studies conducted in the same condition [264]. High metals and
polycyclic aromatic hydrocarbon concentrations were detected, especially when schools were downwind
of a steel plant. The indoor/outdoor (I/O) ratio showed the impact of outdoor pollutants, especially of
industrial markers, such as Fe, Mn, Zn, and Pb, on indoor air quality [265]. A classroom near a busy
intersection on a main arterial road was monitored. It was found that the by-products of motor vehicle
emissions were the main contributor to indoor PM2.5 [266], black carbon, and nanoparticles [268]. Similar
results were obtained for schools in Greece and New Zealand, where combustion products from vehicles
are the critical source of airborne particles [269,270].
Meteorological conditions
The outdoor temperature, relative humidity, and wind speed affect I/O ratios [271]. Based on air pollution
monitoring area data, a strong correlation between air pollutants and meteorological indicators was
observed [272]. In non-winter periods, the outdoor temperature is higher than that indoors. This creates a
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thermal gradient, and so the outdoor air flows indoors, increasing the I/O of particulate matter [273]. The
higher humidity during non-winter seasons reduces the outdoor particulate matter concentrations and
increases the I/O ratio [274]. Further, higher wind speed increases the infiltration of outdoor particulates
indoors. Correlation analyses show that outdoor meteorological factors affect indoor PM2.5 concentrations
[275]. Hence, meteorology plays a vital role in the migration of particulate matter indoors [276].
Airtight buildings have grown rapidly in order to conserve energy, to reduce the infiltration of outside air,
and to make circulation of inside air in the occupied zone. There is a nexus between the ventilation and
indoor air quality in buildings [277] as while airtight buildings can help conserve energy, they accumulate
pollutants inside. The use of natural ventilation not only provides acceptable IAQ levels but also reduces
energy use given the consequent reduction in the use of mechanical ventilation [278]. Outdoor pollution
should be accounted for when making decisions on using a natural ventilation strategy. This is especially
true for developing countries [279]. Hence proper management for school building characteristics is needed.
Considering the mentioned problems, it is essential to develop practical methods of providing pollutant
concentrations using the limited information available from public sources [280]. Taking these measures,
we can get a quick picture of the pollution situation and further make better stratigies for improving IAQ.
Site-specific strategies for different school locations, such as transportation areas and industrial areas,
should be developed to suit the characteristics of the schools in different areas.
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4. Roadmap for the future improvement of classroom air quality
4.1. Raising Awareness
Deepening occpuants’ understanding of IAQ
Occupant behavior is one of the factors affecting CAQ. Therefore, there is a need to encourage and train
school occupants (mostly teachers and children) [281,282]. Scientific activities and seminars are necessary
to improve the occupants' knowledge and perception regarding the importance of indoor air quality [283].
To identify current problems and raise the solutions to solve these problems, children are asked to
conceptualize solution. It is found that children can be valuable contributors in co-designing classroom
environments [284]. By comparing the test-retest repeatability of questionnaires filled by children and
parents, it can be concluded that children can give as, or even more, repeatable information about their
respiratory symptoms and perceived indoor air quality than their parents[285]. Therefore, it may be possible
to learn more about the needs of children and their ideas for improving indoor air quality.
Deepening the understanding of pollutants characteristics
Compared with single pollutant measurements, multiple pollutants are more frequently studied. Most of
these studies are focused on particulate matter, CO2, VOCs, and bioaerosols. However, the sample size for
many measurements is not large. Most of the measurements are conducted for less than one year. Therefore,
further research is needed to analyze the pollutant characteristics in school buildings in the world's different
climatic, social, and cultural regions [286]. Long-term measures are essential for clarifying the hazards of
contaminants. Simultaneous effects of different local factors add complexity, and more studies during
different seasons are needed to identify additional developments in the future [287]. For other types of
pollutants, more in-depth research is required in order to understand the specific mechanism of the impact
on CAQ. Future research should aim at in situ measurements and a source apportionment approach to
investigate CAQ levels within educational buildings to secure healthy conditions for the pupils and staff.
For some pollutants, like the airborne particles, experimental investigation of the indoor school
environment is often difficult and expensive and poses several logistical and practical difficulties. Thus, it
cannot be done frequently; additionally, air quality measurement to clarify uncertainties during early design
stages is not possible. Physical processed are needed to address these situations. Numerical investigation is
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a great alternative in complementing laboratory and on-site measurements. Alternatively, numerical
simulations based on the computational fluid dynamics (CFD) technique can be a powerful tool to
compliment measurement studies and provide valuable information regarding influential parameters in
assessing CAQ [159,161]. In the future, more parameters and specific data should be obtained for CFD
analysis to get a more in-depth understanding at contaminant characteristics.
Deepening the understanding of health and performance effects
Many studies have focused on health and performance effects. Studies on social, economic, and
multiple/synergic impacts are lacking. The main research hotspots are academic achievement performance
and health effects associated with respiratory symptoms. Extending the analysis to other buildings such as
homes seems necessary to determine children's exposure to indoor air with more accuracy and to assess
their lifetime health risks [288]. A cross-sectional study is a commonly used method to investigate the
relationship between health impacts and indoor pollutants. A longitudinal study would help increase the
robustness of the quantitative analysis of the effects of the duration of pollutant exposures on health
symptoms [289]. In addition, toxicological evaluations are recommended to develop practical risk
assessments in future research [25]. More in-depth analysis of contaminants, such as characterizing
particles' chemical composition, is needed to assess toxicology and health impacts [290]. It would also be
helpful to examine how indoor environment quality in homes influences children's sleep quality and,
consequently, whether it affects the next day's performance in schools and learning. Light exposure in
schools and stress caused by exposures in classrooms may result in sleep disturbance of pupils and
consequently poor cognitive performance and learning. It would be useful to examine these issues as well.
Finally, the socio-economic consequences of health and performance effects on children in classrooms
should be considered, including also the impact on teachers. Children staying at home because of health
problems generate absenteeism from parents and guardians. Poor learning may have consequences on future
incomes and thus may have consequences for individuals and society.
4.2. Source Control
Outdoor environment
The primary source of PM contamination in schools is outdoor air, like traffic and industrial emission [112].
The main soure of CO and NO2 is traffic. Infiltration from outdoor air strongly influences indoor levels, in
particular within short distance from roadways or high-density industrial or traffic areas. Studies have
shown that numerous schools are located in areas with high levels of air pollutants [133,259,260]. The
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school's geographical location plays a significant role in formulating its indoor and direct outdoor air
quality. PM10 and total bacteria count levels for schools surrounded by roadways were found to be
significantly lower than surrounded by buildings and mountains [291]. Therefore, suitable management for
school building characteristics is needed. For new schools, reasonable consideration needs to be given to
the location of the school, and for existing schools, pollutant-free control is required based on the
environmental characteristics of the school's surroundings.
Indoor material and activity
VOC exposure in schools is often related to construction materials, furnishings and painting materials , etc.
Some of these emissions can be prevented by using low-emitting materials like improved plastics and paints
(phenol resins instead of urea resins, polyurethane coatings, etc.) and solid wood or old furniture [291]. In
addition, sealing and storing the liquid materials (paints, adhesives, cleaning products,etc.) and minimizing
storage periods can mitigate pollution to some extent [292]. Pupil activities is an important source of particle
resuspension. Vacuum cleaning has a significant effect on reducing resuspension of small and larger
particles, 2.5–10 µm particles [293].
4.3. Mitigation Measures
The use of natural ventilation provides acceptable IAQ levels and reduces energy use given the consequent
reduction in the use of mechanical ventilation [278]; therefore, most of the schools choose natural
ventilation as the primary method for improving IAQ. However, indoor air levels were affected by
surrounding environments [294,295] and improper natural ventilation practices may deteriorate indoor air
quality; thus, it is essential to develop mitigation strategies to improve the IAQ and prevent the transmission
of infectious disease in a naturally ventilated classroom [140]. For example, the proper design of the
window openings, the interior layout, and the fresh air intakes are important to the IAQ of existing buildings
adjacent to roadways [295]. However, the potential capacity of natural ventilation can be reduced by up to
88% considering WHO thresholds for PM2.5 according to a case study in Chongqing, China [279].
Relying on window opening as a tool for ventilation in heavily polluted areas is challenging because
increased ventilation decreases the indoor CO2 levels but increases the NO2 and SO2 levels [296]. Hence
proper management for school building characteristics is needed. Thus, it is essential to develop practical
tools for detecting pollutant concentrations using the limited information available from public sources
[280]. Taking these measures, we can get a quick picture of the pollution situation allowing better strategies
for improving CAQ to be devised.
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Field measurement, as well as numerical evaluation of CAQ, are the two methods frequently used to
evaluate ventilation effectiveness. Good quality ventilation measurements are essential to produce accurate
results. In many studies, the measurement approaches, boundary and climate conditions, and the statistical
analysis of data collected were not described in adequate detail to evaluate their quality, reliability, validity,
replicability, or applicability to the study design. For example, the airtightness of the school building needs
to be considered when evaluating the effect of natural ventilation [297]. The outdoor environment is an
inescapable factor affecting indoor air quality. In the future, site-specific strategies for different school
locations, such as proximity to transportation routes and industrial areas, should be developed to suit the
characteristics of schools in different regions. The research findings and recommendations could thus apply
to many other schools with the same features.
4.4. Integrated control
Building design for balancing energy efficiency and human perceptions
The building itself is one of the main factors in improving IAQ. Optimizing passive design parameters of
buildings (e.g., window to wall ratios, window orientations and sun shading installations) can significantly
reduce the ventilation demands while maintaining indoor thermal comfort [44]. Airtight buildings that have
been designed to conserve energy also reduce the infiltration of outside air. There is a nexus between
ventilation and indoor air quality in buildings [277] since, while airtight buildings can help conserve energy,
they can accumulate pollutants inside. However, when individuals stay indoors for long periods, they will
be at risk of adverse health effects through their exposure to a potentially polluted indoor environment over
a sustained period [298]. While studies are available around transport microenvironments [299], similar
research is needed for school classrooms to fill this existing gap in current understanding.
Choice of ventilation strategy
Different ventilation strategies have different performance in terms of improving IAQ, as well as energy
saving performance. The use of sustainable design, such as solar energy, can improve energy efficiency
while ensuring thermal comfort. Solar air heating technology is combined into the ventilation system. The
average value of hourly solar contributions can be as high as 34.3% over a heating season. Although the
economic effect of the new system is not the best, both its energy saving effect and environmental protection
effect are significant [300]. Some ventilation systems are complex, such as passive with heat recovery. The
feasibility of the system and the effectiveness needs to be taken into account. The assessment of the
ventilation performance of PVHR systems depending solely on wind and buoyancy is complicated as they
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are dynamic systems that constantly balancing with the surrounding conditions, and the operation is highly
correlated to the airtightness of the building's envelope [24]. It is necessary to develop more efficient and
energy-saving systems in the future.
Besides, a solid and quantifiable comparison between the low-cost mitigation measures to enhance the air
quality is recommended to clarify the economic and practical implementation and the effects on energy
sustainability, thermal comfort, health, and security of the occupants [301]. For the future, the application
of more expensive and complex mitigation measures should be evaluated.
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5. Conclusions
This article presents a comprehensive review of the last 50 years of classroom air quality research to
examine, discuss and understand the interaction between classroom air quality and pupils’ performance,
comfort, and health. The published articles summarized here investigated schools’ air quality in 40
countries worldwide.
Most schools worldwide have basic natural ventilation systems; however, inefficient performance is
inadequate for meeting the needs of their users. The design of new schools should require a particular type
of effective ventilation system for achieving good air quality and protection against exposure to airborne
particles and VOCs. When refurbishing existing schools, the challenge comes from finding a feasible
solution to meet the CAQ requirements given the existing infrastructure. Demand-controlled ventilation,
combined with an efficient air distribution system, could reduce the energy use required for mechanical
ventilation and trigger the biggest saving whilst securing the health and well-being of children in schools.
Probably the only general conclusion from the extant literature on thermal comfort is that school pupils
tend to feel comfortable in indoor climates that are generally cooler than environments (e.g. offices) where
adults feel thermally neutral. The classroom temperature that pupils deem comfortable depends on many
factors, including, amongst others, the climatic context of the pupils and their prior exposure to air
conditioning. More studies on the direct associations between indoor classroom thermal conditions and
pupil performance are needed to confirm the suitable temperature-performance model.
In terms of pupils' learning performance, earlier studies consistently show that reduced classroom air quality
will cause a reduction in cognitive performance of pupils with resulting negative consequences for
progressive learning whilst increasing short-term sick leave. Most of the published work relates to the
performance of school work, with the measurements of CO2 concentrations being the proxy for classroom
ventilation and air quality. Little data exists regarding the effects of specific pollutants, and such studies are
much needed. The existing evidence suggests that keeping classroom CO2 levels below 900ppm (absolute
level) reduces the negative impact on learning, but even lower levels may be more conducive; however,
data for lower CO2 levels are scarce. Children also prefer a cooler environment for effective learning.
Exposure to various air pollutants in school buildings risks severe damage to pupils’ health since they inhale
a larger volume of air corresponding to their body weights than do adults. This is especially important as
many studies reported higher pollutant concentrations in schools than in residential and commercial
buildings. The VOC pollutants are among the leading indoor air pollutants causing severe health issues for
children and adults. On the other hand, many schools have identified particulate matter pollution as a major
source of indoor air pollution. In addition, Penicillium, Cladosporium, Aspergillus, and Alternaria were the
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most common fungi found in school indoor environments, and their prevalence varies depending on climate
and location, whether rural or urban.
Worldwide energy consumption is continually growing, and a large proportion of this growth is associated
with non-domestic buildings. While few research studies provide a breakdown of energy consumption by
energy category, including thermal energy and electrical energy, there is limited insight demonstrating
detailed energy use profiles for heating, ventilation, and other building service systems in school buildings.
There is a great need for more comprehensive studies with larger sample sizes, including prospective cohort
studies, with a characterization of strategies to promote indoor school environmental quality on
environmental health exposure, student health and wellness outcomes, indoor satisfaction, and cognitive
performance. Both ecological and behavioral factors affecting classroom air quality should be characterized
along with the effects of indoor environmental controls on energy consumption.
Acknowledgment
This literature review study is inspired by the UK Clean Air Programme TAPAS-Tackling air pollution at
school (NE/V002341/1). The authors from the Joint International Research Laboratory of Green Buildings
and Built Environments would like to thank the sponsors for the research project “School Building and
Children’s Health” [Grant No: B13041]. This research study was financially supported by the Swedish
Research Council Formas [Grant No: 2021-01422]. The authors are also grateful for the input from
Sustainable Energy & Environmental Systems Department at Lawrence Berkeley National Laboratory.
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Highlights
• Most schools worldwide have basic natural ventilation systems; however, inefficient
performance is inadequate for meeting the needs of pupils.
• Exposure to various air pollutants in school buildings risks severe damage to pupils’
health.
• Pupils tend to feel comfortable in indoor climates that are generally cooler than
environments where adults feel thermally neutral.
• In terms of pupils' learning performance, earlier studies show that reduced classroom air
quality will cause a reduction in cognitive performance of pupils.
• There is limited insight demonstrating detailed energy use profiles for heating, ventilation,
and other building service systems in school buildings
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Author Statement
Sasan Sadrizadeh: coordination, paper structure, writing abstract/conclusion; All other authors are
helping in writing a section, commenting on the text, conducting literature survey for their section.
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03-Mar-2022
Dear Editor-in-Chief
The authors declare no conflict of interest.
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