Formaldehyde & Volatile Organic Compounds in Hong Kong Homes

Formaldehyde and volatile organic compounds in Hong Kong
homes: concentrations and impact factors
Introduction
Indoor air pollution caused by volatile organic com-
pounds (VOCs) is attracting international interest as
many indoor materials and utilities contain VOCs
(Godish, 2001). VOCs are classified as organic com-
pounds that have boiling point between 50 and 260°C
(Godish, 2004). There are hundreds of VOCs in the air,
which increases the complexion of indoor air pollution.
Previous studies show that indoor air is contaminated
to various degrees by a wide variety of hydrocarbons
and hydrocarbon derivatives including paraffin, olefin,
aromatics, carbonyls, polycyclic aromatics, and chlo-
rinated hydrocarbons (Guo et al., 2003; Kim et al.,
2001; Lai et al., 2004; Lee et al., 2002a; Park and
Ikeda, 2004, 2006; Sawant et al., 2004). Among the
indoor VOCs, some are toxic i.e., toluene, whereas
some at high levels are carcinogenic such as formalde-
hyde and benzene (Olsen et al., 1984; Godish, 2001;
ATSDR (Agency for Toxic Substances and Disease
Registry), 1994, 2007).
Many studies demonstrate that vehicular exhaust and
industrial emissions are the major sources of ambient
VOCs (e.g. Brocco et al., 1997; Guo et al., 2006;
Mayrsohn and Crabtree, 1976), while the sources of
VOCs are quite numerous within any indoor environ-
ment. These sources include combustion by-products,
cooking, construction materials, furnishings, paints,
Abstract This paper presents formaldehyde and volatile organic compounds
(VOC) concentrations, potential sources and impact factors in 100 homes. The
24-h average formaldehyde concentration in 37 homes exceeded the good class of
the Hong Kong Indoor Air Quality Objectives (HKIAQO), whereas the total
VOCs concentration in all homes was lower than the HKIAQO. Compared to
other East Asian cities, indoor formaldehyde and styrene in Hong Kong was the
highest, reflecting that the homes in Hong Kong were more affected by house-
hold products and materials. The formaldehyde concentration in newly built
apartments was significantly higher than that in old buildings, whereas no
relationship between the concentration and the building age was found for
VOCs. There was no difference for formaldehyde and toluene between smoking
and non-smoking homes, suggesting that cigarette smoking was not the major
source of these two species. Homes of a couple with a child had higher form-
aldehyde and acetic acid concentrations, while homes with more than three
people had higher concentrations of 1-butanol, heptane and d-limonene. When
shoes were inside the homes, heptane, acetic acid, nonane and styrene concen-
trations were statistically higher than that when shoes were out of the homes.
Furthermore, higher levels of 1,2,4-trimethylbenzene, styrene, nonane and hep-
tane were found in gas-use families rather than in electricity-use homes.
H. Guo, N. H. Kwok, H. R. Cheng,
S. C. Lee, W. T. Hung, Y. S. Li
Research Centre for Urban Environmental Technology
and Management, Department of Civil and Structural
Engineering, Hong Kong Polytechnic University,
Hong Kong
Key words: Formaldehyde; Volatile organic compounds;
Homes; Building age; Cigarette smoking; Hong Kong.
H. Guo
Research Centre for Urban Environmental Technology
and Management
Department of Civil and Structural Engineering
Hong Kong Polytechnic University
Hong Kong
Tel.: 852 3400 3962
Fax: 852 2334 6389
e-mail: ceguohai@polyu.edu.hk
Received for review 27 March 2008. Accepted for
publication 6 October 2008.
Ó Indoor Air (2009)
Practical Implications
Long-term exposure to formaldehyde and volatile organic compounds (VOC) in indoor environments may cause a
number of adverse health effects such as asthma, dizziness, respiratory and lung diseases, and even cancers. Therefore,
it is critical to minimize indoor air pollution caused by formaldehyde and VOCs. The findings obtained in this study
would significantly enhance our understanding on the levels, emission sources and factors which affect indoor con-
centrations of formaldehyde and VOCs. The results can help housing designers, builders, home residents, and housing
department of the government to improve indoor air quality (IAQ) by means of appropriate building materials, clean
household products and proper life styles. It can also help policy makers reconcile the IAQ objectives and guidelines.
Indoor Air 2009; 19: 206–217
www.blackwellpublishing.com/ina
Printed in Singapore. All rights reserved
Ó 2009 The Authors
Journal compilation Ó Blackwell Munksgaard 2009
INDOOR AIR
doi:10.1111/j.1600-0668.2008.00580.x
206

varnishes and solvents, adhesives, office equipment,
and consumer products (Godish, 2001; Guo et al.,
2000). The VOCs in domestic microenvironments
originate from a variety of sources. For instance, Risto
(1995) found that domestic furniture i.e., leather sofa
generated high indoor concentrations of trichloro-
ethene and 1,4-dichlorobenzene, whereas aromatic
VOCs such as toluene and benzene were closely related
to the use of consumer products. It was reported that
1,4-dichlorobenzene could also be emitted from moth-
balls and room deodorants, two consumer products
commonly used in Hong Kong homes (Bouhamra
et al., 1997). Studies showed that VOC levels in
smokersÕ homes were higher than those in non-smok-
ersÕ houses (Edwards et al., 2001a; Guerin et al., 1992;
Lee et al., 2002a). The smoke emitted from burning
Chinese incense and the operation of a gas stove and
gas heater led to high levels of formaldehyde in homes
(Garrett et al., 1997; Jia and Yao, 1993; Lee et al.,
2002a).
Hong Kong is one of the most densely populated
cities in the world. Many districts are full of high-rise
buildings surrounding narrow roads with heavy traffic
fleet. The life styles such as cooking patterns and small
living space in Hong Kong homes are closely related to
indoor air quality (IAQ). Because of the potential risk
and problems related to indoor air pollution, the Hong
Kong government has been striving to establish IAQ
objectives for different types of indoor environments.
Many studies have been conducted in different indoor
environments in Hong Kong such as offices, schools,
shopping malls, restaurants, homes, food markets and,
ice-skating rinks (Guo et al., 2003, 2004a,b; Lee and
Chang, 1999; Lee et al., 1999a,b, 2002a,b,c; Li et al.,
2001). Although Hong Kong homes were a target in
previous IAQ study, very limited number of homes (six
homes) was selected for preliminary study and the
target air pollutants were not focused on VOCs (Lee
et al., 2002a).
This project was a collaborative project on indoor
VOCs investigation in East Asian homes among Japan,
China, Korea, Hong Kong and Taiwan. It aimed to
substantially enhance our understanding on the char-
acteristics of indoor environments and the levels of
organic compounds in homes of East Asia, and help us
improve its IAQ. This project was led by the National
Institute of Public Health of Japan (NIPH) and
samplers were delivered to each country and region
for VOCs and formaldehyde sampling in winter 2002.
To better understand the concentration levels, poten-
tial sources and factors which influence the indoor
concentrations of VOCs and formaldehyde in Hong
Kong residences, 100 homes were selected for the
sampling campaign. The results obtained were
compared with the recommended Hong Kong IAQ
Objective (HKIAQO) (Hong Kong Environmental
Protection Department (HKEPD), 2003). They were
also compared with the results obtained in the same
period in other cities in East Asia. The IAQ objectives
for formaldehyde, total VOCs (TVOCs) and individual
VOCs in Hong Kong are presented in Table 1.
Material and methods
Selection of homes and questionnaire survey
Public rental and private housing are two major types
of housing in Hong Kong. There are about 650,000
public rental flats in the Housing AuthorityÕs portfolio,
housing approximately 2 million people (about one-
third of Hong KongÕs population) (Hong Kong Hous-
ing Authority, 2008). By 2007, there were 1.36 million
private domestic households in Hong Kong (Hong
Kong Housing Authority, 2007). The majority of the
housing in Hong Kong has ferroconcrete structure.
Homes were selected on the basis (a) prevalence of
housing type, (b) locations with relatively high popu-
lation densities, and (c) about 20% of newly built
buildings which were built after November 2001. One
hundred homes widely distributed over the urban areas
were then selected. The characteristics of the monitored
homes are presented in Table 2. A package containing
two diffusive samplers, field blank samplers, instruc-
tion for the use and questionnaires was delivered to the
selected homes. For the purpose of comparison among
the participated homes, the diffusive sampling of
indoor air was limited to one room or space in each
home, and the usual sampling location was the living
room. If there was no such a room, a bedroom would
be selected for air sampling. The samplers were placed
in the middle of the sampled rooms with a height of
1.5 m above the floor. Besides, to simulate the real
situation in Hong Kong homes, which usually intend
to minimize the impact of outdoor air in winter, the
windows and doors of the monitored homes were
required to be closed as much as possible during the
Table 1 The IAQ objectives for formaldehyde, TVOCs and individual volatile organic
compunds in Hong Kong (Hong Kong Environmental Protection Department (HKEPD), 2003)
Parameter Unit
8-h average
Excellent class Good class
Formaldehyde lg/m3
<30 <100
TVOC lg/m3
<200 <600
Benzene lg/m3
16.1
Carbon tetrachloride lg/m3
103
Chloroform lg/m3
163
1,2-Dichlorobenzene lg/m3
500
1,4-Dichlorobenzene lg/m3
200
Ethylbenzene lg/m3
1447
Tetrachloroethylene lg/m3
250
Toluene lg/m3
1092
Trichloroethylene lg/m3
770
Xylene (o-, m-, p-isomers) lg/m3
1447
IAQ, indoor air quality; VOCs, volatile organic compunds; TVOCs, total VOCs.
Concentrations and impact factors of formaldehyde and VOCs
207

measurement period. As it was winter season, no
mechanical ventilation systems were turned on. The
instruction described the suitable sampling locations in
the living room. Each participant sampled indoor air
according to the instruction. The questionnaire con-
taining 13 questions about building conditions, resi-
dential life-style and indoor situations was completed
by the participants during the sampling period.
The indoor air samples at each home were collected
for 24 h using two passive samplers in November 2002.
One was a cartridge treated with 2,4-dinitrophenyl-
hydrazine (Sep-Pak DNPH XpoSure; Waters, Milford,
MA, USA) for aldehydes sampling. The other was a
passive charcoal tube (Passive gas tube for organic
solvents, SIBATA, Tokyo, Japan) collecting VOCs. The
two samplers were simultaneously exposed in the same
location as shown in Figure 1. After sampling, the
collected samples and the completed questionnaires
were delivered to NIPH for analysis.
Chemical analysis
Formaldehyde and 15 VOCs were selected for chemical
analysis in this study. These compounds were selected
because they were widely monitored in indoor envi-
ronments and suitable for the passive sampling (Park
et al., 1996).
The description of sampling and species analysis
methods is shown in Table 3. Formaldehyde extracted
with 3 ml of acetonitrile (DuPont, Wilmington, DE,
USA) from the cartridge was analyzed by high-perfor-
mance liquid chromatography (HPLC, Hewlett-Pack-
ard 1100 series; Agilent Technologies, Palo Alto, CA,
USA) equipped with an UV diode array detector. The
UV spectra of formaldehyde were compared with the
respective spectra of standards. The standards were
calibrated in the range of 0.1–1 ng/ll. The VOCs were
extracted with 3 ml of carbon disulﬁde (Toyokaseiko-
gyo Co. Ltd, Osaka, Japan) from the charcoal tube by
shaking, and then analyzed by gas chromatography –
mass selective detection (GC/MSD, Hewlett-Packard
HP6890, HP5973, Agilent Technologies, Inc.). The
target ion of each compound was calibrated with the
standards in the range of 0.1–2.0 ng/ll. Concentrations
of all compounds were calculated as 24-h average
values in the sampling room. The sum of the concen-
tration of the 15 target VOCs was deﬁned as TVOC.
The detection limits were calculated according to the
Table 2 Characteristics of the monitored homes in Hong Kong
Home
type Structure
Number
of homes
House Wooden 0
Ferroconcrete 6
Light steel 1
Others 0
Apartment Wooden 2
Ferroconcrete 82
Stone 3
Light steel 2
Others 1
Others Wooden 0
Ferroconcrete 3
Light steel 0
Others 0
Fig. 1 Example for formaldehyde and volatile organic com-
pound sampling (Left: passive charcoal tube; Right: DNPH-
cartridge)
Table 3 Sampling and species analysis
Aldehydes
Sampler Diffusive 2,4-dinitrophenylhydrazine cartridge
Target compounds Formaldehyde
Sampling method Passive sampling for 24 h
Desorption method Solvent desorption (acetonitrile 3 ml)
Analytical method High-performance liquid chromatography
Detector: Diode array detector at 365 nm (Ref.600 nm)
Column: Eclipse XDB(5 lm · 250 mm)
VOCs
Sampler Passive charcoal tube
Target compounds
(15 compounds)
Heptane, Decane, Nonane, Toluene, p-Xylene,
Trimethyl-benzene, Ethylbenzene, Styrene, Limonene,
a-Pinene, p-Dichlorobenzene, Ethyl acetate, Acetic
acid, m-isobutylketone, Butanol
Sampling method Passive sampling for 24 h
Desorption method Solvent desorption (carbon disulfide)
Analytical method Gas chromatography
Detector: Mass selective detector at scan mode
Column: 5% PH ME Silicone (60.0 m · 250 lm · 1.00 lm)
VOCs, volatile organic compunds.
Guo et al.
208

method of Glazer et al. (1981) and found to be 0.1 lg/m3
for all compounds.
Calibration of the passive samplers
Experiments were carried out to confirm the reproduc-
ibility and sampling rates of the two types of diffusive
samplers in an exposure period of 24 h. Two diffusive
samplers were exposed to three levels of toluene and
formaldehyde mixtures at a time in a large chamber
that simulated a room in a home. Toluene was selected
as the representative compound for VOCs, and form-
aldehyde for aldehydes. Concentrations of the two
compounds in the experiments were determined by
active sampling procedure. The temperature of the
chamber was controlled at three conditions, 15, 20 and
25°C, in order to examine the influence of room
temperature on diffusive sampling rates in an exposure
period of 24 h. It is not unusual that room temperature
changes among homes owing to seasonal variation,
geographical change, and the use of air conditioning
and heating ventilation systems indoors.
It was confirmed from the experiments that diffusive
sampling rates of toluene and formaldehyde given by
the manufacturers of the two types of diffusive sam-
plers are appropriate for a sampling period of 24 h,
although there were about 5% deviations in diffusive
sampling amounts. The change of diffusive sampling
rates due to room temperature variation was about 3%
between 15 and 25°C in both samplers. As a result, the
total deviation in the diffusive sampling methods was
about 5–8% for the exposure period of 24 h and room
temperature between 15 and 25°C.
The diffusive sampling rates of formaldehyde and
VOCs for the passive samplers are as follows, respec-
tively,
DSRVOC ¼ DSRTol
DiffcoVOC
DiffcoTol

ð1Þ
and
DSRald ¼ DSRFor
Diffcoald
DiffcoFor

ð2Þ
where DSR is the 24-h average diffusive sampling rate;
Diffco is the diffusion coefficient of target compound in
air at 20°C; ÔTolÕ represents toluene and ÔForÕ is
formaldehyde; ÔaldÕ means aldehydes.
Statistical analyses
All statistical analyses, including correlation, regres-
sion, t-test, one-way ANOVA and ANOVA with post-
hoc multiple comparison, were performed using the
SPSS statistical software package (SPSS Inc., Chicago,
IL, USA) and Microsoft excel. Non-parametric tests
were undertaken to confirm the parametric results.
That is, the corresponding non-parametric tests led to
the same conclusions of significance/non-significance
as the parametric tests. In addition, the data were
tested for normality by calculating skewness values
and two standard errors for skewness. If data were
not normally distributed, non-parametric tests were
solely used. The non-parametric methods mainly used
included KendallÕs tau_b and SpearmanÕs rho tests.
Results and discussion
Overall description
The descriptive statistics of the formaldehyde and 15
VOCs concentrations in 100 Hong Kong homes is
shown in Table 4. Concentrations of target compounds
below the detection limits were excluded from the
statistical analysis. It can be seen that formaldehyde
was detected in 90% of investigated homes and toluene
was found in 65% of the homes. On the other hand,
decane, d-limonene, ethylbenzene and p-xylene were
detected in only about 20% of the investigated homes.
1-butanol was measured in 11% of all the samples
while the rest VOCs had 5% detection rate in all the
homes. In particular, Ethyl acetate and methyl isobutyl
ketone was found in only one sample.
It was found that the mean values of target
compounds were associated with very large standard
deviations and were higher than the median values.
This reflects wide distribution of formaldehyde and 15
VOCs in Hong Kong homes. Under such circumstance,
median is a better representation. The formaldehyde
concentrations obtained from all measured homes
ranged from 46 lg/m3
(first quartiles) to 142 lg/m3
(third quartiles), with a lower CV of 80%, whereas
toluene level was between 2 lg/m3
(first quartiles) and
10 lg/m3
(third quartiles) with a CV of 300%. The sum
of 15 VOCs concentrations ranged from 4 lg/m3
(first
quartiles) to 45 lg/m3
(third quartiles), while the sum
of all compounds was 162 lg/m3
. Compared to the
median indoor formaldehyde (16.1 lg/m3
) and toluene
(39.3 lg/m3
) levels measured at six Hong Kong homes
in July – October 1999 (hot season), the median indoor
formaldehyde concentration (85.7 lg/m3
) in this study
(cold season) was much higher whereas the median
toluene value (4.4 lg/m3
) was much lower (Lee et al.,
2002a,c), revealing the complexion of indoor VOC
emission sources and impact factors.
Comparing the 24-h average concentration in Hong
Kong homes with the HKIAQO (Table 1), we found
that the formaldehyde level in 18 of the 100 homes was
30 lg/m3
(excellent class, 8-h average); in 45 of the
100 homes, the formaldehyde concentration was
100 lg/m3
(good class) but higher than the excellent
class; and the 24-h average formaldehyde concentra-
tion in 37 homes exceeded the good class of HKIAQO.
The TVOC concentration in all 100 homes was lower
209

than the value of HKIAQO (600 lg/m3
). However,
the formaldehyde levels observed in the study were
much higher than that in Japan (Ohura et al., 2006), in
Australia (Garrett et al., 1999) and in Taiwan (Jia and
Yao, 1993). It is noteworthy that caution should be
taken for comparison as the sampling methods, dura-
tion and periods were diﬀerent in the present and
previous studies.
Comparison with the results obtained in the same period in other
cities in East Asia
Table 5 compares the formaldehyde and VOC concen-
trations in Hong Kong homes with those in other cities
in East Asia. It is worth to emphasize that the results
were obtained from the measurements in these cities in
the same period (November 2002), using the same
Table 5 Comparison of indoor formaldehyde and VOC concentrations in various countries and regions in East Asia
China
(n = 94)
Hong Kong
(n = 100)
Taiwan
(n = 100)
Korea
(n = 96)
Japan
(n = 97)
Formaldehyde 58.2 € 4.8a
112.3 € 9.0 87.1 € 8.5 67.4 € 6.8 37.5 € 3.4
Ethyl acetate 16.7 € 2.0 N.D. 15.0 € 1.6 17.5 € 2.6 21.5 € 3.5
1-Butanol 147.6 € 12.5 82.7 € 10.4 N.D. 79.0 € 14.5 52.7 € 1.4
Heptane 18.1 € 0.9 22.1 € 2.5 14.7 € 0.7 89.9 € 12.4 295.4 € 69.9
Methyl isobutyl ketone N.D. N.D. N.D. 15.9 € 1.0 32.7 € 4.1
Toluene 9.1 € 2.1 15.3 € 4.5 14.5 € 0.9 34.7 € 4.6 7.7 € 1.0
Acetic acid 6.8 € 0.2 14.8 € 1.4 N.D. 34.6 € 5.3 28.8 € 8.5
Ethylbenzene 4.0 € 0.3 4.1 € 0.3 4.3 € 0.1 8.6 € 1.5 4.8 € 0.3
p-Xylene 2.6 € 0.2 3.0 € 0.2 2.6 € 0.1 7.6 € 1.6 3.3 € 0.2
Nonane 15.8 € 1.1 12.0 € 0.4 N.D. 28.9 € 3.1 33.5 € 3.8
Styrene 3.9 € 0.2 10.9 € 1.9 0.1 € 0.002 2.3 € 0.2 0.2 € 0.02
a-Pinene 19.2 € 1.2 11.0 € 0.2 N.D. 20.9 € 1.2 57.0 € 5.5
Decane 24.3 € 3.5 25.0 € 2.4 27.1 € 3.0 25.7 € 3.3 41.8 € 2.8
1,2,4-trimethylbenzene 6.2 € 0.3 5.4 € 0.1 4.2 € 0.1 7.6 € 0.9 7.5 € 0.5
1,4-dichlorobenzene 35.6 € 5.2 5.8 € 0.2 N.D. N.D. 144.6 € 27.4
d-Limonene 49.3 € 5.6 18.1 € 1.0 21.4 € 0.9 42.7 € 4.8 37.0 € 5.2
Sum of VOCsb
186.4 € 14.8 46.1 € 8.8 34.8 € 2.6 180.0 € 21.2 187.7 € 31.4
Sum of ALLc
247.5 € 16.3 162.2 € 13.7 122.0 € 9.4 256.8 € 23.5 263.6 € 36.8
VOCs, volatile organic compunds; N.D., non-detectable; n, number of homes investigated.
a
Mean € 95% confidence interval.
b
Sum of 15 VOCs.
c
Sum of 15 VOCs and formaldehyde.
Table 4 Descriptive statistics of formaldehyde and 15 VOCs in 100 Hong Kong homes (lg/m3
)
Compound Samples
Non-detectable
samplesa
First
quartiles Median Mean
Third
quartiles
Fourth
quartiles s.d.b
C.V. (%)c
Formaldehyde 100 10 46.3 85.7 112.3 141.9 468.9 90.3 80
Ethylacetate 100 99 – – – – – – –
1-Butanol 100 89 14.1 54.6 82.7 87.8 321.0 104.4 126
Heptane 100 96 8.4 11.2 22.1 24.9 59.8 25.4 115
Methyl Isobutyl Ketone 100 99 – – – – – – –
Toluene 100 35 2.2 4.4 15.3 9.8 359.3 45.4 297
Acetic acid 100 96 7.1 9.4 14.8 17.2 34.7 13.5 91
Ethylbenzene 100 78 2.2 3.2 4.1 4.8 12.9 2.9 70
p-Xylene 100 79 1.6 2.1 3.0 3.4 10.9 2.3 77
Nonane 100 97 10.1 12.1 12.0 14.0 15.9 3.9 32
Styrene 100 96 0.2 1.8 10.9 12.6 39.9 19.4 178
a-Pinene 100 98 10.3 11.0 11.0 11.8 12.6 2.2 20
Decane 100 77 13.0 15.5 25.0 24.0 99.2 24.1 96
1,2,4-trimethylbenzene 100 97 4.8 5.0 5.4 5.8 6.6 1.1 20
1,4-dichlorobenzene 100 94 4.3 5.4 5.8 6.9 8.7 1.9 33
d-limonene 100 77 11.8 14.4 18.1 20.7 49.0 10.0 55
Sum of VOCsd
100 – 3.8 15.9 46.1 45.0 537.6 87.8 190
Sum of ALLe
100 – 81.0 114.7 162.2 185.6 706.9 136.9 84
a
Number of samples in which the compound concentration under detection limit.
b
s.d., standard deviation.
c
C.V., coefficient of variation.
d
Sum of VOCs – sum of 15 VOCs.
e
Sum of ALL – sum of 15 VOCs and formaldehyde.
Guo et al.
210

sampling and analytical methods. Thus, the indoor
levels of formaldehyde and VOCs in different countries
and regions were highly comparative. It should be
noted that values presented in Table 5 were arithmetic
means rather than geometric means, although the data
collected in Hong Kong were not normally distributed,
as a result of the fact that the detailed information on
the data from other cities was not available.
The highest 24-h average formaldehyde concentra-
tion was observed in Hong Kong which was higher
than the good class of HKIAQO, whereas the lowest
level was found in Japan. Formaldehyde is mainly
released to indoor air from building materials, smok-
ing, household products, and the use of un-vented,
fuel-burning appliances (Godish, 2001). The variation
of formaldehyde level in different countries and regions
reflects the difference in the use of indoor materials and
living styles. Nevertheless, the formaldehyde levels in
all investigated cities exceeded the excellent class of
HKIAQO (30 lg/m3
).
All the VOCs except styrene in Hong Kong homes
were comparative or lower than that in other countries
and region. The concentration of styrene in Hong
Kong homes was much higher than that in other cities,
showing the significant impact of styrene source
emissions on IAQ in Hong Kong homes (Collins and
Richey, 1992; Daisey et al., 1998; Hodgson et al.,
1996).
Effect of building aging and decoration
Figure 2 shows the effect of building aging and
decoration on formaldehyde and VOC levels in the
investigated homes. Here, we define the age of building
as the construction year or decoration time, whichever
was later. For instance, an apartment was built in 1990
and decorated in 2000. The age of this apartment was
back to 2000. This could better reflect the impact of
indoor materials (not the apartment itself) on indoor
formaldehyde and VOCs. The formaldehyde and VOC
data in each category were tested and all the categories
were found to meet normal distributions. Hence, the
arithmetic mean values were used for comparison.
It was found that the 24-h average formaldehyde
concentration in newly-built apartments in 2002
(235 ± 70 lg/m3
) was significantly higher than that
of 1-year old buildings at 1r level (built in 2001;
134 ± 60 lg/m3
) (P 0.05, two-tail t-test). And the
formaldehyde concentration in 2001 (1-year-old build-
ing) was higher than that in 2000 (2-year-old building,
92 ± 23 lg/m3
), in 1999 (3 years old, 83 ± 36 lg/m3
),
in 1990–1998 (4–12 years old, 85 ± 21 lg/m3
) and
before 1989 (over 13 years old, 89 ± 23 lg/m3
) at 2r
level. However, no significant difference in formalde-
hyde concentration was found in buildings built in or
before 2000. As stated in the previous subsection,
sources of formaldehyde in the home include building
materials, smoking, household products, and the use of
un-vented, fuel-burning appliances, like gas stoves or
kerosene space heaters. Of the above, the most
significant sources of formaldehyde are likely to be
pressed wood products made using adhesives that
contain urea-formaldehyde resins (Godish, 2001). It
was also found that the indoor 24-h average formal-
dehyde level in newly built buildings (235 lg/m3
in2002
and 134 lg/m3
in 2001) was higher than the good class
of the HKIAQO (100 lg/m3
). Even in old buildings
(in and before 2000), the formaldehyde level was higher
than the excellent class of the HKIAQO (30 lg/m3
).
It can be seen that the age of buildings did not have
significant impact on the concentrations of the VOCs
in the homes (Figure 2). For instance, there was no
significant difference in toluene concentration in build-
ings with various ages (P 0.05). This is due to the
Effect of building aging/decoration
0
50
100
150
200
250
300
350
Concentration(µg/m3)
Form
aldehyde
1-Butanol
Toluene
Ethylbenzene
p-X
ylene
D
ecane
d-lim
onene
Sum
ofV
O
Cs
2002, n = 13
2001, n = 11
2000, n = 14
1999, n = 8
1990–1998, n = 31
1989, n = 17
Fig. 2 Effect of building aging and decoration on indoor formaldehyde and volatile organic compounds in Hong Kong homes.
n = number of homes
211

fact that household painting and paint, varnish and
lacquer removal, tobacco smoke, and consumer prod-
ucts such as adhesives, floor polish, inks, coatings and
solvent-thinned products may contain toluene. When
there were activities using toluene-containing products
in homes, the toluene level would show significant
increase. In addition, outdoor sources, ventilation rate
and chemical depletion could have impact on indoor
toluene levels. Thus, the toluene level in homes was not
directly associated with building aging, but associated
with the use of toluene-containing products and other
factors. Similarly, the different concentrations for other
VOCs e.g., 1-butanol, ethylbenzene, p-xylene and
d-limonene, in buildings with different ages were
mainly related to the household activities rather than
the building age. For instance, Home no. 63 is a single
house built in 1996 and thoroughly innovated in 1999.
During the sampling period, the house owner did
painting on a piece of furniture, which caused the
toluene concentration to be as high as 359 lg/m3
.
The concentrations of ethylbenzene (12.9 lg/m3
) and
p-xylene (6.2 lg/m3
) in this house were also recorded
to be the highest among the 100 homes. These VOC
species are all main constituents of decorative paint
(Borbon et al., 2002).
Effect of cigarette smoking
Cigarette smoking is often an indoor source which
emits a variety of organic compounds such as formal-
dehyde, toluene, and styrene (USEPA (United States
Environmental Protection Agency), 2007; Howard,
1990). In order to evaluate the contribution of cigarette
smoking to VOC concentrations in Hong Kong homes,
12 smoking and 82 non-smoking homes were selected.
Figure 3 compares the indoor formaldehyde and VOCs
concentrations in smoking and non-smoking homes.
Although formaldehyde could be emitted from ciga-
rette smoking, no significant difference in formalde-
hyde concentration between smoking and non-smoking
homes was found (119 ± 71 lg/m3
vs. 111 ± 18 lg/
m3
, P = 0.84), which is consistent with our previous
observations (Lee et al., 2002a). This was likely due to
the fact that other indoor sources, such as cooking,
building materials, and consumer products, could emit
formaldehyde as well in the non-smoking homes.
Similarly, toluene showed no statistical difference
between smoking and non-smoking homes, indicating
that cigarette smoking was not the main impact factor
on indoor toluene levels in Hong Kong. In addition to
tobacco smoke, many household products contain
toluene, such as adhesives, paints, coatings, and
particleboard (Department of the Environment, Water,
Heritage and the Arts (DEWHA), 2001). The toluene
level could also be affected by the air exchange rate of
the homes (Chao and Chan, 2001; Kotzias et al., 2004).
Unfortunately, the air exchange rates were not mea-
sured in this work. Further study may focus on the
relationship between indoor VOCs and air exchange
rates. Nevertheless, the finding is similar to the results
reported by Kim et al. (2001), but opposite to our
previous study that toluene level in homes with
smokers was significantly higher than that in homes
without smokers (Lee et al., 2002a).
In contrast, higher concentration of styrene was
found in non-smoking homes (10.9 ± 1.9 lg/m3
) and
no styrene was detected in smoking homes, suggesting
that the elevated styrene in Hong Kong homes was
caused by other sources. Indeed, styrene is often found
in a number of household and building products,
including floor waxes and polishes, paints, varnishes,
adhesives (epoxy resin in particular), metal cleaners,
and carpet backing (USEPA, 1993).
Significant differences in heptane and decane were
observed in smoking and non-smoking homes
(Figure 3) (P 0.001, two-tail t-test). However, these
two VOCs were not the main species emitted from
cigarette smoking (Srivastava et al., 2000). The higher
Effect of smoking
0
20
40
60
80
100
120
140
160
180
200
Form
aldehyde
Ethylacetate1-ButanolH
eptane
M
ethylisobutylketoneToluene
A
ceticacid
Ethylbenzenep-X
yleneN
onaneStyrene
A
lpha-pineneD
ecane
1,2,4-trim
ethylbenzene
1,4-dichlorobenzened-lim
onene
Sum
ofVO
Cs
Smoking, n =12
Non-smoking, n = 82
Fig. 3 Effect of cigarette smoking on indoor formaldehyde and volatile organic compounds in Hong Kong homes. n = number of
homes
Guo et al.
212

levels of heptane and decane in non-smoking homes
were probably attributed to the use of solvent, as these
two VOCs are the main components of adhesives and
solvents related to internal surfaces (Zuraimi et al.,
2006).
Effect of occupants
Number of occupants in homes may affect the indoor
formaldehyde and VOCs levels because of the varia-
tions of indoor activities, living habits and home
volumes (Clobes et al., 1992). During the field mea-
surements, it was found that homes with different
number of residents showed various indoor activities,
which significantly affected the indoor VOC levels. For
example, a home with four people frequently used
cleaning products and air fresheners when the occu-
pants were at home. On the other hand, a home with
one resident had very few indoor activities. The VOC
levels at these two homes showed significant difference
(TVOC: 147 and 40 lg/m3
, respectively). To obtain an
overall picture, the potential impact of occupants on
indoor VOC concentrations in all the monitored homes
was investigated. Table 6 presents the effect of occu-
pant number on indoor formaldehyde and VOCs in
Hong Kong homes. Mean formaldehyde concentration
and acetic acid level in homes with occupants less than
or equal to three was significantly higher than that in
homes with more than three occupants, respectively
(P 0.001, two-tail t-test). Formaldehyde and acetic
acid are often emitted from building materials such as
plywood (Colak and Colakoglu, 2004). Further inves-
tigation found that most of the homes with less than or
equal to three occupants were newly built or recently
decorated. These families were usually composed of a
young couple with/without a child, and the young
couples just started to work and preferred to buy new
apartments or decorate old apartments.
In contrast, statistically higher concentrations of
1-butanol, heptane and d-limonene was observed in
homes with more than three people (P 0.001, two-
tail t-test). 1-butanol is a component of plastics and
paint and is produced by mold and bacterial growth in
home environments (Wolkoff, 1995). Moisture prob-
lems, which lead to the growth of molds, bacteria, and
rot fungi have been documented extensively in houses
because of the sealed nature of the houses (Nevalainen
et al., 1998). Hong Kong is a subtropical region where
the relative humidity is always high with a range of 68–
83% and mean yearly value of 77% (Hong Kong
Observatory (HKO), 2007). As our sampling campaign
was carried out in November and the indoor temper-
atures were generally 22°C, the air conditioners or
heaters in all the homes were not used at all. Thus, it is
reasonable that the indoor temperature and relative
humidity were close to those outdoors when there were
few occupantsÕ activities in homes. It is believed that
the occupantsÕ activities could affect the relative
humidity in homes. Usually, the more occupants are
in home, the higher humidity is created from the
occupantsÕ activities. Furthermore, homes with more
than three people were relatively old buildings. All
these factors favored the growth of mold and bacteria
in humid conditions of Hong Kong. Heptane is a main
component of solvent use (AERIAS, 2007; Zuraimi
et al., 2006), whereas d-limonene is emitted from many
possible sources such as fresh fruits, cosmetic products,
essential oils, wood products, cleaning products, and
room air fresheners (Edwards et al., 2001b; Singer
et al., 2006). The higher levels of these two VOCs in
homes with more than three people reflects the wide
use of these household products in old buildings.
Effect of shoes
During the field measurement, it was found that the
VOC and formaldehyde levels at some homes with
shoes inside were significantly different from those with
shoes outside. As the production of shoe involves the
use of leather, rubber, plastics, coatings, shoe polisher
and adhesives, it is not unusual that VOCs could be
emitted from shoes as well. To facilitate evaluation of
the contribution of shoes to indoor VOC concentra-
tions, all the monitored homes were categorized. As the
information about when the shoes were polished was
not provided by participants, it is not possible to
determine whether the VOC emissions were from shoe
itself or from shoe polisher. Figure 4 shows the effect
of shoes on indoor VOCs concentrations. When shoes
were placed inside the homes, heptane, acetic acid,
Table 6 Effect of occupant number on indoor formaldehyde and VOCs in Hong Kong
homes
No. occupants
3, n = 65
No. of occupants
3, n = 29
Formaldehyde 85.6 € 12.0a
174.5 € 46.5
Ethylacetate N.D. N.D.
1-Butanol 101.5 € 28.2 32.6 € 16.8
Heptane 25.0 € 7.3 N.D.
Methyl isobutyl ketone N.D. N.D.
Toluene 18.7 € 13.7 9.4 € 4.3
Acetic acid 6.6 € 0.3 23.0 € 6.0
Ethylbenzene 3.9 € 0.7 4.9 € 1.1
p-Xylene 2.6 € 0.4 4.3 € 1.4
Nonane 12.0 € 1.0 N.D.
Styrene 14.5 € 5.4 N.D.
a-Pinene N.D. N.D.
Decane 21.7 € 5.2 36.9 € 11.5
1,2,4-trimethylbenzene 5.4 € 0.3 N.D.
1,4-dichlorobenzene 6.5 € 0.5 4.4 € 0.1
d-limonene 16.7 € 1.8 N.D.
Sum of VOCs 41.1 € 22.3 23.8 € 14.5
Sum of ALL 124.1 € 27.0 186.3 € 55.2
VOCs, volatile organic compounds; n, number of homes, N.D., non-detectable.
a
Mean € 95% confidence interval.
213

nonane, and styrene concentrations were statistically
higher than that when shoes were out of the homes
(P 0.001, two-tail t-test). These VOCs are often
identified in the use of adhesives, coatings, and cleaners
(Collins and Richey, 1992; Miller et al., 1994; Samfield,
1992; USEPA, 1993; Wallace, 1996; Zuraimi et al.,
2006). In contrast, the concentration of 1-butanol in
homes with shoes out was significantly higher than that
in homes with shoes in (P 0.001, two-tail t-test),
suggesting the growth of molds, bacteria, and rot fungi
in homes with shoes out (Nevalainen et al., 1998).
Moreover, no difference in formaldehyde level was
found between these two types of homes, implying that
the adhesives, coatings and polishers used for shoe
making might not be the main sources of formalde-
hyde.
Effect of cooking energy
Cooking is an important daily activity in Hong Kong
homes. It occurs at least twice a day in most homes and
each lasts about 1 h. Most kitchens in Hong Kong
homes have cooking range ventilators or fans installed.
During cooking, the ventilator or fan was operated and
windows and doors were closed. In this study, 69% of
the homes use liquefied petroleum gas (LPG) or town
gas as cooking fuel and about 16% of the homes utilize
electricity as cooking energy. As LPG is composed of
VOCs and town gas is made of methane, hydrogen and
carbon oxides, their use in homes could have impact on
indoor VOC levels. Thus, the VOC concentrations at
homes with different cooking fuel use were compared.
Figure 5 illustrates the formaldehyde and VOCs
concentrations in Hong Kong homes with different
cooking energy. It was not surprised that the levels of
1,2,4-trimethylbenzene, styrene, nonane and heptane
in gas-use families were much higher than that in
electricity-use homes (P 0.001, two tail t-test), as
these VOCs are either impurity component of cooking
fuel, i.e., heptane and nonane, or the combustion
by-products such as styrene and 1,2,4-trimethylbenzene
(Miller et al., 1994; Wallace, 1996). It was observed
that toluene concentration in gas-use homes was
moderately higher than that in electricity-use homes
Effect of shoe
0
50
100
150
200
250
Shoe in, n = 71
Shoe out, n = 23
Form
aldehyde
eptane
M
ethylisobutylketoneTolueneA
ceticacid
Ethylbenzenep-X
yleneN
onaneStyrene
A
lpha-pineneD
ecane
1,2,4-trim
ethylbenzene
onene
Sum
ofVO
Cs
Fig. 4 Effect of shoe on indoor formaldehyde and volatile organic compounds in Hong Kong homes. n = number of homes
Effect of cooking energy
0
50
100
150
200
250
300
Gas, n = 69
Electricity, n = 16
Form
aldehyde
eptane
M
ethylisobutylketone
TolueneA
ceticacid
Ethylbenzenep-X
ylene
N
onane
Styrene
A
lpha-pinene
D
ecane
1,2,4-trim
ethylbenzene
onene
Sum
ofVO
Cs
Fig. 5 Effect of cooking fuels on indoor formaldehyde and volatile organic compunds in Hong Kong homes. n = number of homes
Guo et al.
214

at 2r level, which is consistent with our previous
observation (Guo et al., 2003). In addition to the
sources of solvent for paints, coatings, gums, and
resins, toluene is emitted from petroleum production.
In contrast, the formaldehyde level in gas-use homes
was significantly lower than that in electricity-use
homes (P 0.001, two-tail t-test), reflecting that the
type of cooking fuel was not the dominant source of
formaldehyde in Hong Kong homes.
Summary and conclusions
Indoor formaldehyde and VOCs were monitored in
November 2002 in 100 Hong Kong homes using
passive samplers for 24 h, measured with other East
Asian countries and regions in the same period.
Formaldehyde was detected in 90% homes whereas
toluene was found in 65% homes. The mean values of
target compounds were associated with very large
standard deviations, reflecting wide distribution of
formaldehyde and 15 VOCs in Hong Kong homes. The
24-h average concentration of formaldehyde was
112.3 ± 9.5 lg/m3
(Mean ± 95% Confidence Inter-
val), which is higher than the good class of HKIAQO.
In contrast, the total VOC concentration was
46.1 ± 8.8 lg/m3
, much lower than that of HKIAQO
(600 lg/m3
). Compared to other East Asian cities,
the levels of formaldehyde and styrene were much
higher in Hong Kong homes.
The possible influence of the building age and
decoration, cigarette smoking, number of occupants,
location of shoes, and cook fuel on the indoor
formaldehyde and 15 VOCs concentrations in Hong
Kong homes was discussed. The level of formaldehyde
was higher in newly built homes and decreased with
the age of buildings. No significant impact of cigarette
smoking was found on indoor formaldehyde and
toluene, indicating the multiple sources of these
chemicals in residential environments. Different
groups of population were found to have close
association with indoor formaldehyde and VOCs.
The indoor formaldehyde and acetic acid were higher
in homes with a couple plus a child, whereas the
concentrations of 1-butanol, heptane and d-limonene
were higher in homes with more than three family
members.
When shoes were placed inside the homes, heptane,
acetic acid, nonane, and styrene concentrations were
statistically higher. However, the concentration of
1-butanol in homes with shoes out was significantly
higher. On the other hand, the levels of 1,2,4-trimethyl-
benzene, styrene, nonane and heptane in gas-use
families were much higher than that in electricity-use
homes (P 0.001, two tail t-test), but the opposite
observation was obtained for formaldehyde.
Acknowledgements
The authors thank the National Institute of Public
Health of Japan for the support of field measurement
and chemical analysis. Special thanks go to the 100
homes for their collaboration in air sampling. The data
analysis presented in this study is supported by the
Research Grants Council of the Hong Kong Special
Administrative Region (Project No. PolyU 5163/07E),
and the Research Grant (87PK) of the Hong Kong
Polytechnic University. The invaluable comments of
anonymous reviewers are greatly appreciated.
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