2009 The Authors Journal compilation Blackwell Munksgaard 2009 Indoor Air 2009; 19: 206–217 www.blackwellpublishing.com/ina Printed in Singapore. All rights reserved INDOOR AIR doi:10.1111/j.1600-0668.2008.00580.x Formaldehyde and volatile organic compounds in Hong Kong homes: concentrations and impact factors 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 household 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 formaldehyde 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 concentrations were statistically higher than that when shoes were out of the homes. Furthermore, higher levels of 1,2,4-trimethylbenzene, styrene, nonane and heptane 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: [email protected] Received for review 27 March 2008. Accepted for publication 6 October 2008. 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 concentrations 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. Introduction Indoor air pollution caused by volatile organic compounds (VOCs) is attracting international interest as many indoor materials and utilities contain VOCs (Godish, 2001). VOCs are classified as organic compounds that have boiling point between 50 and 260C (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 chlorinated hydrocarbons (Guo et al., 2003; Kim et al., 206 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 formaldehyde 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 environment. These sources include combustion by-products, cooking, construction materials, furnishings, paints, Concentrations and impact factors of formaldehyde and VOCs 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 trichloroethene 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 mothballs 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-smokersÕ 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 characteristics 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, potential 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 Table 1 The IAQ objectives for formaldehyde, TVOCs and individual volatile organic compunds in Hong Kong (Hong Kong Environmental Protection Department (HKEPD), 2003) 8-h average Parameter Formaldehyde TVOC Benzene Carbon tetrachloride Chloroform 1,2-Dichlorobenzene 1,4-Dichlorobenzene Ethylbenzene Tetrachloroethylene Toluene Trichloroethylene Xylene (o-, m-, p-isomers) Unit 3 lg/m lg/m3 lg/m3 lg/m3 lg/m3 lg/m3 lg/m3 lg/m3 lg/m3 lg/m3 lg/m3 lg/m3 Excellent class Good class <30 <200 <100 <600 16.1 103 163 500 200 1447 250 1092 770 1447 IAQ, indoor air quality; VOCs, volatile organic compunds; TVOCs, total VOCs. 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 onethird of Hong KongÕs population) (Hong Kong Housing 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 population 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, instruction 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 207 Guo et al. Table 2 Characteristics of the monitored homes in Hong Kong Home type House Apartment Others Structure Number of homes Wooden Ferroconcrete Light steel Others Wooden Ferroconcrete Stone Light steel Others Wooden Ferroconcrete Light steel Others 0 6 1 0 2 82 3 2 1 0 3 0 0 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 containing 13 questions about building conditions, residential 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-dinitrophenylhydrazine (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. Fig. 1 Example for formaldehyde and volatile organic compound sampling (Left: passive charcoal tube; Right: DNPHcartridge) Table 3 Sampling and species analysis Aldehydes Sampler Target compounds Sampling method Desorption method Analytical method 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 environments 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-performance liquid chromatography (HPLC, Hewlett-Packard 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 (Toyokaseikogyo Co. Ltd, Osaka, Japan) from the charcoal tube by shaking, and then analyzed by gas chromatography – mass selective detection (GC/MSD, Hewlett-Packard 208 VOCs Sampler Target compounds (15 compounds) Sampling method Desorption method Analytical method Diffusive 2,4-dinitrophenylhydrazine cartridge Formaldehyde Passive sampling for 24 h Solvent desorption (acetonitrile 3 ml) High-performance liquid chromatography Detector: Diode array detector at 365 nm (Ref.600 nm) Column: Eclipse XDB(5 lm · 250 mm) Passive charcoal tube Heptane, Decane, Nonane, Toluene, p-Xylene, Trimethyl-benzene, Ethylbenzene, Styrene, Limonene, a-Pinene, p-Dichlorobenzene, Ethyl acetate, Acetic acid, m-isobutylketone, Butanol Passive sampling for 24 h Solvent desorption (carbon disulfide) 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. 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 concentration of the 15 target VOCs was defined as TVOC. The detection limits were calculated according to the Concentrations and impact factors of formaldehyde and VOCs 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 reproducibility 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 formaldehyde 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 25C, 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 samplers 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 25C 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 25C. The diffusive sampling rates of formaldehyde and VOCs for the passive samplers are as follows, respectively, DiffcoVOC DSRVOC ¼ DSRTol ð1Þ DiffcoTol 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 20C; ÔTolÕ represents toluene and ÔForÕ is formaldehyde; ÔaldÕ means aldehydes. Statistical analyses All statistical analyses, including correlation, regression, t-test, one-way ANOVA and ANOVA with posthoc 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 concentration in 37 homes exceeded the good class of HKIAQO. The TVOC concentration in all 100 homes was lower 209 Guo et al. 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 Ethylacetate 1-Butanol Heptane Methyl Isobutyl Ketone Toluene Acetic acid Ethylbenzene p-Xylene Nonane Styrene a-Pinene Decane 1,2,4-trimethylbenzene 1,4-dichlorobenzene d-limonene Sum of VOCsd Sum of ALLe 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 10 99 89 96 99 35 96 78 79 97 96 98 77 97 94 77 – – 46.3 – 14.1 8.4 – 2.2 7.1 2.2 1.6 10.1 0.2 10.3 13.0 4.8 4.3 11.8 3.8 81.0 85.7 – 54.6 11.2 – 4.4 9.4 3.2 2.1 12.1 1.8 11.0 15.5 5.0 5.4 14.4 15.9 114.7 112.3 – 82.7 22.1 – 15.3 14.8 4.1 3.0 12.0 10.9 11.0 25.0 5.4 5.8 18.1 46.1 162.2 141.9 – 87.8 24.9 – 9.8 17.2 4.8 3.4 14.0 12.6 11.8 24.0 5.8 6.9 20.7 45.0 185.6 468.9 – 321.0 59.8 – 359.3 34.7 12.9 10.9 15.9 39.9 12.6 99.2 6.6 8.7 49.0 537.6 706.9 90.3 – 104.4 25.4 – 45.4 13.5 2.9 2.3 3.9 19.4 2.2 24.1 1.1 1.9 10.0 87.8 136.9 80 – 126 115 – 297 91 70 77 32 178 20 96 20 33 55 190 84 a Number of samples in which the compound concentration under detection limit. 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. b 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, duration 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 concentrations 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) Formaldehyde Ethyl acetate 1-Butanol Heptane Methyl isobutyl ketone Toluene Acetic acid Ethylbenzene p-Xylene Nonane Styrene a-Pinene Decane 1,2,4-trimethylbenzene 1,4-dichlorobenzene d-Limonene Sum of VOCsb Sum of ALLc 58.2 16.7 147.6 18.1 N.D. 9.1 6.8 4.0 2.6 15.8 3.9 19.2 24.3 6.2 35.6 49.3 186.4 247.5 € € € € 4.8a 2.0 12.5 0.9 € € € € € € € € € € € € € 2.1 0.2 0.3 0.2 1.1 0.2 1.2 3.5 0.3 5.2 5.6 14.8 16.3 Hong Kong (n = 100) Taiwan (n = 100) Korea (n = 96) 112.3 N.D. 82.7 22.1 N.D. 15.3 14.8 4.1 3.0 12.0 10.9 11.0 25.0 5.4 5.8 18.1 46.1 162.2 87.1 15.0 N.D. 14.7 N.D. 14.5 N.D. 4.3 2.6 N.D. 0.1 N.D. 27.1 4.2 N.D. 21.4 34.8 122.0 67.4 17.5 79.0 89.9 15.9 34.7 34.6 8.6 7.6 28.9 2.3 20.9 25.7 7.6 N.D. 42.7 180.0 256.8 € 9.0 € 10.4 € 2.5 € € € € € € € € € € € € € 4.5 1.4 0.3 0.2 0.4 1.9 0.2 2.4 0.1 0.2 1.0 8.8 13.7 VOCs, volatile organic compunds; N.D., non-detectable; n, number of homes investigated. Mean € 95% confidence interval. b Sum of 15 VOCs. c Sum of 15 VOCs and formaldehyde. a 210 € 8.5 € 1.6 € 0.7 € 0.9 € 0.1 € 0.1 € 0.002 € 3.0 € 0.1 € 0.9 € 2.6 € 9.4 € € € € € € € € € € € € € € Japan (n = 97) 6.8 2.6 14.5 12.4 1.0 4.6 5.3 1.5 1.6 3.1 0.2 1.2 3.3 0.9 € 4.8 € 21.2 € 23.5 37.5 21.5 52.7 295.4 32.7 7.7 28.8 4.8 3.3 33.5 0.2 57.0 41.8 7.5 144.6 37.0 187.7 263.6 € € € € € € € € € € € € € € € € € € 3.4 3.5 1.4 69.9 4.1 1.0 8.5 0.3 0.2 3.8 0.02 5.5 2.8 0.5 27.4 5.2 31.4 36.8 Concentrations and impact factors of formaldehyde and VOCs 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 building) 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 formaldehyde 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 formaldehyde 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 buildings with various ages (P > 0.05). This is due to the 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 concentration 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, smoking, 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 Effect of building aging/decoration 350 Concentration (µg/m3) 2002, n = 13 300 2001, n = 11 250 2000, n = 14 1999, n = 8 200 1990–1998, n = 31 150 <1989, n = 17 100 50 of m Su d- lim V on O en Cs e an e ec D en yl pX lb hy e e en ze n e en To Bu 1- lu Et Fo rm al de hy ta no l de 0 Fig. 2 Effect of building aging and decoration on indoor formaldehyde and volatile organic compounds in Hong Kong homes. n = number of homes 211 Guo et al. fact that household painting and paint, varnish and lacquer removal, tobacco smoke, and consumer products 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 200 180 160 140 120 100 80 60 40 20 0 Effect of smoking Smoking, n =12 Non-smoking, n = 82 Fo rm al de hy Et de hy la ce 1- tate Bu M ta et n hy H ol li e so p ta bu n ty lk e et on To e lu e A ce ne Et tic a hy c lb id en ze p- ne X yl en N e on an St e yr A e lp ha ne -p 1, in 2, en 4tri D e m e ca et 1, 4- hylb ne di e ch nz lo en ro be e nz en de l Su imo m ne of ne VO Cs Concentration (µg/m3) Cigarette smoking is often an indoor source which emits a variety of organic compounds such as formaldehyde, 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 cigarette 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 measured 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 Fig. 3 Effect of cigarette smoking on indoor formaldehyde and volatile organic compounds in Hong Kong homes. n = number of homes 212 Concentrations and impact factors of formaldehyde and VOCs 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 variations of indoor activities, living habits and home volumes (Clobes et al., 1992). During the field measurements, 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 occupants 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 occupant 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 investigation found that most of the homes with less than or Table 6 Effect of occupant number on indoor formaldehyde and VOCs in Hong Kong homes No. occupants >3, n = 65 Formaldehyde Ethylacetate 1-Butanol Heptane Methyl isobutyl ketone Toluene Acetic acid Ethylbenzene p-Xylene Nonane Styrene a-Pinene Decane 1,2,4-trimethylbenzene 1,4-dichlorobenzene d-limonene Sum of VOCs Sum of ALL 85.6 N.D. 101.5 25.0 N.D. 18.7 6.6 3.9 2.6 12.0 14.5 N.D. 21.7 5.4 6.5 16.7 41.1 124.1 € 12.0a € 28.2 € 7.3 € € € € € € 13.7 0.3 0.7 0.4 1.0 5.4 € € € € € € 5.2 0.3 0.5 1.8 22.3 27.0 No. of occupants <3, n = 29 174.5 N.D. 32.6 N.D. N.D. 9.4 23.0 4.9 4.3 N.D. N.D. N.D. 36.9 N.D. 4.4 N.D. 23.8 186.3 € 46.5 Effect of shoes € 16.8 € € € € 4.3 6.0 1.1 1.4 € 11.5 € 0.1 € 14.5 € 55.2 VOCs, volatile organic compounds; n, number of homes, N.D., non-detectable. Mean € 95% confidence interval. a 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, twotail 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 problems, 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 temperatures were generally <22C, 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. 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 concentrations, 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, 213 Guo et al. Concentration (µg/m3) 250 Effect of shoe 200 150 Shoe in, n = 71 Shoe out, n = 23 100 50 Fo rm al d Et ehy d hy la e ce ta te 1Bu M ta et n hy H ol li ep so ta bu n ty lk e et on To e lu en A e ce Et tic hy ac lb id en ze ne pX yl en N e on an S e A tyre lp ha ne -p 1, in 2, en 4e tri D m e c et 1, a h ne 4di ylb ch en lo ze ro be ne nz e dlim ne on Su e m of ne VO Cs 0 Fig. 4 Effect of shoe on indoor formaldehyde and volatile organic compounds in Hong Kong homes. n = number of homes 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 formaldehyde. 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 Concentration (µg/m3) 300 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 cooking energy 250 200 150 100 Gas, n = 69 Electricity, n = 16 50 Fo rm al de hy Et de hy la ce ta te 1Bu M ta et no hy l li H ep so bu ta n ty lk e et on e To lu e ne A ce t Et ic a hy c lb id en ze ne pX yl en e N on an e St yr A en lp ha e -p 1, in 2, en 4e tri D m ec et an 1, h yl 4e b di ch enz lo en ro be e nz en dlim e o Su ne m ne of VO Cs 0 Fig. 5 Effect of cooking fuels on indoor formaldehyde and volatile organic compunds in Hong Kong homes. n = number of homes 214 Concentrations and impact factors of formaldehyde and VOCs 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 Interval), 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-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), 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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