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Environment
Review Paper
Air quality inside motor vehicles’
cabins: A review
Bin Xu
1
, Xiaokai Chen
2
and Jianyin Xiong
3
Abstract
Among many environments, the motor vehicle cabin microenvironment has been of particular public
concern. Although commuters typically spend only 5.5% of their time in vehicles, the emissions from
various interior components of motor vehicles as well as emissions from exhaust fumes carried by
ventilation supply air are significant sources of harmful air pollutants that could lead to unhealthy
human exposure due to their high concentrations inside vehicles’ cabins. This review summarizes sig-
nificant findings in the literature on air quality inside vehicle cabins, including chemical species, related
sources, measurement methodologies and control measures. More than 90 relevant studies performed
across over 10 countries were carefully reviewed. These comprised more than 2000 individual road trips,
where concentrations of numerous air pollutants were determined. Ultrafine particles, aromatic hydro-
carbons, carbonyls, semi-volatile organic compounds and microbes have been identified as the primary
air pollutants inside vehicle cabins. Air recirculation with high-efficiency air filter has been reported as
the most effective measure to lower air pollutant concentrations. Future work should focus on investi-
gating the health risks of exposure to various air pollutants inside different vehicles and further develop-
ing advanced air filter to improve the in-cabin air quality.
Keywords
Air pollution, Vehicle cabin, Particulate matter, Volatile organic compounds, Formaldehyde, Carbonyls
Accepted: 23 October 2016
Introduction
In the past several decades, the effects of air pollution
exposure on human health have been extensively stu-
died, indicating that air pollution is a vital cause of
increased morbidity and mortality.
1–3
People spend
most of their time in indoor environments, including
vehicle cabins.
4
Similar to the indoor environments
inside buildings, the in-cabin microenvironment has
become a significant source of exposure to various air
pollutants, such as particulate matters (PMs), volatile
organic compounds (VOCs), semi-volatile organic com-
pounds (SVOCs), carbon monoxide, nitrogen
oxides.
5–10
Although people spend only an average of
about 5.5% of time daily in automobiles,
4
the health
risks associated in-cabin air quality are high for some
pollutants. Particle concentrations observed in the vehi-
cle cabins have been typically reported in the range of
100,000 to 500,000 particles/cm
3
.
5–8
These measure-
ments were much higher than those measured for the
related ambient environment, implying high levels of
exposure to PM inside vehicle cabins. Although con-
centrations of VOCs inside vehicles could vary under
different driving conditions, the mean concentrations of
VOCs in a vehicle cabin could be much higher than
normal ambient levels.
9,10
The inhalation intake expos-
ure to polybrominated diphenyl ethers (PBDEs), a
flame retardant, during an 80-min drive has been esti-
mated to be approximately equivalent to 16.5-h expos-
ure at home.
11
During commuting time, drivers and
1
State Key Laboratory of Pollution Control and Resource Reuse,
Tongji University, Shanghai, China
2
College of Civil Engineering, Kunming University of Science
and Technology, Kunming, China
3
School of Mechanical Engineering, Beijing Institute of
Technology, Beijing, China
Corresponding author:
Jianyin Xiong, School of Mechanical Engineering, Beijing
Institute of Technology, Beijing, 100081, China
Email: xiongjy@bit.edu.cn
Indoor and Built Environment
2018, Vol. 27(4) 452–465
!The Author(s) 2016
Reprints and permissions:
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journalsPermissions.nav
DOI: 10.1177/1420326X16679217
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passengers could be exposed to short periods of high
pollutant concentrations either emitted from surround-
ing mobile sources or interior fittings.
This review focuses on air quality in vehicle cabins.
The aim of the study is to review the current situation
of in-cabin air pollution and methodologies used for
assessing air quality in vehicles’ cabins based on main
characteristics of in-cabin environments.
The scientific literature was collected by searching the
following databases: Web of Science, ScienceDirect and
Google Scholar. The following keywords were used
during the search: vehicle, motor vehicle, vehicle cabin,
in-cabin, particle, particulate matter, VOCs, formalde-
hyde, SVOCs, interior material and chamber. Over 90
relevant articles were selected and reviewed. In this
review paper, pollutant species, their sources and con-
centrations, control measures inside different motor
vehicles’ cabins (new and used vehicles; cars, suburban
utility vehicle (SUVs) and buses; driving and stationary
vehicles) are discussed.
Vehicle cabin microenvironment and
air pollutants
In-cabin environment
Hazardous air pollutants exist in the in-cabin micro-
environment, which is mainly ascribed to the emissions
from interior materials, fuel leakage, exhaust fume
leakage, and infiltration from out-vehicle polluted air.
The wide range of vehicle cabin designs has led to a
large variation in occurrence of in-cabin pollutants spe-
cies, levels and hence personal exposure levels.
12
Different manufacturers designed vehicles with different
interior spaces, ventilation settings, cabin interior fit-
tings, etc. As a result, commuters’ exposure to indoor
vehicle pollution is subject to the variation in vehicle
cabin characteristics.
Figure 1 shows the typical air pollutant species, their
sources and transportation of air pollutants under dif-
ferent ventilation settings. Yoshida et al.
13
investigated
101 Japanese private-use cars, and identified a total of
275 pollutants existing in the in-cabin microenviron-
ment. The in-cabin pollutant concentrations are
often high for newly manufactured cars, at high interior
temperatures, or with low air exchange rate.
14–17
This
section reviews the research advances on in-cabin air
pollution from two aspects: the effect of ventilation
and interior emission characteristics.
Effect of ventilation
Ventilation is the process of air exchange between in-
cabin and the outside, and has an important effect on
pollutant transport dynamics: (a) from outside atmos-
phere into cabins; (b) emission from surfaces of various
interior fittings.
18
The effect of ventilation on the in-
cabin air quality has been studied under four ventila-
tion modes in previous literature:
19
(i) windows open;
(ii) fan off-recirculation (RC) off; (iii) fan on-RC off;
and (iv) fan on-RC on. For the latter three ventilation
modes, windows were considered closed.
Some literature have reported an air exchange
rate as large as 120 h
1
when windows are open and
the in-cabin air pollutants level could become equili-
brated with outside pollutants level immediately.
20,21
However, air pollutants level could vary significantly
in response to ventilation modes when windows are
closed.
19
Zhu et al.
6
measured the ultrafine particle
(UFP) concentrations inside three vehicles under differ-
ent ventilation modes. The lowest UFP concentration
was observed under the ventilation mode of fan on-RC
on due to the least air exchange between in-cabin
and outside. Xu and Zhu
19
developed a mass-balance
in-cabin particle dynamic model to study the vehicle
ventilation settings affecting in-cabin UFP level.
Figure 1. Schematic representation of in-cabin air pollutant sources and transport.
BC: black carbon; HC: hydrocarbon.
Xu et al. 453
Similar to the measurement data, the modelled UFP
in-cabin to on-road (I/O) ratios were found to be the
lowest (0.10) under the ventilation mode of fan
on-RC on. Low air exchange rate, filtration by cabin
air filters, and particle deposition on cabin interior sur-
faces were identified as primary causes.
19
Besides ven-
tilation mode, ventilation airflow rate and air exchange
rate were found as key parameters that determined air
pollutants exposure levels inside vehicle cabins.
22–24
Under the ventilation mode of fan on-RC off, larger
airflow rate could result in greater air exchange rate,
which could lead to more particle entry from outside
atmosphere and higher in-cabin particle concentration.
Besides ventilation, infiltration air (leakage air) through
the vehicle body frame could also account for the on-
road air pollutant entry into cabins.
25
Previous studies
reported that air leakage could mainly occur at the rear
body part of cars and airflow leakage rates could be
between 10 and 40 m
3
/h.
25–27
Since the ambient
carbon dioxide (CO
2
) concentration is usually about
400–500 ppm and VOC concentrations are much
lower than in-cabin levels, the air exchange between
inside-cabins and outside not only brings particles
into vehicle cabins but also dilutes CO
2
from exhaust
fume and passengers’ respiration and other pollutants
emitted from interior emissions. Table 1 summarizes
the air exchange flow under various ventilation
modes. Mechanical airflow through ventilation system
and infiltration airflow through joints and leaks in vehi-
cle envelopes are two predominant airflows that could
affect pollutant transportation inside vehicle cabins.
The mechanical airflow rates of 0–400 m
3
/h and infil-
tration airflow rates of 0–50 m
3
/h had been
reported.
20–24
The cabin air exchange rates had been
calculated as 0–70 h
1
.
20–24
The cabin air exchange
rate can be defined as ‘airflow rate/cabin interior
volume’.
21,22
Interior emission
A typical passenger compartment in a car commonly
comprises upholstery, steering wheel, dashboard,
ceiling, floor, doors and various fittings. The presence
of pollutants inside a vehicle is greatly associated with
emissions from interior materials used to equip the
compartment, including leather, plastics, fabrics, car-
pets, sealants, adhesives, paints, foam cushions and so
on.
16,28,29
Studies on emission characteristics of pollu-
tants from interior materials can be divided into two
categories: in-cabin measurement and chamber tests.
In-cabin measurement
For this approach, the in-cabin pollutant concentration
is measured under certain environmental conditions.
The air collected from the vehicle cabin is commonly
sampled by Tenax-TA tube based on ISO 16000-6:2011
or Tedlar bag or Summa canister for VOCs and then
analysed by gas chromatography-mass spectrometry
(GC/MS). For SVOCs, polyurethane foams (PUF)
sampler is used and then analysed by GC/MS. DNPH
(2,4-Dinitrophenylhydrazine) cartridge is used for
sampling volatile carbonyls including formaldehyde
based on ISO 16000-3:2011, and analysed by high-
performance liquid chromatography (HPLC). In
addition to the off-site analysis for speciation and quan-
tification of VOCs/SVOCs, the on-site monitoring tech-
nique is also applicable in some scenarios. For example,
the in-cabin PM concentrations are generally measured
in the field using portable instruments (optical particle
counters, condensed particle counters). The in-cabin
measurement can be easily performed in the field and
thus are widely used. Yoshida et al.
30
examined the influ-
ence of interior materials on vehicles’ cabin air pollution
by in-cabin measurements. The authors observed
that the in-cabin emission concentrations of alcohols
(2-(2-ethoxyethoxy)ethanol, 2-(2-butoxyethoxy)ethanol
and 1-decanol) and 1-methyl-2-pyrrolidone in 6 vehicles
with leather upholstery were higher than those with
fabric upholstery in 95 vehicles. Moreover, the in-cabin
concentrations of airborne ketones, furans, styrene and
1-methyl-2-pyrrolidone emitted from leather steering
wheels in 35 vehicles were higher than those with poly-
urethane steering wheels in 66 vehicles. Brodzik et al.
31
Table 1. Summary of vehicle ventilation airflow information.
20–24
Ventilation mode Type of airflow
Airflow
rate, m
3
/h
Air exchange
rate, h
1
Fan off RC off Mechanical ventilation 0 5–10
Infiltration leakage 35–50
Fan on RC off Mechanical ventilation 0–400 0–70
Infiltration leakage 0–20
Fan on RC on Mechanical ventilation 0–400 3–6
Infiltration leakage 20–40
454 Indoor and Built Environment 27(4)
measured the in-cabin pollutant compositions in several
unconditioned, newly produced cars, and indicated that
the presence of a sunroof could increase the in-cabin
concentrations of aliphatic hydrocarbons due to emis-
sions from sealing materials and adhesives around the
sunroof. These could further increase the TVOC
(total VOCs) concentration by 30%. Additionally, the
use of black and white fabric upholstery could add more
than 30% of in-cabin compounds.
The above studies all focused on experimental inves-
tigations. Xiong et al.
32
took a different approach by
theoretically examining the in-cabin VOC emissions
from interior materials and derived a correlation. In
their work, the emission mechanisms of materials
used in the vehicular environment were assumed to be
similar to those used in normal building environments.
By applying the physical model describing building
material emissions and performing some simplifica-
tions, a theoretical correlation (equation (1)) between
the in-cabin VOC concentration and temperature can
be derived:
32
ln Ca
T0:75 ¼C1C2
Tð1Þ
where C
a
is the pollutant concentration in the vehicle
cabin, mg/m
3
;Tis the air temperature in the vehicle
cabin, K; C
1
,C
2
are positive constants, which are inde-
pendent of temperature and are only related to the phys-
ical and chemical properties of the material-pollutant
combinations.
As shown in Figure 2, good agreements between the
correlation prediction and experimental data for eight
pollutants (benzene, toluene, ethylbenzene, xylene, styr-
ene, formaldehyde, acetaldehyde, acraldehyde) in a cer-
tain vehicle cabin have demonstrated the goodness of
the correlation.
Generally speaking, the in-cabin environment could
contain many kinds of interior materials, and the con-
centration of a certain pollutant could be the sum of
contribution from all sources of materials. This makes
it difficult to attribute the pollution level to individual
interior material and to predict the in-cabin pollution
level at the vehicle design stage. This promotes the
development of chamber test approach.
Chamber test
In this approach, individual interior material that is used
in a vehicle is tested in an environmental chamber one at
a time. During the test, the chamber condition (e.g., tem-
perature, relative humidity and air exchange rate) can be
accurately controlled. In addition, emission characteris-
tics (i.e., concentration, emission rate and key param-
eters) of interior materials can be measured individually.
Once these emission characteristics of all interior mater-
ials in a vehicle cabin are obtained, they can be used to
estimate the in-cabin pollution levels both in the design
and usage stages by mathematical modelling, which
should be informative to engineering applications.
Hoshino et al.
33
used this approach to measure the
emissions of VOCs and SVOCs from interior materials.
Figure 2. A comparison between the correlation (equation (1)) and experimental data for eight pollutants tested in a vehicle.
Xu et al. 455
During the test, a piece of an interior material was
firstly put into a quartz chamber placed in an oven.
Helium was then supplied into the chamber, and the
outlet air was sampled. This procedure was mainly
used for VOC emission tests (procedure 1). Secondly,
the test sample was removed from the chamber. The
empty chamber was heated to 250 C to desorb com-
pounds adsorbed on chamber’s interior surfaces. This
procedure was used for SVOC emission tests (proced-
ure 2). The emission rates of the sum of all quantified
VOCs and SVOCs of three tested materials in proced-
ure 1 at 65 C were observed to span from 6 to
39.34 mg/(m
2
.h). These were much higher than those
obtained in procedure 2 (the emission rate in this pro-
cedure was determined from the desorbed mass) which
ranged from 2.39 to 3.91 mg/(m
2
.h).
Kim et al.
34
applied three kinds of chamber meth-
ods, i.e. the thermal desorption method, the field and
laboratory emission cell (FLEC) method, and the 20-L
small chamber method to investigate VOC emissions
from automobile interior materials. The TVOC emis-
sion rate for the neat polylactic acid (PLA) increased
from 0.26 to 4.11 mg/(m
2
.h) when the chamber tem-
perature was increased from 30 Cto90
C. While for
two kinds of PLA bio-composites (PLA with pineapple
flour and PLA with destarched cassava flour), TVOC
emission rates were, respectively, in the range of 0.30–
3.72 mg/(m
2
.h) and 0.19–8.74 mg/(m
2
.h) in the above
temperature range.
The material emissions in vehicles are similar
to those in buildings,
32
and the emission rates are con-
trolled by three key parameters: the initial emittable
concentration (C
m,0
), the diffusion coefficient (D
m
)
and the material/air partition coefficient (K).
Although these three key parameters for building
materials are well determined by various chamber
methods,
35–38
these parameters for materials commonly
used in vehicles are seldom studied. This can be
ascribed to the fact that most of these vehicular envir-
onmental studies have been at the macro level (i.e. mea-
suring the in-cabin concentration or emission rate)
instead of the micro level (i.e. determination of key
parameters that could characterize material emissions
from the viewpoint of physics).
Particulate matter
Among numerous in-cabin air pollutants, particulate
matter has been associated with adverse respiratory
and cardiovascular effects. The aerosol exposure via
the respiratory route could lead to a major potential
risk of acute respiratory system responses such as
inflammation, allergy, asthma
39,40
and long-term
health problems including lung cancer and cardiovas-
cular diseases.
41,42
The PM number concentrations in the on-road
atmosphere are typically in the range of 100,000 to
500,000 particles/cm
3
.
43–45
In the absence of tobacco
smoking, most of the in-cabin PM could be from
outside air. Since the PM emitted from the engine is
typically in the size range of 3–300 nm, there is a
growing interest in the study of exposure to UFPs
(diameter <100 nm) inside vehicles’ cabins. Previous
in-cabin UFP exposure studies were mainly conducted
in three types of vehicles: saloon cars (sedan in
American English), vans and buses. Typical UFP con-
centrations in the order of 10
4
particles/cm
3
have been
reported.
6,46–52
UFP concentration in outside air, ven-
tilation settings, leakage airflow rate, cabin air filter
quality and driving speed were identified by previous
studies as key parameters that could determine the
in-cabin UFP exposure levels.
6,46–48
The in-cabin
human exposure to PM concentrations in automobile
vehicles was compared to other transportation modes
(e.g. walking, cycling, train, bus, and ferry) in different
countries.
49–51
Figure 3 shows PM concentrations
reported from literature. PM was identified as an
important air pollutant inside vehicle cabins.
6,8,50
Besides PM size distribution, chemical speciation of
in-cabin PM has also been studied to some extent.
53
The PM components were measured in highway
patrol vehicles. Some metals (Al, Ca, Ti, V, Cr, Mn,
Fe, Cu and Sr) were observed at high levels in the
in-cabin airborne PM.
VOCs
Many airborne VOCs have been detected inside cabins
of passenger vehicles, including benzene, toluene, ethyl-
benzene, xylenes, styrene, butyl acetate, undecane, for-
maldehyde, acetaldehyde, acrolei, and so on,
17,32,54–56
Los Angeles
Portland
BeeCounty
London
Sydney
Basel
WakeCounty
UFP number concentration, cm-3
0.0
2.0e+4
4.0e+4
6.0e+4
8.0e+4
1.0e+5
1.2e+5
PM2.5 mass concentration, µg/m
3
0
20
40
60
80
100
120
UFPs
PM2.5
Figure 3. PM concentrations measured in different cities
worldwide.
43–51
456 Indoor and Built Environment 27(4)
which can lead to adverse health risks.
57
VOCs found
inside new or in-use vehicles could be emitted from
interior materials and exterior sources. This review
mainly focuses on previous findings on VOC species
and their exposure levels. As a limitation of this
study, we were unable to distinguish between new and
in-use vehicles in some cases.
Aromatic hydrocarbons
Aromatic hydrocarbons, including benzene, toluene,
ethylbenzene, xylenes, styrene, and so on, are key
VOCs found in vehicle cabins.
17,32,54,55
For example,
50 aromatic hydrocarbons were detected in the tested
Japanese cars under static condition with the engine
stopped and windows, doors and vents closed.
13
The
median and maximum total concentrations of aromatic
hydrocarbons were 112.0 and 595.0 lg/m
3
, respectively,
and the maximal concentrations of benzene, toluene,
ethylbenzene, styrene, m/p-xylene and o-xylene were
33.0, 356.0, 59.0, 79.0, 65.0 and 25.0 lg/m
3
, respect-
ively.
13
The concentration of aromatic hydrocarbons
can be influenced by many factors such as interior tem-
perature, ventilation rate, ventilation mode, relative
humidity, solar radiation, vehicle age, travel distance,
vehicle grades (brand and price), cabin volume, interior
trims and air conditioner.
17,32,54,55,57–59
For instance,
the aromatic hydrocarbon concentrations were higher
in cars with leather trims than with fabric trims, in air-
conditioned buses than in non-air-conditioned ones, in
high-grade buses than in low grade ones and in vehicles
with small cabins than with larger volume cabins.
54,55
Aromatic hydrocarbon pollution could increase with a
rise in in-car temperature or relative humidity, and
decrease with car age or total car travel mile-
age.
17,32,54,55
The concentrations of benzene, toluene,
xylenes and ethylbenzene were higher in new vehicles
than in old vehicles by 12.89%, 103.54%, 123.14% and
104.20%, respectively.
54
The benzene concentration in
vehicles at 29 C was about 28.8% higher than that at
24 C, and the 6 C temperature difference from 29 C
to 35 C could lead to an increase of 102% of in-cabin
benzene concentration. The magnitude of this concen-
tration increase was much higher than the increase seen
at the lower temperatures.
32
Moreover, the aromatic
hydrocarbon concentrations could be increased by
50.46% when the ventilation condition changed from
fan on to fan off, and an increase of 51.38% when the
ventilation condition was changed from RC off to
RC on.
54
An increase in ventilation rate would
remove aromatic hydrocarbons out of the automobile
cabin more rapidly, and would also enhance the con-
vective mass transfer coefficient over the material sur-
face, which, in turn, could affect emissions of the
aromatic hydrocarbons.
32
The cancer and non-cancer health risks of people
exposing to aromatic hydrocarbons in vehicle cabins
could vary between different groups of receptors such
as male drivers, female drivers, male passengers and
female passengers. The health risk of male drivers is
the highest and is 1.04, 6.67 and 6.94 times higher
than female drivers, male passengers and female pas-
sengers, respectively.
57
The health risk of drivers is
higher than passengers due to their direct exposure to
emission sources at the driver’s position. For transient
passengers (e.g. in public transport), the risk is obvi-
ously lower, whereas for the professional driver, their
occupational exposure would be high due to their long-
exposure time. Moreover, the aromatic hydrocarbons
could lead to cancer health risk for drivers as the aver-
age cancer index is 1.21E-04 which is 1.21 times more
than the unacceptable cancer health risk published by
U.S. EPA.
60
The cancer health risk of male drivers and
passengers associated with exposure to aromatic hydro-
carbons is shown in Figure 4.
57
Furthermore, for the in-car airborne benzene con-
centration (X,lg/m
3
) exposure to male drivers,
female drivers, male passengers and female passengers,
the cancer health risk equations are shown in equations
(2) to (5), respectively,
57
and the non-cancer health risk
equations are shown in equations (6) to (9), respect-
ively.
57
For the same group of receptor (people), ben-
zene has the highest non-cancer health risk among all
the aromatic hydrocarbons, followed by xylenes, tolu-
ene, ethylbenzene and styrene. Meanwhile, benzene is
the foremost the most potent cancer health risk among
the hydrocarbon compounds.
57
Therefore, airborne
benzene is the main chemical pollutant of interest
among the aromatic compounds which could cause
health risk to passengers and drivers in vehicle cabins,
and some effective measures should be taken to protect
the health of male and female drivers, particularly in
respect to occupational exposure.
57
YCHRMD ¼1:48E06 Xð2Þ
YCHRFD ¼1:42E06 Xð3Þ
YCHRMP ¼2:22E07 Xð4Þ
YCHRFP ¼2:13E07 Xð5Þ
YNHRMD ¼1:70E03 Xð6Þ
YNHRFD ¼1:63E03 Xð7Þ
YNHRMP ¼2:55E04 Xð8Þ
YNHRFP ¼2:45E04 Xð9Þ
Xu et al. 457
where Y
CHR
and Y
NHR
are the dependent variables on
cancer health risk and non-cancer health risk, respect-
ively; the subscript
MD
,
FD
,
MP
and
FP
are abbreviations
of male drivers, female drivers, male passengers and
female passengers, respectively; Xis the independent
variable on benzene mass concentration (mg/m
3
)in
vehicles’ cabins.
Formaldehyde and other carbonyl
compounds
Emissions from interior materials especially upholstery
and ceiling are regarded to be the main sources of
formaldehyde and other carbonyls,
61–63
since these
compounds are important precursors for manufacture
of these interior materials. Fedoruk and Kerger
14
mea-
sured carbonyl concentrations in five vehicles including
three rental saloons (sedans) (less than 6 months old)
and two used saloons (about 4 years old). Two car-
bonyls (hexanal and nonyl aldehyde) were reported
among the 10 most abundant airborne VOCs in some
vehicle cabins. When the experimental conditions chan-
ged from static moderate-heat condition (32.2–42.8 C;
the definition of static condition is the same as men-
tioned previously) to static high-heat condition (47.8–
62.8 C), the in-cabin nonyl aldehyde concentration
increased by 12.5-folds (from 4.5 to 61 lg/m
3
) in the
same vehicle.
14
Yoshida and Matsunaga
15
studied the
carbonyl concentrations in the cabin of a new Japanese
estate car (station wagon in American English). On the
day after the delivery (about two weeks after manufac-
ture), the in-cabin concentrations of formaldehyde, n-
nonanal, methylethylketone and methylisobutylketone
were determined to be 46.4, 2.4, 5.2 and 48.9 lg/m
3
,
respectively. The in-cabin formaldehyde concentration
was reported to decline from a higher concentration in
summer to a lower concentration in winter and the con-
centration was reported to increase from winter to
summer.
15
Yoshida et al.
30
tested the pollutant concentrations
in 50 new cars and observed that the in-cabin formal-
dehyde concentration was in the range of 17–67 lg/m
3
.
In addition, the in-cabin formaldehyde concentration
became significantly high when drivers or passengers
smoked in these vehicles.
Pang and Mu
61
investigated the characteristics of
carbonyl compounds in 29 vehicles including taxi, bus
and subway in Beijing. In the 12 tested taxis, the mean
in-cabin formaldehyde and acetaldehyde concentra-
tions were found to be 26 lg/m
3
and 30 lg/m
3
, respect-
ively. In three tested buses,
61
the existence of interior
materials including wood products, carpet, leather and
paint in the cabin was shown to cause relatively high
formaldehyde concentration. Zhang et al.
28
examined
Figure 4. Cancer health risk (C
HR
) of male receptors due to exposure to aromatic hydrocarbons in vehicles.
57
458 Indoor and Built Environment 27(4)
the air pollution in the microenvironment of 802
parked new cars and found that the formaldehyde con-
centration was within 20–1110 lg/m
3
, with the average
value of 80 lg/m
3
. Analyses indicated that about 24%
of surveyed vehicles had exceeded the Chinese regula-
tion standard, given in GB/T 27630-2011.
64
Geiss et al.
10
investigated the in-vehicle compounds
in 23 used private cars during the summer and winter.
The mean in-cabin concentrations of formaldehyde,
acetaldehyde, propanal and hexanal in summer were
observed to be higher than those in winter, while acet-
one was an outlier. Mapou et al.
63
analysed the field
data concerning aldehydes in passenger vehicles col-
lected during the relationship of indoor, outdoor and
personal air (RIOPA) study involving participation of
non-smoking adults from communities in California,
New Jersey and Texas in the USA. The mean in-vehicle
formaldehyde and acetaldehyde concentrations mea-
sured in 115 tested vehicles were 39.7 and 17.6 lg/m
3
,
respectively. Highest in-cabin concentrations of alde-
hydes in vehicles tested in New Jersey were recorded.
Xiong et al.
32
sampled three kinds of aldehydes (for-
maldehyde, acetaldehyde, acraldehyde) in three vehicles
at different temperatures. The in-cabin aldehydes con-
centration demonstrated an associated effect correlat-
ing with temperature. When the temperature increased
from 24 Cto35
C, a corresponding increase in in-
cabin formaldehyde, acetaldehyde and acraldehyde
concentrations was recorded; these were increased
by 1.5, 1.1 and 6-folds, respectively. Detailed informa-
tion on the in-cabin concentrations of carbonyls and
tested conditions mentioned above are summarized
in Table 2.
In the Chinese National Standard, GB/T 27630-
2011,
64
the acceptable exposure guidelines for some
typical carbonyls in vehicle cabins are specified. The
threshold values for formaldehyde, acetaldehyde, acral-
dehyde are specified as 100 mg/m
3
,50mg/m
3
and 50 mg/
m
3
, respectively. According to the measured in-cabin
formaldehyde and other carbonyl concentrations in lit-
erature given in Table 2 (including the concentration
range and mean concentration), the concentration
levels reported in some studies exceeded the threshold,
implying potential adverse health effect on passengers
and drivers.
Other VOCs
In addition to the above VOCs, a large variety of ali-
phatic and cyclic hydrocarbons were also found in
vehicular interior air. For example, 74 aliphatic hydro-
carbons including 42 alkanes, 24 cycloalkanes, 6
alkenes and 2 cycloalkenes were found in the air sam-
ples from vehicle cabins, and aliphatic hydrocarbons
accounted for 42% of the TVOC concentration
(136–3968 mg/m
3
).
13
Furthermore, the interior concen-
trations of alkanes containing n-hexane, n-heptane,
n-dodecane, n-tridecane and n-tetradecane were low in
small cabins, the concentrations of alkanes, cycloalk-
anes and cycloalkenes were significantly higher in cars
parked in built-in garages than those in outdoor places,
and the total concentration of alkanes was lower in cars
with smaller cabins though the concentrations of
ketones and benzothiazole were lower in big cars.
13
The aliphatic hydrocarbon concentration was estimated
to be approximately 5 to 10 times of the guideline value
(300 mg/m
3
)
65
after one month from delivery as a new
car, and aliphatic hydrocarbons were considered to be
major contaminants in car cabins, regardless of time
elapsed since production or country of production of
these cars.
13
Moreover, some terpenes (mainly a-pinene and d-
limonene) were detected in new vehicles,
15,30,31,66
with
limonene generally showing higher concentration than
a-pinene.
15,30,66
SVOCs
SVOCs are ubiquitous in vehicular environments,
which can be redistributed from their original sources
to airborne particles, settled dust and on human skin.
The main SVOCs found in vehicle cabins are phthalate
esters (PEs), brominated flame retardants (BFRs) and
polycyclic aromatic hydrocarbons (PAHs).
11,33,67,68
PEs are typically added to a variety of interior materials
(e.g. plastics and leather) to enhance their flexibility and
extensibility, while the BFRs are added to materials to
reduce the flammability and slow the rate of combus-
tion. Exposure to certain SVOCs is associated with ser-
ious adverse health effects, e.g. reproductive disorders,
reduced growth, lower birth weight, external malforma-
tions, and elevated risks of asthma, allergies and lung
cancer,
69–71
which has attracted interests from research-
ers and drawn concerns from government regulators,
industries and publics
56
. As SVOCs generally account
for a large proportion of the material weight and their
emission rates are relatively slow,
68
they tend to exist in
both new and old vehicles for a long time. According to
the report released by Ecology Center in the U.S.A. in
2012, 40% of tested vehicles contained BFRs in interior
materials.
72
Hoshino et al.
33
detected several phthalate
esters including DEP (diethyl phthalate), DBP (dibutyl
phthalate), DEHP (di(2-ethylhexyl) phthalate), BHT
(butylated hydroxyltoluene) and others in a small
chamber study for three kinds of interior materials.
Mandalakis et al.
11
collected air samples from automo-
bile cabins and found that the concentration of total
PBDEs varied from 0.4 to 2644 pg/m
3
, with a median
value of 201 pg/m
3
. The SVOC concentrations were
observed to decline with the vehicle’s age and increase
Xu et al. 459
with a rise in temperature. A comparison with overall
exposure via inhalation, dust ingestion and dietary
intake
73
has indicated that in-vehicle exposure to
BDE-209 is a significant pathway.
74
PAHs have been identified in both new and old vehi-
cle cabins.
75
These compounds mainly originated from
combustion of fuel and could enter into the cabin
through infiltration and ventilation. Li
75
measured
PAHs concentrations in 11 vehicles, including 8 new
and 3 old ones and found that the sum of PAHs con-
centrations were in the range of 689.08-923.74 ng/m
3
in
new vehicles, which was about five times of those found
in old vehicles.
Microbes
Airborne microbe pollution is always an occupa-
tional and public health concern for its association
with lung impairment, respiratory allergies, infections
and other health problems. Microbial exposure inside a
vehicle has attracted great attention in recent years.
76–79
For example, cladosporium, penicillium, aspergillus and
alternaria were found to be the dominant fungal genera
in vehicles, and the maximum bacterial aerosol concen-
tration was 2550 CFU/m
3
.
76
Moreover, the average
concentrations of bacteria and fungi in commuting
trains were 417 and 413 CFU/m
3
, respectively,
77
and
Table 2. Measured in-cabin formaldehyde and other carbonyl concentrations reported in literature.
Pollutants
Environmental
conditions
Vehicle
numbers
Concentration
range, lg/m
3
Mean
concentration,
lg/m
3
Formaldehyde 30 C 1 – 46.4
15
32 C 50 17–67 31 (median
value)
30
– 12 taxi 13–34 26
61
– 9 buses 13–94 26.7
61
18 C 802 20–1110 80
28
Summer 23 13.6–43.6 21.3
10
Winter 23 2.6–14.7 5.6
10
– 115 2.3–1095.6 39.7
63
24 C 3 62.8–111.1 86.8
32
29 C 3 64.0–115.7 93.4
32
35 C 3 172.8–251.6 215.8
32
Acetaldehyde – 12 taxi 18–84 30
61
– 9 buses 14–29 18.3
61
Summer 23 10.8–65.1 21.2
10
Winter 23 5.2–38.9 10.0
10
– 115 1.8–1031.2 17.6
63
24 C 3 13.7–25.7 20.9
32
29 C 3 18.8–28.9 24.9
32
35 C 3 23.1–57.0 44.6
32
Acraldehyde 24 C 3 3.4–4.5 4.1
32
29 C 3 4.8–7.5 6.1
32
35 C 3 6.8–14.2 28.7
32
Hexanal Summer 23 5.5–44.0 17.0
10
Winter 23 1.9–13.3 5.4
10
Propanal Summer 23 3.6–41.4 11.9
10
Winter 23 1.1–6.0 2.5
10
n-nonanal 30 C 1 – 2.4
15
Nonyl aldehyde 32.2–62.8 C 5 4.5–61 32.8
14
Acetone Summer 23 9.3–56.0 22.9
10
Winter 23 14.4–39.7 23.7
10
Methylethylketone 30 C 1 – 5.2
15
Methylisobutylketone 30 C 1 – 48.9
15
460 Indoor and Built Environment 27(4)
the combined maximum level of bacterial and fungal
aerosols was 1000 CFU/m
3
in public buses and passen-
ger cars.
78
In addition, the in-vehicle bacterial concen-
trations were significantly higher in summer in public
buses than in passenger cars, and the in-vehicle fungal
concentrations were generally higher in summer than in
winter,
77
as shown in Figure 5. The respirable fractions
of bacteria and fungi were both higher than 50% of
total airborne bacterial and fungal concentrations in
every season, increased from spring to autumn, and
then decreased in winter. Additionally, the respirable
fraction was higher for fungi than for bacteria, as
shown in Figure 5(a). The bacterial concentration
reached its highest level in autumn and its lowest level
in winter, as shown in Figure 5(b).
77
As for microbial pollution control, the use of clean-
ing air conditioner in vehicle cabin was shown to have
the capacity to reduce the total number of microorgan-
isms, such as bacterial and fungal spores, by over
80%.
76
However, Li et al.
79
proved that automobile
air conditioning filters are often heavily contaminated
with various microbial agents, including many human
opportunistic pathogens and high levels of endotoxin.
The bacteria and fungi filtered from the air stream by
the air conditioner could proliferate under high humid-
ity conditions such as raining or snowing.
79
When the
air conditioner is turned on, the air stream passing
through the vehicle filtration system could re-aerosolize
air conditioning filter-borne bacteria and fungi and
subsequently carry the microbes into vehicle cabins.
79
For those automobiles which have been dormant in
a high humidity condition and have not been used for
a long time, the use of air conditioning system is not
recommended when re-use for driving again, until the
car has been serviced, by thoroughly cleaning the air
conditioning system, e.g. removing dusts and/or disin-
fecting the air filter.
79
For convenience, before turning
on air conditioning of the car, the cabin windows
should be open for 15 min or longer, which could min-
imize the microbial exposure risk of passengers and the
driver in a vehicle’s cabin.
79
Pollution control measures
Ventilation with filtration unit is a most efficient method
to avoid outside pollutants entering into vehicles’ cabins
and capture air pollutants, emitted from combusted fuel
and interior materials into the cabin air. Xu and Zhu
80
compared effects of numerous factors on reducing in-
cabin UFPs and found that ‘driving at the speed limit
using the largest ventilation airflow rate with the fan-on
and RC-on and using a high-efficiency cabin filter’ had
led to the lowest in-cabin exposure to UFPs.
80
An in-
cabin air purifier was applied to reduce in-cabin PM
level.
81
The effects of the air purifier on particle concen-
trations and average size inside a vehicle have been eval-
uated. A significant reduction by 95–99% of particle
concentrations was observed. However, the air purifier
could also cause CO
2
accumulation in the vehicle
cabin due to the occupants’ exhalation. Similar results
were found in school buses when a high-efficiency cabin
air purifier was installed.
81
Lee and Zhu
82
installed a
high-efficiency cabin air filter in the ventilation system
to simultaneously reduce UFP and carbon dioxide
exposure levels. An average in-cabin UFP reduction of
93% was reported and the cabin CO
2
concentration
remained in the range of 620–930 ppm.
82,83
Currently,
most in-cabin air cleaners are cabin air filters that consist
of fibrous or porous or both materials. Also, high-effi-
ciency particle air (HEPA) filters and stand-alone air
cleaner units can be tentatively applied in vehicles’
cabins. As an important topic of future research, we rec-
ommend the development of air cleaners with high
removal rates for both PM and gaseous pollutants.
Figure 5. The seasonal distribution of respirable fractions (a) and pollution levels (b) bacterial and fungal concentrations in
vehicles.
77
Xu et al. 461
Source control is a promising measure to control and
improve the in-cabin air quality, which mainly focuses
on reducing emissions from interior materials. Kim
et al.
34
used a bake-out technology for source control.
In that study, some cabin materials were baked in an
oven at 70 C for 5 h, and then placed in a contaminant-
free chamber for testing. Results indicated that TVOC
emissions from two polylactic acid (PLA) composites
were reduced by 57% and 72%, respectively, after
baking in an oven, demonstrating the effectiveness of
the bake-out treatment. In addition, reducing the for-
maldehyde content in adhesives is also an effective
measure to control the formaldehyde emission from
materials in both vehicular and indoor
environments.
84,85
Health significance
In-cabin exposure to air pollutants has become an
important public concern due to the significant time
people spent inside vehicles. Positive correlations
between exposure to air pollutants and adverse health
effects have been identified by a series of epidemio-
logical studies. Exposure to PM has been linked to
adverse respiratory and cardiovascular health effects.
86
Recent studies found that UFPs may enter the circula-
tory system and deposit in the brain and showed
high toxicity in laboratory animals.
87,88
The adverse
impact of PM exposure on health has been extensively
studied, showing that PM is a significant cause of car-
diovascular disease and atherosclerosis.
89
Exposure
to in-cabin VOCs has also become a public concern
due to the potential high exposure levels. Recent studies
indicated that short- and long-term exposure to mix-
tures of VOCs may cause mucosal irritation, non-
specific symptoms,
90,91
and even more severe health
problems such as neurological system damage and
leukaemia.
92
At present, there is not enough evidence to support
the hypothesis that the exposure in in-transit environ-
ments is greater than that in other environments (i.e.,
indoor environments, aircraft environments). However,
Yoshida and Matsunaga
15
pointed out those airborne
concentrations of pollutants in cabins are expected to
be generally higher than those in residences due to the
high ratio of material volume to space volume in vehicles
and possible higher cabin temperature, especially in sum-
mer, due to a high heat gain in the cabin. In addition,
as mentioned previously, the inhalation exposure to
PBDEs during an 80 min drive was approximately
equivalent to that of staying in a home for 16.5 h.
11
Given that, on average, commuters spend about 5.5%
of their time daily (equivalent to 79 min) in automobiles,
the exposure in vehicles’ cabins could account for a con-
siderable proportion of the total exposure.
Conclusions
This review summarizes major findings reported in lit-
erature on air quality inside passenger vehicles’ cabins,
including chemical species, related sources, measure-
ment methodologies and control measures.
Information given in literature has provided solid evi-
dence that air pollutants commonly observed inside
cabins are at high exposure levels and can pose adverse
effects on passengers’ health. Different air pollutants
emitted from different sources are at different levels
under different ventilation or driving conditions.
Ventilation mode and airflow rate, the age and air-
tightness of the vehicle, interior materials, number of
passengers and ambient pollution level outside the vehi-
cle could play important roles in determining the in-
cabin pollutant concentrations. To reduce the in-cabin
exposure levels of air pollutants, some guidelines,
national standards or protocols have been derived to
ensure a better safeguard of drivers and passengers
during transit and travel. The development of manufac-
turing standards based on the environmental health
perspective would be an important improvement of
more environmentally friendly vehicles with a consider-
ation of the wellbeing of passengers. The measures
could include using the largest ventilation airflow rate
with fan-on and RC-on mode, installation of an air
purifier unit inside vehicle cabin, applying a high-effi-
ciency cabin air filter in the ventilation system, pre-
bake-out of interior materials prior to installation to
vehicles, etc.
Authors’ contribution
All authors contributed equally in the preparation of this
manuscript.
Declaration of conflicting interests
The author(s) declared no potential conflicts of interest with
respect to the research, authorship and/or publication of this
article.
Funding
The author(s) disclosed receipt of the following financial sup-
port for the research, authorship, and/or publication of this
article: This research was supported by the National Natural
Science Foundation of China (Grant Nos. 51568026 and
51476013) and the Fundamental Research Funds for the
Central Universities.
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