Predicting indoor emissions of cyclic volatile methylsiloxanes from the

Transcription

1 Supporting Information Predicting indoor emissions of cyclic volatile methylsiloxanes from the use of personal care products by university students Tao Yang 1, Jianyin Xiong 1,2,*, Xiaochen Tang 3, Pawel K. Misztal 2,4 1 School of Mechanical Engineering, Beijing Institute of Technology, Beijing , China 2 Department of Environmental Science, Policy and Management, University of California, Berkeley, California 94720, United States 3 Indoor Environment Group, Energy Technologies Area, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, United States 4 Centre for Ecology & Hydrology, Edinburgh, Midlothian, EH26 0QB, UK * Corresponding author. Tel.: ; Fax: ; address: xiongjy@bit.edu.cn This supporting information (11 pages) includes 2 sections, 8 figures and 2 tables. S1

2 Section S1. Comparison between different models We compare the simulated results of D5 concentrations between the present model and two other simple models, i.e., the multi-pathway exposure model and the adsorption/desorption model. 1,3 The multi-pathway exposure model is applied to predict chemical emission from personal care products which considers mass transfer from the product on skin surface to air and into the stratum corneum. The adsorption/desorption model is applied to predict chemical emission from indoor materials and products, which neglects the internal diffusion process (i.e., chemical is assumed to be always evenly distributed in the skin lipids). In the multi-pathway exposure model, the mass transfer of D5 inside the skin lipids and in the indoor air can be described by the following two equations (equation (S1) is taken from the reference 1 ; equation (S2) is the mass balance equation indoors): dmc () t kp,a mc ( t) kp,s mc ( t) (S1) dt dc dt a V Q Cin Q Ca kp,a mc t + ( ) (S2) where, m c (t) is the remaining mass of D5 in the skin lipids at time t, µg; k p,a and k p,s are the product to air rate constant and the product to skin (dermal intake) rate constant, respectively, h -1, which can be calculated by the following equations: 1 k 1/ ( L / K L / ) (S3) aq p,s p w k =1/ ( L / L / ) p,a a w (S4) where, aq Kp is the skin permeation coefficient, m/h; φ w is the transfer coefficient through the aqueous film, m/h; φ a is the transfer coefficient from the aqueous film to the air, m/h; L is the thickness of the skin lipids, m. Base on formula in the reference 1 and D5 physical-chemical property data in EPI Suite, the parameters aq K p, φ w and φ a can be calculated, and the results are summarized in the following Table S1. This table indicates that the ratio of k p,a to k p,s for D5 is about 27, meaning that most of D5 are volatilized from the skin lipids surface. Therefore, dermal intake can be S2

3 neglected. Based on this analysis, the solution of equations (S1) and (S2) can be derived as: M k Q C ( t) C (exp( k t) exp( t)) Q k V V 0 p,a a in p,a p,a (S5) where, M 0 (= C 0 A L) is the initial mass of D5 inside the skin lipids, µg. For the adsorption/desorption model, by neglecting the internal diffusion process, the indoor gas phase D5 concentration from skin lipids can be represented as: 3 M A h LF C ( t) [exp( rt) exp( r t)] 0 m a 1 2 Ke ( r2 r1 ) (S6) where, r 1 and r 2 are the two roots of the following equation: 2 r hm Ke N hm L r Nhm Ke ( / ) / 0 (S7) where, K e (=K L) is the is the skin/air partition coefficient, m; LF (=A/V) is the loading factor, m 2 /m 3 ; N (= Q/V) is the air-exchange rate, h -1. Based on the determined C 0 and K, and the data in Table S1, the gas phase D5 concentration can be calculated by virtue of equations (S5) and (S6) for the multi-pathway exposure model (model 1) and the adsorption/desorption model (model 2). Figure S2 shows the calculated gas phase D5 concentrations for class session 1 on Nov 6 based on different models. This figure indicates that the present model agrees better with experimental data, which further demonstrates the effectiveness of the present model. When applying the adsorption/desorption model to calculate the D5 concentrations, if the parameter K e is not obtained from D5 physical property data (i.e., based on equation (16) and skin lipids thickness) but derived from nonlinear regression by fitting equation (S6) to experimental data (fitted K e =1.63 m), the prediction can be improved, as shown in Figure S2. However, the nonlinear regression technique might lead to multiple solution of K e as pointed out by Zhang et al. 4 In addition, the fitted K e cannot be obtained by independent experiments, thus cannot be extended to other environmental conditions. Compared with the adsorption/desorption model, the parameters in the present model have more distinct physical meanings, and can be measured through independent experiments (e.g., by performing independent S3

4 environmental chamber tests). Moreover, the key parameters determined here are the physical parameters of the skin lipids, which reflect the essence of the D5 mass transfer process, thus can be generalized to other indoor environment with different settings (e.g., varied room volume, air-exchange rate, occupants with similar usage habit of personal care products). Section S2. Analysis on the remaining amount of D5 on the skin The remaining amount of D5 on the skin over time can also be predicted using the emission model. Figure S6 gives the calculated results for class session 1. The model predicts that the remaining amount of D5 on the skin decreases from about 7000 µg/person (the first 3 min is excluded) to 1200 µg/person in 1.5 h. The sharp decrease in the D5 amount is due to the high D5 emission (or loss) rate in the initial short-term period. The rapid decay of D5 from skin lipids in the initial period has also been reported in prior studies. Gouin et al. 5 found that only 3.74% of D5 was left in the skin after 1 h from the use of personal care products in their tests, and that over 99.9% of D5 emitted from the skin lipids within 24 h. While in Tang et al. s test, 2 the D5 emissions only slowly declined over the course of the monitoring days. A possible explanation for these results is that some students may repeatedly apply personal care products ( times/day), which will increase the emission rate after the new application of D5 taking place throughout the day, as examined in the previous sections. S4

5 Figure S1. Inlet D5 concentrations for different class sessions on Nov 6. Figure S2. Comparison between different models for predicting D5 concentrations for class session 1 on Nov 6. S5

6 Figure S3. Impact of remaining students on the D5 emission characteristics. Figure S4. Impact of re-use of personal care products (PCPs) on the D5 emission characteristics (class session 3 on Nov 6). S6

7 Figure S5. Calculated D5 emission rate for class session 1 on Nov 6. Figure S6. Calculated remaining amount of D5 on the skin for class session 1 on Nov 6. S7

8 (a) D m (b) K S8

9 (c) h m Figure S7. Sensitivity analysis of some key parameters (a) D m, (b) K and (c) h m. Figure S8. Update of K by using the short-term emission data for class session 2 on Nov 6. S9

10 Table S1. Determined D5 transfer coefficient Parameters Unit Value aq K m/h p φ w m/h φ a m/h 40 kp,a kp,s L m/h L m/h Table S2. Calculated average emission rate for the first three class sessions on Nov 6 and Nov 13, and comparison with a prior study 2 Date Class sessions D5 emission rate (µg/(person. h)) Results calculated in this study Results in reference Relative deviation a Nov 6 Class session 1 (8:10-9:10) % Class session 2 (9:40-11:10) % Nov 13 Class session 1 (8:10-9:10) % Class session 2 (9:40-11:10) % a Relative deviation is calculated as: E 1 -E 2 /E 2 ; E 1 is the average emission rate calculated in this study; E 2 is the average emission rate in the reference. References (1) Ernstoff, A.S.; Fantke, P.; Csiszar, S.A.; Henderson, A.D.; Chung, S.; Jolliet, O. Multi-pathway exposure modeling of chemicals in cosmetics with application to shampoo. Environ. Int. 2016, 92-93, (2) Tang, X.; Misztal, P.K.; Nazaroff, W.W.; Goldstein, A.H. Siloxanes are the most abundant volatile organic compound emitted from engineering students in a classroom. Environ. Sci. Technol. Lett. 2015, 2, (3) Xiong, J.Y.;, Cao, J.P.; Zhang, Y.P. Early stage C-history method: Rapid and accurate S10

11 determination of the key SVOC emission or sorption parameters of indoor materials. Build. Environ. 2016, 95, (4) Zhang, J.S.; Zhang, J.S.; Chen, Q.Y.; Yang, X.D. A critical review on studies of volatile organic compound (VOC) sorption by building materials. ASHRAE Trans. 2002, 108, (5) Guoin, T.; van Egmond, R.; Sparham, C.; Hastie, C.; Chowdhury, N. Simulated use and wash-off release of decamethylcyclopentasiloxane used in anti-perspirants. Chemosphere 2013, 93, S11