Environ. Sci. Technol. 2010, 44, 6081–6087
Occurrence of Cyclic and Linear Siloxanes in Indoor Dust from China, and Implications for Human Exposures Y A N L U , †,‡ T A O Y U A N , † S E H U N Y U N , ‡ WENHUA WANG,† QIAN WU,‡ AND K U R U N T H A C H A L A M K A N N A N * ,‡,§ School of Environmental Science and Engineering, Shanghai Jiao Tong University, Shanghai, 200240, China, Wadsworth Center, New York State Department of Health, and Department of Environmental Health Sciences, School of Public Health, State University of New York at Albany, Empire State Plaza, PO Box 509, Albany, New York 12201-0509, and International Joint Research Center for Persistent Toxic Substances, State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, Harbin 150090, China
Received April 27, 2010. Revised manuscript received July 2, 2010. Accepted July 6, 2010.
Siloxanes are used in a wide variety of personal-care and other consumer products. Although there is clearly a potential for contamination of indoor dust with siloxanes, reports of occurrence of siloxanes in indoor dust were not available, prior to the present study. Here, we have determined the concentrations and profiles of four cyclic siloxanes, octamethylcyclotetrasiloxane (D4), decamethylcyclopentasiloxane (D5), dodecamethylcyclohexasiloxane (D6), and tetradecamethylcycloheptasiloxane (D7), as well as 11 linear siloxanes, from L4-L14, in 100 dust samples collected in China. Cyclic and linear siloxanes were found in all dust samples, with the linear siloxanes L9-L14 being the predominant compounds. Concentrations of total siloxanes in dust ranged from 21.5 to 21 000 (mean: 1540 ( 2850) ng g-1. The highest concentration of the individual linear siloxanes, L9-L14, ranged between 2680 and 6170 ng g-1. Concentrations of total linear siloxanes (TLS) were 1-2 orders of magnitude higher than concentrations of total cyclic siloxanes (TCS), in all indoor dust samples. Siloxane concentrations in dust were associated with the number of electrical/electronic appliances, number of occupants, and smokers living in the house. Based on the measured siloxane concentrations and on estimated daily ingestion rates of dust by toddlers and adults, we calculated the daily intake of siloxanes. For adults, daily exposure to total siloxanes, based on an average dust intake rate and median exposure concentration, was calculated to be 15.9 ng day-1; the corresponding value for toddlers was 32.8 ng d-1.
Introduction Cyclic and linear siloxanes are synthetic organosilicone compounds, consisting of a backbone of alternating silicons oxygen [SisO] bonds, with organic side chains attached to each silicon atom (1). These compounds have been widely used in industrial appliances and a vast range of consumer products, such as electrical devices, health-care products, cosmetics, cookware, and household cleaning products (2, 3). Horii and Kannan (4) determined the concentrations of cyclic and linear siloxanes in 76 consumer products sampled in Albany, NY, and showed widespread occurrence of octamethylcyclotetrasiloxane (D4, where D refers to the dimethylsiloxane unit, and the subscript refers to the number of SisO bonds), decamethylcyclopentasiloxane (D5), and dodecamethylcyclohexasiloxane (D6). Recently, Wang et al. (5) reported the occurrence of high concentrations (on the order of hundreds of µg g-1) of D5 in cosmetics sampled in Canada. Because of their widespread use and their characteristic of high volatility, siloxanes are presumably present in various environmental media. Until now, almost all investigations relating to siloxanes have focused on the distribution and fate in the outdoor environment. Previous studies suggested the occurrence of siloxanes in suspended solids and activated sludge from wastewater treatment plants (2, 6), in sludgeamended soils and sediments (6-10), as well as in landfill biogases (11). Despite the widespread usage of products containing siloxanes in the indoor environment, little is known on the distribution and profiles of these compounds in indoor dust. Owing to siloxanes’ high particle-binding affinities (12), indoor dust can be a potential environmental sink as well as a source of human exposures. A few studies have suggested toxic potentials of siloxanes in laboratory animals. Exposure to D4 impaired the fertility and reproductive health of rats, in a series of inhalation and oral exposure studies (13-17). Inhalation exposures (18, 19) to D4 and D5 in rats caused histopathological changes in lungs. Meeks et al. (20) reported that a single 6 h inhalation exposure to D4, on the day prior to mating, significantly reduced fertility. Some evidence suggests potential carcinogenicity of D5 (21). In view of the potential for occurrence and toxicity, determination of siloxanes in indoor dust, and evaluation of human exposures arising through the ingestion of dust, are imperative, to enable the assessment of risks, and the development of strategies for reducing exposures. To our knowledge, no earlier study has investigated the occurrence of siloxanes in the indoor environment. In the present work, we determined four cyclic siloxanes (D4, D5, D6, and tetradecamethylcycloheptasiloxane or D7), and 11 linear siloxanes, L4-L14 (L refers to the linear dimethylsiloxane unit, and the subscript refers the number of silicon bonds) in 88 indoor dust samples from homes, offices, laboratories, and dormitories, and in 12 dust samples collected from within electrical/electronic devices (personal computers or PCs and air conditioners) in houses. Exposure of adults and toddlers to siloxanes via the ingestion of dust was then estimated based on the concentrations measured.
Materials and Methods * Corresponding author: K. Kannan, Wadsworth Center, Empire State Plaza, PO Box 509, Albany, NY 12201-0509, Tel: +1-518-4740015, Fax: +1-518-473-2895, E-mail:
[email protected]. † Shanghai Jiao Tong University. ‡ Wadsworth Center and State University of New York at Albany. § Harbin Institute of Technology. 10.1021/es101368n
2010 American Chemical Society
Published on Web 07/21/2010
Standards and Chemicals. Individual standards of D4, D5, and D6 were purchased from Tokyo Chemical Industry (Wellesley Hills, MA). A polydimethylsiloxane mixture (PDMS; viscosity of 5 cSt), which contains D7 and linear siloxanes (L4-L14), was obtained from Sigma-Aldrich (St. Louis, MO). VOL. 44, NO. 16, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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The internal standard, tetrakis (trimethylsiloxy)-silane (M4Q; purity 97%) was purchased from Aldrich. All of the organic solvents and reagents used in this study were analytical grade. Sampling. Indoor dust samples were collected during JulySeptember, 2009 from living rooms of a number of houses (n ) 56), student dormitories (n ) 17), offices (n ) 9), and chemical laboratories (n ) 6). The sampling locations of house dust were divided into three geographic areas (northern China, n ) 18; eastern China, n ) 22; central China, n ) 24). Twelve indoor house dust samples were collected in rural areas, and others were sampled in urban areas. Dust samples from student dormitories, offices, and chemical laboratories were all collected in Shanghai, China. Sampling was conducted with household vacuum cleaners. Carpet or floor surfaces in living rooms where the dwellers spent most of the time resting or working, were vacuumed. In addition, dust was collected, using brushes, from inside of electrical devices (peripheral corners) after the casings/housings were opened (PCs, n ) 5; air conditioners, n ) 7). Each dust sample was placed in precleaned aluminum foil, and then sealed in a plastic bag and stored at -20 °C until analysis. A questionnaire was filled in, to provide information regarding age of the house, number of large electrical devices (such as TV sets, air conditioners, PCs, DVD players) in the room where dust was sampled, number of occupants, and smoking habits of occupants (Yes/No). Chemical Analysis. Prior to the analysis, nondust particles, such as hair and pet fur, were removed, and the samples were sieved through a 500 µm mesh sieve. Three hundred to five hundred milligrams of dust sample were weighed accurately and spiked with 100 µL of 1 ppm M4Q (as an internal standard). The extraction procedure was similar to the method reported earlier (4), with some modifications. Briefly, sieved dust samples were shaken with 5 mL of n-hexane for 15 min. After shaking, samples were centrifuged at 4000 rpm for 5 min, and the solvent layer was transferred into a flat-bottomed flask. The samples were re-extracted three times with ethyl acetate/n-hexane mixture (1:1) as mentioned above. To confirm the extraction efficiency, after the first two extractions, we soaked the samples in 5 mL of solvent mixture (ethyl acetate/n-hexane; 1:1) overnight. Each extract was concentrated to 1-2 mL using a rotary evaporator and then purified by passage through a solid phase extraction (SPE) cartridge topped with 0.2 g of sodium sulfate and 0.5 g of silica gel (Sigma-Aldrich; 100-200 mesh, 150 Å). Six milliliters of n-hexane and 5 mL of dichloromethane/nhexane mixture (1:1) were eluted through the cartridge. The fractions were collected in a polypropylene tube and concentrated to 500 µL under a gentle stream of nitrogen, for GC-MS analysis. Cyclic and linear siloxanes were identified and quantified by gas chromatography-mass spectrometry (GC-MS; Agilent 6890 GC and 5972 MS; Foster City, CA), and separation was achieved by a 30 m DB-5MS column (0.25 mm i.d., 0.25 µm film thickness; J&W Scientific, Folsom, CA). Two microliters of sample extract were injected in splitless mode at 200 °C. The oven temperature was programmed as follows: 40 °C (2 min) to 220 °C at a rate of 20 °C min-1 and to 280 at 5 °C min-1, with a hold for 10 min (postrun at 300 °C for 5 min). The MS was operated in electron impact-selected ion monitoring (EI-SIM) mode. The ions monitored for individual compounds have been reported previously (4). Concentrations of D4, D5, and D6 were quantified based on the external calibration standards prepared for each of these compounds. Concentrations of D7 and L4-L14 were based on the composition and concentrations of these compounds found in the PDMS calibration standard, as reported earlier (4). Quality Assurance/Quality Control (QA/QC). Cyclic and linear siloxanes are present in many consumer commodities and laboratory products. The analyst took care not to use 6082
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TABLE 1. Limit of Quantification (LOQ, ng g-1) for All Target Siloxanes (D4-D7 and L4-L14), in Dust Samples Collected in China compound
LOQ
compound
LOQ
compound
LOQ
D4 D5 D6 D7 L4
1.25 0.82 1.11 0.87 0.69
L5 L6 L7 L8 L9
0.58 1.41 1.28 1.47 1.24
L10 L11 L12 L13 L14
1.08 0.81 0.52 0.24 0.18
hand lotions or other possible sources of contamination before or during the analysis. In addition, efforts to reduce background levels of siloxanes in products used in the laboratory, including vial septa, inlet septa, and capillary columns have been reported in our earlier study (4). Considering SPE cartridge may contain siloxanes, blank solvent (hexane) was used to evaluate the possible influence by cleanup. After cleanup, only D4-D6, L6, and L8 could be detected with ignorable concentrations in blank tests (all of their concentrations were below 0.33 ng g-1), and other compounds could not be found in all of the blank tests. The accuracy and precision of the analytical method was determined by spiking of known concentrations of target siloxanes (D4-D6, and PDMS) into Milli-Q water and dust samples, and passing them through the entire analytical procedure. The mean recoveries of D4-D6 from the two matrices were 75 ( 6.4%, and 89 ( 9.8%, respectively. For D7, and for L4-L14, the recoveries ranged between 68 ( 1.7% (D7) and 108 ( 16.3% (L11). Procedural blanks were analyzed by extraction of anhydrous sodium sulfate (as a surrogate for dust) and passage through the entire analytical procedure for every 12 samples. The limit of quantification (LOQ) was set at 3 times the mean values found in blank samples (See Table 1). All of the reported concentrations in dust samples were subtracted from the mean value found for the procedural blanks. The internal standard (M4Q) recoveries in all samples ranged from 67.8 to 112%. The reported data were not corrected for the recovery of M4Q. Total linear siloxane (TLS) refers to the sum of the concentrations of all 11 linear siloxanes and total cyclic siloxane (TCS) refers to the sum of all four cyclic siloxanes analyzed. Statistical Analysis. Statistical analysis of the data was conducted using Microsoft Excel (Microsoft Office 2007) and SPSS version 13.0. For values below the LOQ, the concentration was set to one-half of the LOQ. The siloxane concentrations in dust samples were log-normally distributed. Hence, ANOVA and t tests were performed on log-transformed concentrations.
Results and Discussion Frequency of Occurrence and Concentrations of Siloxanes. Cyclic (D4-D7) and linear (L4-L14) siloxanes were found in all dust samples analyzed. The detection frequencies and concentrations of individual siloxanes in indoor dust are shown in Figure 1. Concentrations of total siloxanes (TLS+TCS) in dust ranged from 21.5 to 21 000 (mean: 1540 ( 2850) ng g-1. The concentration data for the floor dust collected using vacuum cleaners, and for the dust recovered from inside electrical/electronic devices, are reported separately. For floor dust samples, the detection frequencies for L5 and L6 were 63.6% and 55.7%, respectively; the corresponding values for dust from inside electrical devices were 50% and 58.3%, respectively. Relatively higher frequencies of occurrences and concentrations were found for linear siloxanes, L9-L14, than cyclic siloxanes and L4-L8, in floor dust and electrical device dust. This pattern suggests that high-molecular-weight linear siloxanes are widely present in products used indoors in China. Cyclic siloxanes are
FIGURE 1. Frequency of occurrence and concentrations of cyclic (D4-D7) and linear (L4-L14) siloxanes in floor dust (A) and dust from electrical devices (B). The box plot shows 5th, 10th (lower whisker), 25th (bottom edge of the box), 75th (top edge of the box), 90th (upper whisker), and 95th percentiles. The median concentration is given as the line within the box. primarily used in personal care products and linear siloxanes are widely used in paints, varnishes, lacquers, and furniture polishes (4). High levels of linear siloxanes have also been found in face creams (4). The highest concentrations of L9-L14 (ranging from 2680 to 6170 ng g-1) were an order of magnitude higher than the concentrations of the other target compounds analyzed (ranging from 151 to 924 ng g-1). L10 was the commonest compound, found in more than 50% of the dust samples analyzed. In contrast to our results found for dust from China, consumer products collected from the U.S., in a previous study, contained predominant proportions of cyclic siloxanes, D5-D7 (4). However, it should be noted that personal care and consumer products typically contain blends of several cyclic and linear siloxanes (4). Elevated concentrations of the linear siloxanes L9-L14 found in dust samples analyzed in the present study suggest that sources other than personal care products contributed to the contamination of indoor dust. As mentioned above, paints, varnishes, and furniture polishes contain high levels of linear siloxanes. Further studies are needed to examine the patterns of siloxanes in consumer products in China. Among the four cyclic siloxanes analyzed here, the concentrations of D5 and D6 were slightly higher than the concentrations of D4 and D7, in the indoor dust samples; that pattern is similar to what was reported for consumer products from Canada (5). Dust samples collected from electrical/electronic devices (PCs, n ) 5; air conditioners, n ) 7) exhibited significant differences in D5-D7 concentrations. Dust collected from PCs contained significantly lower D5-D7 concentrations than did dust collection from air conditioners (p < 0.05; detailed information is supplied in SI Table S1). It is possible that low-molecular-weight siloxanes are volatilized or thermally degraded at the relatively high temperatures prevailing in PCs during usage (22, 23). The indoor dust samples were classified into four categories, based on the sampling environment (living rooms of houses n ) 56; students’ dormitories, n ) 17; offices, n ) 9; chemical laboratories, n ) 6). Based on this classification, the sampling locations could be further grouped into two categories, namely living environment (house and dormitory) and working environment (office and laboratory). Before the high level grouping, we tested the differences in the concentrations of siloxanes between house dust and dormitory dust, and between office dust and laboratory dust. D4
concentrations were significantly higher (p < 0.05) in house dust than in dormitory dust (SI Table S2); no significant difference was found for the other siloxanes. No significant difference was found in the concentrations of all siloxanes between office dust and laboratory dust (SI Table S2). Concentrations of total linear siloxanes (TLS) were 1-2 orders of magnitude higher than concentrations of total cyclic siloxanes (TCS), in all indoor dust samples (Figure 2). L9-L14 were the predominant siloxanes found in dust samples. Significantly higher concentrations of TCS were found in dust from the working environment (p < 0.05) than in dust from the living environment (mean: 87.9 ( 103 ng g-1 and 170 ( 116 ng g-1 for living and working environments, respectively). The result suggests the potential for high exposure to cyclic siloxanes in the working environment. Concentrations of TLS in dust samples were not significantly different (p > 0.05) between living and working environments (mean: 1260 ( 2810 ng g-1 and 2340 ( 2870 ng g-1 for living and working environments, respectively). Relationship Between Concentrations of Siloxanes and Number of Electrical/Electronic Devices. Information regarding the number of electrical/electronic devices used in the living/working environment was obtained when dust samples were collected. A general tendency of increasing siloxane concentrations in dust samples (median) with the number of electrical/electronic products used in the indoor environment was observed in present study (Table 2). Significantly higher concentrations of D4, L8, L9, and L10 (p < 0.05; one-way ANOVA) were found in dust collected from rooms with more electrical devices (N > 5), than from rooms with fewer such appliances (N < 3). The electrical/electronic devices can be a source of siloxane contamination in the indoor dust (6, 24). Electronic components are coated in siloxanes to increase stability against mechanical and electrical shock, radiation, and vibration (http://www.chemidex.com/). These compounds can volatilize and accumulate in dust. Relationship between Siloxane Concentrations and Number of Occupants and Smokers. The concentrations of both TCS and TLS increased with increasing number of occupants in the house, although the differences in concentrations were not statistically significant (p > 0.05) (Figure 3). The general trend suggests a greater use of siloxanecontaining products, for larger numbers of occupants in the house. VOL. 44, NO. 16, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. Distribution of concentrations of total cyclic siloxanes (TCS) and total linear siloxanes (TLS) between living (house and dormitory) and working (office and laboratory) environments.
TABLE 2. Median Concentrations (ng g-1) of Silicones in Indoor Dust As Related to the Number of Electrical/Electronic Devices (N) Present in the Indoor Environment
D4 D5 D6 D7 TCSa L4 L5 L6 L7 L8 L9 L10 L11 L12 L13 L14 TLSb
N < 3 (n ) 32)
N ) 3-5 (n ) 31)
N > 5 (n ) 25)
5.10 14.8 11.0 5.31 44.9 43.6 1.12 1.56 1.53 12.1 22.7 38.1 29.9 16.8 11.1 9.30 242.1
10.4 20.2 17.5 12.3 65.1 25.8 1.66 0.05). To further assess whether or not smoking contributes notably to siloxanes in indoor dust, we evaluated the data using the 75th-percentile approach developed by Kersten and Reich (25). In this model (see eq 1), the quotient (q) is defined as the 75th percentile of the category that contains the source (p (75) contains), to the 75th percentile of the category that does not contain the source (p (75) free). The influence of this specific source is confirmed when the value of q exceeds 2. This approach has been successfully applied in the identification of sources of phthalates in house dust (26). q)
p(75)contains fqg2 p(75)free
(1)
The results of the analysis suggested that smoking is a source for some linear siloxanes in the indoor dust, since q values for these compounds were >2 (see SI Table S3). The calculated 75th percentiles for TCS were 98.9 ng g-1 for the nonsmoker group and 132 ng g-1 for the smoker group. 6084
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The calculated 75th percentiles for TLS were 916 and 2190 ng g-1 for the nonsmoker group and the smoker group, respectively. The q values for L11-L14 ranged from 2.05 to 3.66 (mean: 2.39), suggesting that the concentrations of these linear siloxanes in dust are positively associated with smokers in the house. These results further suggest that smokers may use some products that contain siloxanes. Correlations among Siloxanes in Dust. Relationships among the concentrations of various siloxanes in all dust samples were examined using Pearson correlation analysis (Table 3). A significant positive correlation existed among four cyclic siloxanes in dust samples, except between D4 and D6 (p < 0.05). Cyclic siloxanes were previously found to be the predominant compounds in a large number of consumer products from North America (4, 5). A significant correlation was found between D5 and D6 in personal care products such as fragrances, hair care products, nail polishes, as was the case for our indoor dust samples; this suggests that personal care products are a source of cyclic siloxanes in the indoor environment. D6 and D7 are the predominant siloxanes in rubber products (e.g., nipples, cookware, sealants) (4, 27); the significant correlation between concentrations of D6 and D7 in our samples indicates that rubber products were sources of these compounds in indoor dust (r ) 0.60, p < 0.01). Various combinations of pairs of concentrations of highmolecular-weight linear siloxanes, L9-L14, were significantly correlated with one another. In particular, each pair wise comparison, within the L11-L14 subset, showed a significant correlation, with Pearson correlation coefficients (r) close to 1, indicating a similarity in sources of these compounds. L13 and L14 were not found in an analysis of consumer products from the U.S. (4); from that we could infer that the main source of these highest-molecular-weight linear siloxanes in our Chinese dust samples is not consumer products. Due to their excellent electrical insulating capacity and thermal stability, high-molecular-weight linear siloxanes are widely used in transformer dielectric and heat-transfer components (28). Nevertheless, further research on the composition of siloxanes in personal care products from China is needed. Human Exposure to Siloxanes via the Ingestion of Indoor Dust. Several parameters such as age, time spent indoors, and the amount of dust intake can affect exposure of humans to contaminants present in house dust. Klepeis et al (29) proposed a typical time-activity pattern of 63.8% in home, 22.3% in office, 5.1% in public microenvironments, and 4.7% in car for adults. Although this activity pattern was developed for the US population, a similar pattern can be expected for urban residents in China; the pattern has already
FIGURE 3. Concentrations of TCS and TLS in house dust and the number of occupants in nonsmoker house as well as the smokers living in the house. (Bar chart represents median concentrations of TCS and TLS. 95th percentile is shown as the upper whisker.
TABLE 3. Pearson Correlation Coefficients (r) among Concentrations of Individual Siloxanes Measured in Indoor Dust From China compound D5 D6 D7 L4 L5 L6 L7 L8 L9 L10 L11 L12 L13 L14 a
D4
D5
D6
D7
L4
L5
L6
L7
L8
L9
L10
L11
L12
L13
0.62b 0.38b 0.24a 0.51b 0.42b 0.32b 0.14 0.03 –0.02 –0.03 –0.02 –0.05 –0.05
0.60b 0.05 0.28b 0.83b 0.78b 0.27b 0.01 –0.02 –0.02 –0.04 –0.04 –0.04
–0.10 0.06 0.55b 0.62b 0.53b 0.29b 0.17 0.09 0.03 0.02 0.01
–0.00 –0.03 –0.05 –0.15 –0.12 –0.14 –0.14 –0.11 –0.09 –0.09
0.30b 0.07 –0.04 –0.08 –0.09 –0.09 –0.08 –0.06 –0.05
0.95b 0.29b –0.03 –0.06 –0.06 –0.06 –0.05 –0.04
0.44b 0.10 0.04 –0.00 –0.02 –0.02 –0.03
0.84b 0.63b 0.29b 0.13 0.07 0.04
0.88b 0.52b 0.36b 0.31b 0.27b
0.84b 0.65b 0.60b 0.56b
0.90b 0.87b 0.83b
0.95b 0.94b
1.00b
a
0.23 0.09 0.29b 0.29b –0.02 –0.01 0.03 0.11 0.10 0.01 –0.03 –0.01 –0.03 –0.01
Significance of correlation coefficient: p < 0.05.
b
Significance of correlation coefficient: p < 0.01.
TABLE 4. Calculated Daily Exposure (ng day-1) to Total Cyclic Siloxanes (TCS), Total Linear Siloxanes (TLS), and Total Siloxanes via Ingestion of Indoor Dust by Adults and Toddlers in China living environment
working environment
total personal exposure via ingestiona
TCS
TLS
total siloxanes
TCS
TLS
total siloxanes
TCS
TLS
total siloxanes
0.84 3.13
7.89 57.2
8.37 30.4
0.61 1.63
7.31 28.9
7.53 58.1
1.45 4.79
15.2 86.1
15.9 88.5
3.29 12.3
30.9 224
32.8 228
3.58 11.9
38.0 215
40.3 221
13.2 49.0
124 896
131 910
b
average dust intake profile adults median high (95th percentile) toddlers
median high (95th percentile)
3.29 12.3
30.9 224
32.8 228
high dust intake profilec adults median high (95th percentile)
2.10 7.82
19.7 143
20.9 145
124 896
131 910
toddlers
median high (95th percentile)
13.2 49.0
1.48 4.08
18.3 72.4
19.4 76.0
a For adults, total exposure via dust)concentration in living environment × ingestion rate ×63.8%+concentration in working environment × ingestion rate ×22.3%; for toddlers, total exposure via dust ) concentration in living place × ingestion rate ×100%. b Average dust intake rate for adults is 20 mg d-1, for toddlers is 50 mg d-1. c High dust intake rate for adults is 50 mg d-1, for toddlers is 200 mg d-1
been applied for dust ingestion/inhalation exposure estimations in various countries (30-32). Generally, more than 85% of the time is spent in home and office for adults. Based on
the concentrations of siloxanes measured in dust from the living (home and dormitory) and working (office and laboratory) environments, we calculated the exposures to VOL. 44, NO. 16, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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siloxanes via dust ingestion. For adults, average and highexposure scenarios for dust ingestion rate are 20 and 50 mg per day, respectively (30); corresponding values for toddlers are 50 and 200 mg per day (33). The daily intake rate of siloxanes was determined for median and high-exposure concentrations. For the median exposure, the median concentration of total siloxanes in dust samples was used, whereas for high exposure group, the 95th percentile concentration value was used (for details see SI Table S4). At an average dust ingestion rate of 20 mg d-1 and a median concentration of total siloxanes (656 and 1690 ng g-1 for living and working environments, respectively), intake of total siloxanes through dust was calculated to be 15.9 ng day-1 for adults; the corresponding value for toddlers (50 mg d-1 ingestion and median total siloxane concentration of 656 ng g-1) was 32.8 ng d-1 (Table 4). Despite the high concentrations of siloxanes found in dust samples from offices and laboratories, contamination in homes and dormitories contributed the preponderance of the daily exposures. Dermal exposure is another important pathway for siloxanes exposure, since many personal care products that are applied directly to the skin contain these compounds. Exposure rates for siloxanes from the use of personal care products in the U.S. were previously estimated to be as high as 256 mg d-1 for TCS, and 50 mg d-1 for TLS (4). However, dust ingestion could be a relatively more important route of exposure to siloxanes for toddlers, since they spend a lot of time on the floor, and since this age group is expected not to have heavy use of consumer products on a daily basis. In summary, concentrations of siloxanes in indoor dust are reported here for the first time. Remarkably high rate of occurrences and concentrations of high-molecular-weight linear siloxanes (L9-L14) were found in indoor dust in China. Siloxane concentrations in dust were found to be correlated with the number of electrical/electronic appliances, the number of occupants, and whether smokers were living in the house. Based on an estimated average dust intake rate and on the median concentration found here, total siloxane intake from indoor dust ingestion was calculated to be 15.9 and 32.8 ng d-1, for adults and toddlers, respectively.
Acknowledgments We acknowledge all of the participants who provided dust samples and information related to the samples. The China Scholarship Council (CSC) provided a scholarship to the first author (Y.L.), to perform this study. The sampling of indoor dust was supported by a grant from The National Natural Science Foundation of China (No. 50608050). The Centers for Disease Control and Prevention (CDC), Atlanta, Georgia, provided funding for sample extraction and analysis portion of the study through a Biomonitoring grant (1U38EH00046401) to Wadsworth Center, New York State Department of Health.
Supporting Information Available Specific details related to concentrations of siloxanes in dust collected from electrical/electronic devices, statistics of individual siloxane concentration in indoor dust as well as exposure profiles of siloxanes concentration from dust ingestion are summarized in tables. This material is available free of charge via the Internet at http://pubs.acs.org.
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