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Abstract

COVID-19 disease is caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), which originated in Wuhan, China and spread with an astonishing rate across the world. The transmission routes of SARS-CoV-2 are still debated, but recent evidence strongly suggests that COVID-19 could be transmitted via air in poorly ventilated places. Some studies also suggest the higher surface stability of SARS-CoV-2 as compared to SARS-CoV-1. It is also possible that small viral particles may enter into indoor environments from the various emission sources aided by environmental factors such as relative humidity, wind speed, temperature, thus representing a type of an aerosol transmission. Here, we explore the role of relative humidity in airborne transmission of SARS-CoV-2 virus in indoor environments based on recent studies around the world. Humidity affects both the evaporation kinematics and particle growth. In dry indoor places i.e., less humidity (< 40% RH), the chances of airborne transmission of SARS-CoV-2 are higher than that of humid places (i.e., > 90% RH). Based on earlier studies, a relative humidity of 40–60% was found to be optimal for human health in indoor places. Thus, it is extremely important to set a minimum relative humidity standard for indoor environments such as hospitals, offices and public transports for minimization of airborne spread of SARS-CoV-2.
Special Issue on COVID-19 Aerosol Drivers, Impacts and Mitigation (V)
Aerosol and Air Quality Research, 20: 18561861, 2020
ISSN: 1680-8584 print / 2071-1409 online
Publisher: Taiwan Association for Aerosol Research
https://doi.org/10.4209/aaqr.2020.06.0302
Copyright The Author(s). This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits
unrestricted use, distribution, and reproduction in any medium, provided the original author and source are cited.
An Overview on the Role of Relative Humidity in Airborne Transmission of
SARS-CoV-2 in Indoor Environments
Ajit Ahlawat1*, Alfred Wiedensohler1, Sumit Kumar Mishra2
1 Leibniz Institute for Tropospheric Research (TROPOS), Permoserstraße, 15 Leipzig, Germany
2 CSIR-National Physical Laboratory, New Delhi, India
ABSTRACT
COVID-19 disease is caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), which originated in
Wuhan, China and spread with an astonishing rate across the world. The transmission routes of SARS-CoV-2 are still
debated, but recent evidence strongly suggests that COVID-19 could be transmitted via air in poorly ventilated places. Some
studies also suggest the higher surface stability of SARS-CoV-2 as compared to SARS-CoV-1. It is also possible that small
viral particles may enter into indoor environments from the various emission sources aided by environmental factors such
as relative humidity, wind speed, temperature, thus representing a type of an aerosol transmission. Here, we explore the role
of relative humidity in airborne transmission of SARS-CoV-2 virus in indoor environments based on recent studies around
the world. Humidity affects both the evaporation kinematics and particle growth. In dry indoor places i.e., less humidity
(< 40% RH), the chances of airborne transmission of SARS-CoV-2 are higher than that of humid places (i.e., > 90% RH).
Based on earlier studies, a relative humidity of 4060% was found to be optimal for human health in indoor places. Thus, it
is extremely important to set a minimum relative humidity standard for indoor environments such as hospitals, offices and
public transports for minimization of airborne spread of SARS-CoV-2.
Keywords: Aerosol; COVID-19; SARS-CoV-2; Indoor; Humidity.
INTRODUCTION
The World Health Organization (WHO) declared a global
pandemic for the outbreak of novel coronavirus disease
(nCOVID-19), which is a highly transmittable and pathogenic
viral infection caused by severe acute respiratory syndrome
coronavirus 2 (SARS-CoV-2) (Sanders et al., 2020; WHO,
2020b). There are more than 7 million confirmed COVID-19
cases worldwide through June 10, 2020 (Worldometer, 2020)
since its first reported case in Wuhan, China in December,
2019 (WHO, 2020a). The overall geographic range of
COVID-19 spread is much larger as compared with the
epidemic of the severe acute respiratory syndrome (SARS)
in 2003 (WHO, 2004). SARS-CoV-2 is identified as an
enveloped, non-segmented, positive ribonucleic acid (RNA)
virus with a diameter of 65125 nm, containing single
strands of RNA with spikes in a crown shape on the outer
surfaces (Astuti and Ysrafil, 2020). SARS-CoV-2 is majorly
transmitted from human-to-human via direct or indirect
contact between people and with contaminated surfaces
* Corresponding author.
Tel.: +49-163-3019-921
E-mail address: ahlawat@tropos.de
(Prather et al., 2020). It is clear that airborne transmission of
SARS-CoV-2 is likely although many countries have not
acknowledged this possibility (Tellier et al., 2019; Asadi et
al., 2020; Hadei et al., 2020; Hsiao et al., 2020; Morawska
and Cao, 2020; National Academies of Sciences, Engineering,
and Medicine, 2020; Prather et al., 2020; Setti et al., 2020;
van Doremalen et al., 2020). Similarly, Paules et al. (2020)
recently pointed out that the airborne transmission of SARS-
COV-2 may also occur besides close distance contacts.
Recently, SARS-CoV-2 was found in aerosols in the
hospitals in Wuhan, China farther than 6 ft. distance (Liu et
al., 2020). Generally, respiratory infections occur through
the transmission of aerosols (< 5 µm) and viral droplets
(> 510 µm) exhaled from the infected persons. The larger
respiratory viral droplets will fall due to gravitational
settling, which leads to contact transmission, whereas smaller
droplets will lose mass through evaporation and remain in
the air for a longer time (Prather et al., 2020). It has been
shown in a recent study that small droplets of radius approx.
5 µm will take 9 minutes to reach the ground when produced
at height of 160 cm i.e., average speaking height. These small
droplets are of specific interest because of their association
with aerosol transmission of SARS-CoV-2 (Somsen et al.,
2020). These exhaled droplets are basically dilute saline
solutions with salts, water and some organics material along
with the attached virus (Kumar and Morawska, 2020). The
Ahlawat et al., Aerosol and Air Quality Research, 20: 18561861, 2020
1857
large population of these fine droplets originating from
coughing, sneezing and speech remain airborne for many
hours and can infect healthy persons (Prather et al., 2020).
The coronavirus transmission can also be affected by various
factors such as climate conditions (majorly temperature,
humidity and wind speed), population density, available
medical facilities (Dalziel et al., 2018). It is shown in previous
studies that wintertime climate and host behavior can favor the
influenza transmission (Shaman et al., 2011; Chattopadhyay et
al., 2018) and other human coronaviruses (Killerby et al.,
2018; Neher et al., 2020). Current studies indicate that
temperature and humidity have a significant influence on the
number of confirmed cases for a certain location (Bukhari
and Jameel, 2020). Therefore, precise understanding of the
influence of humidity on the transmissibility of COVID-19
in indoor environments is important for general public
awareness. For indoor areas with poor ventilation facilities,
people inhale the recirculating air. In cold and temperate
climates, within an indoor environment, the RH values are
typically low. Due to low RH, the droplets will evaporate at
a more rapid rate forming particles with smaller sizes (Feng
et al., 2020). The smaller size could lead to more airborne
suspension time of viral droplets and ultimately, they could
be transported to farther distances depending on ventilation
conditions (Bourouiba, 2020). But, in humid places, as the
humidity increases, the viral droplet size increases and falls
from the air faster providing less chances for other people to
breathe in the infectious viral droplets. The role of humidity
seems to be extremely important to the airborne spread of
COVID-19 in indoor environments.
ROLE OF RELATIVE HUMIDITY IN AIRBORNE
TRANSMISSION OF SARS-COV-2 IN INDOOR
CONDITIONS
As the exhaled droplets comes out from an infected person,
they will start either evaporating or there will be some
droplet growth. Both these scenarios are relative humidity
dependent (Feng et al., 2020). In dry indoor conditions,
when the aerosol droplets containing viruses and other fluids
are expelled in the air, they evaporate so that the water vapor
pressure at aerosol surface equilibrates with the ambient
conditions. The resulting water loss causes change in solute
concentration like proteins, salts or changes in other properties
such as pH (Marr et al., 2019). After evaporation of water,
the microdroplets will become quite small and suspended in
the air for longer durations. After some time, the suspended
viral particles concentration will increase depending on the
stagnant air and poor ventilation facilities, thus increasing
the infection risk in public places such as hospitals, restaurants
(Kumar and Morawska, 2020).
Based on literature, we have found that there are three
different scenarios where RH affects virus transmission in
the indoor surroundings (a) fate of microorganisms inside the
viral droplets (b) survival or inactivation of virus on surfaces
(c) role of dry indoor air in airborne transmission of viruses.
Fate of Microorganisms inside the Viral Droplets
Highly infectious diseases transmission such as COVID-19
requires pathogens to remain active outside of the host body.
RH affects the survival of some of these microorganisms in
the environment. A recent study explained that the viruses
survived well at RHs below 33% and at 100%, whereas, at
the intermediate RHs the viability was considerably reduced
(Lin and Marr, 2020). Lin and Marr (2020) investigated the
effect of RH on the viability of viruses both in suspended
aerosols and in droplets using culture-based approaches.
Based on the Lin and Marr (2020) findings, the viability is
typically much lower at a RH around 60% (~55%). This is
because evaporation kinetics plays an important role in
modulating the survival of the microorganism within the
droplets or aerosols. RH controls the evaporation kinetics of
the droplets. The enrichment factor (i.e., the calculated
concentration of the solute given a certain amount of water
loss or the fold increase in concentration of the solutes, as
the droplets evaporate) is an important parameter while
explaining the evaporation kinetics process. At lower RH,
i.e., at 43% and below, Lin and Marr (2020) found the
enrichment factor could increase rapidly with droplet
evaporation and dry out completely, while at higher RH,
evaporation occurred more slowly leading to a gradual
increase in the enrichment factor and the droplet never drying
out. Lin and Marr (2020) characterized the impact of this
evaporation with concentrations of solutes harmful to virus
viability (e.g., salts) by calculating their cumulative dose, or
sum of the products of the solute concentration and time. At
lower RH, due to rapid evaporation, solute concentrations
increased but then became irrelevant after the droplets dried
out, allowing virus viability to remain high. At the highest
RH levels, the cumulative dose increased slowly and did not
greatly impact virus viability, while at intermediate RH,
cumulative dose was a crucial factor to reduce virus viability
as the solute concentrations significantly increased while the
droplet never completely evaporated. Thus, virus inactivation
within droplets or aerosols is linked to the cumulative dose
of a harmful substance in solution, which itself has a
nonlinear response to RH.
Survival or Inactivation of Virus on Surfaces
A report based on humidity’s role on virus survival and
inactivation on surfaces showed that high temperature at
high relative humidity has a collegial impact on inactivation
of SARS-CoV-1 viability (Chan et al., 2011). Whereas,
lower temperatures and low humidity support prolonged
survival of virus on contaminated surfaces. Another important
point to mention here is that, the virus transmission has often
occurred in well air-conditioned environments such as
hospitals or hotels in some countries which has intensive use
of air-conditioning (Chan et al., 2011).
The Role of Dry Indoor Air in Airborne Transmission of
Viruses
There is a significant contribution of dry indoor air in both
disease transmission and poor resident health. During cold
winters, an outdoor air is drawn indoors and then heated to a
comfortable temperature level. This process will significantly
lower the indoor RH, which creates an extremely dangerous
situation for indoor residents, particularly during the
Ahlawat et al., Aerosol and Air Quality Research, 20: 18561861, 2020
1858
COVID-19 pandemic. When the indoor RH is less than
40 percent, humans becomes more vulnerable to viral
respiratory infections making SARS-CoV-2 virus more
infectious in the inhaled air. Earlier studies have shown that
for human health, a relative humidity between 40 to 60% is
optimum (Condair Ltd., 2007). When the indoor RH is lower
than 40 percent, the resulting moisture-free air yields optimal
route for long distance transmission of small infectious
aerosols. These viral airborne particles will further travel,
become inhaled by other residents, or finally settle on surfaces
where they can survive for many days. The infectivity of
many viruses, including SARS-CoV-2 are actually enhanced
due to low RH levels. Dry air also causes a significant
impact on our respiratory immunity. During the inhalation
of low RH air, the mucus in our nose and throat becomes dry
and more viscous, which diminish cilia’s capability to expel
viral aerosols. The low RH compromises the immune
system’s ability to effectively respond to microorganisms
(Taylor, 2020). Human ear, nose and throat areas are more
effective as virus fighter at high RH values rather than when
room air is very dry (Hohmann-Jeddi, 2019). While social
distancing reduces the risk of getting COVID-19 from other
inhabitants through short range contamination by large
droplets, it does less to prevent the transmission of tiny
infectious aerosols in the air.
COMPARISON OF INDOOR AND OUTDOOR
(AMBIENT) RH RELATIONSHIP WITH COVID-19
A quick comparison based on recent literature between
indoor and outdoor (ambient) RH relationship with COVID-19
will provide more insights into this topic. An indoor
environment is a microenvironment in which most people
spend the major portion of their daily life. As a result, indoor
air contributes to population exposures more than those
outdoors, although of course being influenced by factors
present at indoors as well as outdoors. In dry indoor places i.e.,
less humidity (< 40%), the chances of airborne transmission
of COVID-19 are high. Based on an indoor experiment from
Chinese cities during Jan-March 2020, the airborne spread
of SARS-CoV-2 was reduced by increasing RH from
23.33% to 82.67% (Yao et al., 2020). Feng et al. (2020)
recently investigated the influence of RH using numerical
modeling. In the study, they considered 40% RH as lower
bound and 95% RH as upper bound. They found that 40%
RH activates the evaporation of water in the cough droplets,
leading to droplet shrinkage and prolonged suspension in air
whereas high RH at 95% will increase the droplet size due
to hygroscopic growth with higher deposition fractions both
on humans and on ground. Biktasheva (2020) emphasized
on the air humidity control for indoor environment as a
feasible way to mitigate patients’ SARS-CoV-2 exposure.
When considering ambient air humidity, an important role
of humidity was found in rapid transmission of COVID-19
within the New York city (Bashir et al., 2020). Pani et al.
(2020) found the positive correlation of absolute humidity (AH)
with COVID-19 spread based on the daily data provided by
Ministry of Health, Singapore. Similarly, a positive correlation
was found between COVID-19 and RH (r = 0.106, p = 0.001)
in Kuala Lumpur, Malaysia (Suhaimi et al., 2020). Because,
in more humid outdoor environments, the population is more
likely to use drier indoor air and thus promote more COVID
viability. Considering outdoor absolute humidity factor
during cold winters, it was found that 73% of confirmed
cases in region of study with AH in range 310 g m3 (Huang
et al., 2020). When the outdoor temperature is low and the
indoor environment is heated, indoor RH is closely correlated
with outdoor AH, resulting in more COVID-19 cases.
Another study pointed out that COVID-19 spread was found
to be significant in US with AH in range 46 g m3 (Gupta
et al., 2020).
For better understanding, Table 1 depicts the influence of
RH on the survival, transmission and infection of H1N1,
SARS-CoV-1, MERS and SARS-CoV-2 viruses.
POLICIES FOR CONTROLLING THE OUTBREAK
OF SARS-COV-2 INCLUDING THE RH FACTOR
Nowadays, there is an immense need for rapid development
of effective vaccination and anti-viral medications which will
save humanity from a brutal pandemic. But, apart from that,
the building supervisors and government officials to play an
extremely important role in reducing the viral transmission
of these deadly diseases, such as SARS-CoV-2, by issuing
guidelines and standards. Governments around the world have
already set some indoor air quality standards for temperature
and indoor pollutants, but to the best of our knowledge there
are no such regulations and policies worldwide that require
a minimum RH standard in public buildings and indoor
environments. Based on research findings, for future scenarios,
setting a minimum RH standard of 40% for public buildings
will not only reduce the impact of COVID-19, but it will also
reduce the impact of further viral outbreaks, both seasonal
and novel. Though it is not an easy task to predict the
outbreaks of viral infections, gathering enough knowledge on
how these viral infections spread and developing counter plans
accordingly will certainly prevent us from such large-scale
pandemic like SARS-CoV-2. For countries in colder climates,
minimum RH standard for the indoor environments should be
kept into consideration. While, for tropical and typical hot
countries, humidity control measures are recommended while
avoiding extreme cooling of indoor places. Hygroscopic
growth at high RH will play an important role in reducing
the airborne spread of virus. Although virus viability will not
be minimized at high RH, the large droplet size will ensure
the of being airborne is minimized. Overall, air humidifying
is advised.
In order to implement the abovementioned guidelines, we
need a concrete plan along with the relationship between
different communities such as medical professionals, policy
makers, planners and government officials. In order to curb
the disease outbreaks, we must focus on the role of indoor air
on disease transmission and resident health. Other precautions
apart from RH optimization is to increase natural ventilations
like opening of windows during indoor stay, using proper face
masks (face shields along with face mask could provide better
results), avoid staying in direct periphery of the infected or
other persons, and maintaining social distancing.
Ahlawat et al., Aerosol and Air Quality Research, 20: 1856–1861, 2020
1859
Table 1. The influence of RH on the survival, transmission and infection of H1N1, SARS-CoV-1, MERS and SARS-CoV-2 viruses.
Reference Viruses Type of Study Typical Conditions Remarks
Lowen et al.,
2007
H1N1, Influenza Experimental, Indoor, Chamber
study
Transmission in cold and dry environment
(i.e., low RH of 20%–35%) conditions.
Transmission found to be completely stopped
at high RH of 80%.
Range which was tested (RH 20%–80%).
Low RH due to indoor heating in winter
supports virus spread in humans.
Tamerius et al.,
2013
H1N1, Influenza Modeling, Globally Temperate regions show a seasonal cycle with
low humidity in the winter and occurs in
some tropical locations during the rainy
season.
Low specific humidity (SH) conditions
favors the airborne survival and
transmission in temperate regions
during the cold-dry season.
Yuan et al., 2006 SARS-CoV-1 Meteorological data and
statistical analysis, Outdoor
(Ambient), Beijing, China
The peak transmission was found at mean RH
52.2%.
RH was found to be an important
meteorological parameter affecting the
transmission.
Cai et al., 2007 SARS-CoV-1 Meteorological data and
statistical analysis, China
Association of daily RH was found up to
certain extent.
Contribution of heaters and air
conditioning to the long-lasting
outbreak.
Chan et al., 2011 SARS-CoV-1 Experimental, Individual plastic
plate representing non-porous
surfaces
Prolonged survival of viruses was found at low
humidity on contaminated surfaces.
There were no major community
outbreaks found in Asian countries in
tropical area with high RH
environment
van Doremalen
et al., 2013
MERS Experimental Stability for a long time (as droplets on solid
surface and as aerosol) in low-humidity
Potential to be transmitted via contact or
aerosol transmission due to long
environmental presence.
van Doremalen
et al., 2020
SARS-CoV-2 Experimental, Stability on
aerosols and surfaces (plastic,
stainless steel, copper, and
cardboard)
Stable on plastic and stainless steel (65% RH),
Poor stable on copper and cardboard
(65% RH).
Viable virus was detected up to 72 h after
application in all surfaces.
Bukhari and
Jameel, 2020
SARS-CoV-2 Meteorological data and
statistical analysis
Lower number of cases in tropical countries
due to warm-humid conditions.
High absolute humidity (> 10 g m
–3
) was
a factor for slowdown in transmissions.
Yao et al., 2020 SARS-CoV-2 Experimental, Indoor, China The spread of SARS-CoV-2 was reduced by
increasing RH from 23.33% to 82.67%
RH being an important factor in reducing
the airborne transmission
Ma et al., 2020 SARS-CoV-2 Meteorological data and
statistical analysis, Wuhan,
China
Absolute humidity is negatively associated
with daily death counts.
Patients during therapy felt quite stable
and a comfortable environment
Ahlawat et al., Aerosol and Air Quality Research, 20: 18561861, 2020
1860
ACKNOWLEDGEMENTS
The authors confirm that no funding was received for this
work. The authors declare that there are no competing
interests.
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Received for review, June 16, 2020
Revised, July 17, 2020
Accepted, July 21, 2020
... All available hourly, population-weighted ERA5 and NLDAS values since January 1, 2020 were extracted, aggregated to daily mean or total values, and matched by date and district to the Rt values. The following variables were included as the main exposures of interest based on their documented or hypothesized associations with SARS-CoV-2: near surface air temperature (℃) (Morris et al., 2021;Rubin et al., 2020); relative humidity (%) (Ahlawat et al., 2020); solar radiation (KJ/m 2 ) (Y et al., 2020); total precipitation volume (mm) (Shenoy et al., 2022); average 10-m above ground wind speed (m/s) (Majumder and Ray, 2021). In addition, average volumetric soil moisture (m 3 /m 3 ) was included as a negative control exposure (Sanderson et al., 2018), since it is a variable presumed to affect infectious disease transmission through its influence on pathogen survival on surfaces and fomites (Colston et al., 2019), which is thought to be at most only a secondary mode of SARS-CoV-2 transmission (Karia et al., 2020). ...
... The primary direct, person-to-person mode of transmission of the pathogen is via virus-laden aerosols exhaled by infectious individuals, while an indirect route via contact with contaminated fomites is thought to make a minor contribution (Karia et al., 2020;Zhang et al., 2020). Small-scale atmospheric conditions such as the temperature, pressure and humidity of the air affect the rates at which aerosolized respiratory droplets are formed, suspended, and dispersed and thus influence disease transmission in complex ways (Ahlawat et al., 2020;Colston et al., 2019). The negative association of relative humidity on SARS-CoV-2 Rt identified here, among the largest absolute effect sizes of the hydrometeorological variables analyzed (though lower ranking by ALE), is consistent with one of the most widely documented of the disease's environmental sensitivities as well as current understanding regarding the virus' modes of transmission (Ahlawat et al., 2020;Hosseini, 2020). ...
... Small-scale atmospheric conditions such as the temperature, pressure and humidity of the air affect the rates at which aerosolized respiratory droplets are formed, suspended, and dispersed and thus influence disease transmission in complex ways (Ahlawat et al., 2020;Colston et al., 2019). The negative association of relative humidity on SARS-CoV-2 Rt identified here, among the largest absolute effect sizes of the hydrometeorological variables analyzed (though lower ranking by ALE), is consistent with one of the most widely documented of the disease's environmental sensitivities as well as current understanding regarding the virus' modes of transmission (Ahlawat et al., 2020;Hosseini, 2020). Whether quantified by absolute or relative measures, humidity has been shown to be an influential COVID-19 driver across many contexts (Majumder and Ray, 2021;Paraskevis et al., 2021), with very dry atmospheric conditions appearing to favor transmission as has been shown for other respiratory (Lin and Marr, 2020) and nonrespiratory (Colston et al., 2022) viruses. ...
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Background The COVID-19 pandemic has caused societal disruption globally and South America has been hit harder than other lower-income regions. This study modeled effects of 6 weather variables on district-level SARS-CoV-2 reproduction numbers (Rt) in three contiguous countries of Tropical Andean South America (Colombia, Ecuador, and Peru), adjusting for environmental, policy, healthcare infrastructural and other factors. Methods Daily time-series data on SARS-CoV-2 infections were sourced from health authorities of the three countries at the smallest available administrative level. Rt values were calculated and merged by date and unit ID with variables from a Unified COVID-19 dataset and other publicly available sources for May – December 2020. Generalized additive models were fitted. Findings Relative humidity and solar radiation were inversely associated with SARS-CoV-2 Rt. Days with radiation above 1,000 KJ/m² saw a 1.3%, and those with humidity above 50%, a 0.9% reduction in Rt. Transmission was highest in densely populated districts, and lowest in districts with poor healthcare access and on days with least population mobility. Wind speed, temperature, region, aggregate government policy response and population age structure had little impact. The fully adjusted model explained 4.3% of Rt variance. Interpretation Dry atmospheric conditions of low humidity increase, and higher solar radiation decrease district-level SARS-CoV-2 reproduction numbers, effects that are comparable in magnitude to population factors like lockdown compliance. Weather monitoring could be incorporated into disease surveillance and early warning systems in conjunction with more established risk indicators and surveillance measures. Funding NASA's Group on Earth Observations Work Programme (16-GEO16-0047).
... In addition to hygrothermal comfort, RH and T a have been proven to impact the SARS-CoV-2 transmission. The risk of transmission of this airborne virus in dry indoor areas is higher than in humid ones and higher in cold regions [81][82][83]. For example, a negative correlation was found between the average temperature per country and the number of cases of SARS-CoV-2 infection [84]. ...
... Thus, indoor RH and T a are also essential parameters to be monitored under real conditions, i.e., with real occupancy and ventilation conditions to estimate the aerosol transmission risk of SARS-CoV-2. As shown in Fig. 3, the indoor RH is generally within the optimal range for human health, 40%-60% [81,82], except for some days in March, where RH were below 40% and the chances of airborne transmission of SARS-CoV-2 were higher. Table 3 shows the percentage of occupied time in which the minimum values of thermal comfort [52] and thermal stress [55] were achieved. ...
Article
Despite the risk of transmission of SARS-CoV-2, Spanish educational centers were reopened after six months of lockdown. Ventilation was mostly adopted as a preventive measure to reduce the transmission risk of the virus. However, it could also affect indoor air quality (IAQ). Therefore, here we evaluate the ventilation conditions, COVID-19 risk, and IAQ in secondary school and university classrooms in Toledo (central Spain) from November 2020 to June 2021. Ventilation was examined by monitoring outdoor and indoor CO2 levels. CO2, occupancy and hygrothermal parameters, allowed estimating the relative transmission risk of SARS-CoV-2 (Alpha and Omicron BA.1), Hr, under different scenarios, using the web app COVID Riskairborne. Additionally, the effect of ventilation on IAQ was evaluated by measuring indoor/outdoor (I/O) concentration ratios of O3, NO2, and suspended particulate matter (PM). University classrooms, particularly the mechanically ventilated one, presented better ventilation conditions than the secondary school classrooms, as well as better thermal comfort conditions. The estimated Hr for COVID-19 ranged from intermediate (with surgical masks) to high (no masks, teacher infected). IAQ was generally good in all classrooms, particularly at the university ones, with I/O below unity, implying an outdoor origin of gaseous pollutants, while the source of PM was heterogeneous. Consequently, controlled mechanical ventilation systems are essential in educational spaces, as well as wearing well-fitting FFP2–N95 masks indoors is also highly recommended to minimize the transmission risk of COVID-19 and other airborne infectious diseases.
... The anti-epidemic performance of buildings is emphasized in this paper. For example, setting a minimum RH standard of 40% for public buildings will not only reduce the impact of COVID-19, but will also reduce the impact of further viral outbreaks, both seasonal and novel [146]. Guidelines may also need to be improved to meet risk-control requirements during a pandemic and address the concerns of building practitioners. ...
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The COVID-19 pandemic has lasted from 2019 to 2022, severely disrupting human health and daily life. The combined effects of spatial, environmental, and behavioral factors on indoor COVID-19 spread and their interactions are usually ignored. Especially, there is a lack of discussion on the role of spatial factors in reducing the risk of virus transmission in complex and diverse indoor environments. This paper endeavours to summarize the spatial factors and their effects involved in indoor virus transmission. The process of release, transport, and intake of SARS-CoV-2 was reviewed, and six transmission routes according to spatial distance and exposure way were classified. The triangular relationship between spatial, environmental and occupant behavioral parameters during virus transmission was discussed. The detailed effects of spatial parameters on droplet-based, surface-based and air-based transmission processes and virus viability were summarized. We found that spatial layout, public-facility design and openings have a significant indirect impact on the indoor virus distribution and transmission by affecting occupant behavior, indoor airflow field and virus stability. We proposed a space-based indoor multi-route infection risk assessment framework, in which the 3D building model containing detailed spatial information, occupant behavior model, virus-spread model and infection-risk calculation model are linked together. It is also applicable to other, similar, respiratory infectious diseases such as SARS, influenza, etc. This study contributes to developing building-level, infection-risk assessment models, which could help building practitioners make better decisions to improve the building’s epidemic-resistance performance.
... Numerous [88]. At moderate humidity between 40 and 60%, salt concentrations can inactivate the virus due to evaporation and smaller droplets [89]. At humidity below 30%, the salts crystallize and the virus survives [90]. ...
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COVID-19 originated in Wuhan city of Hubei Province in China in December three years ago. Since then, it has spread to more than 210 countries and territories. This disease is caused by Severe Acute Respiratory Syndrome Coronavirus 2. The virus has a size of one to two nanometers and a single-stranded positive RNA. Droplets spread the virus from coughing and sneezing. This condition causes coughing, fever, acute respiratory problems, and even death. According to the WHO, the virus can survive outside the body for several hours. This research aimed to determine how environmental factors influenced the COVID-19 virus’s survival and behavior, as well as its transmission, in a complex environment. Based on the results, virus transmissions are influenced by various human and environmental factors such as population distribution, travel, social behavior, and climate change. Environmental factors have not been adequately examined concerning the transmission of this epidemic. Thus, it is necessary to examine various aspects of prevention and control of this disease, including its effects on climate and other environmental factors.
... But the ability to be transported in air particles and find their way to get in contact with potential hosts is just the first step in the etiopathogenesis of airborne diseases. Indeed, microbes must survive multiple environmental barriers, such as temperature (2) and relative humidity (3). In addition, microbes must overcome the hostile environment of the host mucous membranes of the respiratory tract, from the nostrils and upper respiratory tract down to the lungs, each with its arsenal of defensive strategies that encompass the local immune system, the epithelial barrier lining as well as structured communities of commensal microorganisms, or microbiota (4). ...
Article
The recent COVID-19 pandemic has dramatically brought the pitfalls of airborne pathogens to the attention of the scientific community. Not only viruses, but also bacteria and fungi may exploit air transmission to colonize and infect potential hosts and be the cause of significant morbidity and mortality in susceptible populations. The efforts to decipher the mechanisms of pathogenicity of airborne microbes have brought to light the delicate equilibrium that governs the homeostasis of mucosal membranes. The microorganisms already thriving in the permissive environment of the respiratory tract represent a critical component of this equilibrium and a potent barrier to infection by means of direct competition with airborne pathogens or indirectly via modulation of the immune response. Moving down the respiratory tract, physicochemical and biological constrains promote site-specific expansion of microbes that engage in cross-talk with the local immune system to maintain homeostasis and promote protection. In this review, we critically assess the site-specific microbial communities that an airborne pathogen encounters in its hypothetical travel along the respiratory tract and discuss the changes in the composition and function of the microbiome in airborne diseases by taking fungal and SARS-CoV-2 infections as examples. Finally, we discuss how the technological and bioinformatics advancements may turn microbiome analysis into a valuable tool in the hands of clinicians to predict the risk of disease onset, the clinical course and the response to treatment of the individual patients in the direction of personalized medicine implementation.
... Materials such as masks, purifiers, or even filters for air filtration with biocide activity can play an important role in airborne filtration. It can improve the air quality and human health, especially in indoor environments, in which the probability of human transmission of pathogens microorganisms, and diseases is higher than in outdoor environments (Ahlawat et al. 2020;Yao et al. 2020). ...
Article
Antimicrobial air filtration techniques have recently been widely studied to enhance indoor air quality and mitigate hazardous airborne microorganisms. Here, CuNPs were incorporated into a commercial polyester fiber surface and Scanning Mobility Particle Sizer was used to measure the adherence between fibers and nanoparticles. An acid pretreatment previous CuNP incorporation was effective against the particle release and enhanced the adhesion between particle and fiber. CuNP was a mixture of Cu0 and Cu2O with a diameter size of 90 nm (SEM micrographs). The permeability of the filter was low, in order of 10–9 m2. The activity against pathogens was tested in loco in a real environment using a filtration prototype apparatus. It was observed that the presence of CuNP mitigated the fungi and bacteria growth not only on the surface but also reduced microbe concentrations after passing through the filter. These results show that CuNP can be used as an inhibitor of various microorganisms, making them a good alternative for indoor environments to control indoor air quality.
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Mounting evidence suggests the primary mode of SARS-CoV-2 transmission is aerosolized transmission from close contact with infected individuals. While transmission is a direct result of human encounters, falling humidity may enhance aerosolized transmission risks similar to other respiratory viruses (e.g., influenza). Using Google COVID-19 Community Mobility Reports, we assessed the relative effects of absolute humidity and changes in individual movement patterns on daily cases while accounting for regional differences in climatological regimes. Our results indicate that increasing humidity was associated with declining cases in the spring and summer of 2020, while decreasing humidity and increase in residential mobility during winter months likely caused increases in COVID-19 cases. The effects of humidity were generally greater in regions with lower humidity levels. Given the possibility that COVID-19 will be endemic, understanding the behavioral and environmental drivers of COVID-19 seasonality in the United States will be paramount as policymakers, healthcare systems, and researchers forecast and plan accordingly.
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Air sterilizer is one of the essential components in combating the Covid-19. A wind tunnel model of the air sterilizer using a dielectric barrier discharge plasma is proposed to destroy the virus by direct contact with the plasma. Dangerous ozone production in the plasma reactor should be controlled to a safe level. Two parameters affecting the ozone concentration, i.e., electrical power and airflow, were investigated. The DBD reactor was a cell constructed from an array of alternate electrodes. The plasma was generated by an AC high voltage generator with a range of 2kV -3kV. The power and the high voltage were varied by controlling the DC input voltage of the generator. The airflow was varied by controlling the speed of an exhaust fan from 0.5 m/s to 3.0 m/s. The state was characterized using optical emission spectroscopy in the range of 200 nm – 1000 nm. The results showed that both parameters played a significant role in ozone concentration. The trend of the ozone is strongly correlated with the OH species, which reacts with oxygen. The highest ozone concentration of 4.51 ppm was observed at the DC voltage around 19 volts or the power of 34.2 watts. However, a decrease of the ozone concentration at a voltage higher than 19 volts related to 2.9 kV was observed. In general, the data showed that faster airflow decreases ozone concentration. A drastic decrease of the nitrogen species sustaining the plasma occurred at the airflow higher than 2 m/s.
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Meteorological parameters are the critical factors affecting the transmission of infectious diseases such as Middle East Respiratory Syndrome (MERS), Severe Acute Respiratory Syndrome (SARS), and influenza. Consequently, infectious disease incidence rates are likely to be influenced by the weather change. This study investigates the role of Singapore's hot tropical weather in COVID-19 transmission by exploring the association between meteorological parameters and the COVID-19 pandemic cases in Singapore. This study uses the secondary data of COVID-19 daily cases from the webpage of Ministry of Health (MOH), Singapore. Spearman and Kendall rank correlation tests were used to investigate the correlation between COVID-19 and meteorological parameters. Temperature, dew point, relative humidity, absolute humidity, and water vapor showed positive significant correlation with COVID-19 pandemic. These results will help the epidemiologists to understand the behavior of Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) virus against meteorological variables. This study finding would be also a useful supplement to help the local healthcare policymakers, Center for Disease Control (CDC), and the World Health Organization (WHO) in the process of strategy making to combat COVID-19 in Singapore.
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Air pollution is the culprit to yearly millions of deaths worldwide, deteriorating human health. What is not yet clear is the impact of environmental factors on susceptibility to getting infected by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). The study aimed to determine associations between air quality, meteorological factors, and COVID-19 cases in Kuala Lumpur, Malaysia. Air pollutants and meteorological data in 2018–2020 were obtained from the Department of Environment Malaysia, while daily new COVID-19 cases in 2020 were obtained from the Ministry of Health Malaysia. Data collected were statistically analyzed using the Statistical Package for Social Sciences (SPSS). There were significant differences between PM10, PM2.5, SO2, NO2, CO, O3, and solar radiation in 2019 and 2020 since movement control order (MCO) was implemented on 18 March 2020. Spearman’s correlation test showed that COVID-19 cases were positively correlated with PM10 (r = 0.131, p < 0.001), PM2.5 (r = 0.151, p < 0.001), SO2 (r = 0.091, p = 0.003), NO2 (r = 0.228, p < 0.001), CO (r = 0.269, p = 0.001), and relative humidity (RH) (r = 0.106, p = 0.001), whereas ambient temperature (AT) was negatively correlated with COVID-19 cases (r = -0.118, p < 0.001). Further, multiple linear regression suggested that NO2 and AT (R2 = 0.071, p < 0.001, f2 = 0.08) were the most significant air pollutant and meteorological factors that influenced the incidence of COVID-19 cases in Kuala Lumpur. In general, better air quality, lower RH, higher AT, along with the targeted approach implemented thus far, have proven to curb the spread of this virus infection in Malaysia. This study supports future research in studies documented to understand the potential of transmission, survival, and infection of SARS-CoV-2. Keywords: SARS-CoV-2; Lockdown; Air pollution; Traffic; Tropical country.
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The World Health Organization declared the infectious spread of SARS-CoV-2 (severe acute respiratory syndrome coronavirus 2) an epidemic during its initial outbreak in Wuhan (China) and has since declared it a pandemic and, more recently, an endemic infection that may remain in our communities. A vaccine for COVID-19 is expected to take several months, meaning that the spread may continue in future, in the absence of the most effective measures of social distancing and self-isolation. While these measures have worked well under lockdowns, the potential of airborne transmission of COVID-19 under the eased restrictions has not been considered important enough. We discuss the need to acknowledge the airborne spread of COVID-19 inside built spaces under eased movement restrictions and the potential steps that can be taken to control it.
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Transient local over-dry environment might be a contributor and an explanation for the observed asynchronous local rises in Covid-19 mortality. We propose that a habitat's air humidity negatively correlate with Covid-19 morbidity and mortality, and support this hypothesis on the example of publicly available data from German federal states.
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It has been confirmed that the coronavirus disease 2019 (COVID-19) can transmit through droplets created when an infected human coughs or sneezes. Accordingly, 1.83-m (6-feet) social distancing is advised to reduce the spread of the disease among humans. This is based on the assumption that no air circulation exists around people. However, it is not well investigated whether the ambient wind and relative humidity (RH) will cause SARS-CoV-2 laden droplets to transport farther in the air, making the current social distancing policy insufficient. To provide evidence and insight into the “social distancing” guidelines, a validated computational fluid-particle dynamics (CFPD) model was employed to simulate the transient transport, condensation/evaporation, and deposition of SARS-CoV-2 laden droplets emitted by coughs, with different environmental wind velocities and RHs. Initial droplet diameters range from 2 to 2000 μm, and the wind velocities range from 0 to 16 km/h, representing different wind forces from calm air to moderate breeze. The comparison between a steady-state wind and a gust with a constant frequency has also been performed. Ambient RHs are 40% and 99.5%. The distances between the two virtual humans are 1.83 m and 3.05 m (6 feet and 10 feet). The facial covering effect on reducing the airborne transmission of the cough droplets has also been evaluated. Numerical results indicate that the ambient wind will enhance the complexity of the secondary flows with recirculation between the two virtual humans. Microdroplets follow the airflow streamlines well and deposit on both human bodies and head regions, even with the 3.05-m (10-feet) separation distance. The rest of the microdroplets can transport in the air farther than 3.05 m (10 feet) due to wind convection, causing a potential health risk to nearby people. High RH will increase the droplet sizes due to the hygroscopic growth effect, which increases the deposition fractions on both humans and the ground. With the complex environmental wind and RH conditions, the 6-feet social distancing policy may not be sufficient to protect the inter-person aerosol transmission, since the suspending micro-droplets were influenced by convection effects and can be transported from the human coughs/sneezes to the other human in less than 5 s. Thus, due to the complex real-world environmental ventilation conditions, a social distance longer than 6 feet needs to be considered. Wearing masks should also be recommended for both infected and healthy humans to reduce the airborne cough droplet numbers.
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It is essential to know the environmental parameters within which the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) can survive to understand its global dispersal pattern. We found that 60.0% of the confirmed cases of coronavirus disease 2019 (COVID-19) occurred in places where the air temperature ranged from 5 °C to 15 °C, with a peak in cases at 11.54 °C. Moreover, approximately 73.8% of the confirmed cases were concentrated in regions with absolute humidity of 3 g/m³ to 10 g/m³. SARS-CoV-2 appears to be spreading toward higher latitudes. Our findings suggest that there is an optimal climatic zone in which the concentration of SARS-CoV-2 markedly increases in the ambient environment (including the surfaces of objects). These results strongly imply that the COVID-19 pandemic may spread cyclically and outbreaks may recur in large cities in the mid-latitudes in autumn 2020.
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The COVID-19 pandemic is creating a havoc situation across the globe that modern society has ever seen. Despite of their paramount importance, the transmission routes of SARS-Cov-2 still remain debated among various sectors. Evidences compiled here strongly suggest that the COVID-19 could be transmitted via air in inadequately ventilated environments that are housing the infected by SARS-Cov-2. Existing experimental data showed that coronavirus survival was negatively impacted by ozone, high temperature and low humidity. Here, regression analysis showed that the spread of SARS-Cov-2 was reduced by increasing ambient ozone concentration level (48.83–94.67 μg/m³) (p-value = 0.039) and decreasing relative humidity (23.33–82.67%) (p-value = 0.002) and temperature (−13.17-19 °C) (p-value = 0.003) observed for Chinese cities during Jan-March 2020. Besides using these environmental implications, social distancing and wearing a mask are strongly encouraged to maximize the fight against the COVID-19 transmission. At no other time than now are the scientists in various disciplines around the world badly needed by the society to collectively confront this disastrous pandemic.