United States Environmental Protection Agency Environmental Monitoring Systems Laboratory Las Vegas, NV 89193-3478 Research and Development EPA/600/SR-92/094 September 1992 EPA Project Summary Tests of Indoor Air Quality Sinks James Quackenboss, Janet Remmers, James McHugh and Karin Bauer Experiments were conducted in a room-size test chamber to determine the sink effects of selected materials on indoor air concentrations of p- dichlorobenzene (PDCB). These effects might alter pollutant behavior from that predicted using simple indoor air qual- ity models, by reducing the peak con- centrations during source usage and by increasing the ventilation rates needed to reduce pollutant concentra- tions. Four experimental conditions were tested: empty chamber (painted gypsum wallboard), carpeting only, car- peting and drapes, and carpeting plus a full-size bed with a comforter. Cham- ber temperature and relative humidity were controlled; atmospheric pressure was monitored. Air exchange rates were monitored using the tracer-gas decay method. Chamber air, sampled at six minute intervals, was analyzed using a gas chromatograph. Sink effects were estimated by comparing decay curves fit to the tracer gas (SF6) and the PDCB concentrations. The influence of sink effects on indoor air concentrations was illustrated by estimating the ventilation rates required to reduce PDCB concen- trations from 14 ppm to 2 ppm. In the absence of sink effects, the theoretical rate was 1.95 air changes per hour (ACH). With sinks, the rates predicted for each experimental condition were: 2.5 (empty test chamber), 3.21 (carpet only), 6.64 (carpet and drapes), and 3.75 ACH (carpet and bed). This Project Summary was developed by EPA's Environmental Monitoring Systems Laboratory, Las Vegas, NV, to announce key findings of the research project that is fully documented in a separate report of the same title (see Project Report ordering information at back). Introduction The Toxic Substances Control Act (TSCA) mandates the assessment of risks associated with the manufacture and use of new and existing chemicals. In 1985, the Interagency Committee on Indoor Air Quality called for developing an under- standing of human exposures to indoor air pollutants, the contributions of various energy conservation measures, and the impact of introducing new consumer prod- ucts and building materials. This effort re- quires assessing exposure to chemical re- leases from consumer products and build- ing materials in indoor environments un- der various conditions. Due to the number and variety of prod- ucts on the market, it would be extremely costly to rely exclusively on the monitoring of indoor air concentrations in order to determine all possible exposure implica- tions of these products. A preferable op- tion is the use of predictive indoor air quality models to estimate the air concen- trations or. personal exposures that can be expected under various conditions. One of the most commonly used pre- dictive mathematical models is the simple dilution model. In a well-mixed chamber with a constant air exchange rate (A), the air concentration of a nonreactive pollut- ant in air (C(), generated at a constant Printed on Recycled Paper ------- emission rate (G), can be predicted at time (t) by the following mathematical model1: C, - G(1 - e-*1) / (AV) (1) where C, - air concentration at time (t), G « emission rate, A - air exchange rate, V - exchange volume, and t - time. After the source is removed, the air concentration decays exponentially and is defined by the following equation: ff* (2) where C0 is the initial air concentration at t-0, the time at which the source is re- moved. The dilution mode! assumes that no pol- lutant is tost within the chamber from any mechanisms other than dilution (e.g., through chemical decomposition, chemi- cal reaction, or mass transfer between the gas or vapor phase and solid surfaces inside the chamber). These other mecha- nisms are often referred to as "sinks." The dilution model also assumes that the inlet air stream delivers "clean" air to the chamber at the same temperature and pressure as those of the outlet stream, and that there is complete mixing of the chamber air. The application of the dilution model to predict exposure to a pollutant in an in- door environment is complex if materials (such as ftoor and wall coverings, window drapes, and upholstered furniture) appre- ciably adsorb or absorb the pollutant. For purposes of this study, a "sink effect" is defined as the mass transfer of an air pollutant between its gas or vapor phase and any surfaces inside the chamber. The adsorption of the nonreactive pollutant to, and subsequent desorption from, nonreactive surfaces is assumed to be a reversible process. The objectives of this project were two- fold. First, experimental tests in a environ- mental chamber were performed to as- sess the sink effects of selected surfaces and building materials on the concentra- tions of an indoor air pollutant. Second, the importance of these sink effects to existing predictive mathematical models of indoor air pollutants was to be deter- mined. The project report presents the data for 12 experimental runs using one chemical, p-dichlorobenzene (PDCB). Re- sulls of 11 of these runs are presented in the body of the report. Due to problems in 1 Ootm.J.E. 1987. Models and Statistical Methods for Gaseous Emission Testing of Finite Sources in Well- mixed Chambers. Atmos. Environ. 21:425-430. run 11, all data and results pertaining to that run are considered suspect and are presented separately (in Appendix H of the project report). PDCB was selected for study because it is widely used in the indoor environment as an insecticide (moth repellant), a disin- fectant, or a deodorant (room freshener). It was also selected to minimize the sink effects from chemical reaction or degra- dation mechanisms, allowing the study to focus on adsorption and desorption mecha- nisms. Procedure Experiments were conducted in a room- size (1,261-ft3) test chamber at the Mid- west Research Institute's (MRI's) Air Con- sumer Exposure (ACE) Laboratory under stable environmental conditions. The test chamber simulates a residential room where household consumer products would be used. Interior surfaces were cov- ered with gypsum wallboard. The wall- board was sealed with ready-mix joint com- pound and painted with one coat of inte- rior latex primer and one coat of semi- gloss interior latex paint. Twelve experi- mental runs were conducted, three tests for each of the following configurations: empty test chamber, test chamber with carpeting, test chamber with carpeting and drapes, and test chamber with carpeting and a full-size bed covered with a com- forter. The carpet pile was made of 100% polyester fibers. The drapes were made of 72% rayon and 28% polyester. The mattress consisted of 45% cotton felt, 40% polyurethane foam, 15% polyester fibers on a wire spring unit. The box spring con- sisted of a wire spring unit covered with 100% polyester fabric. The mattress was covered with a comforter consisting of 50% cotton, 50% polyester fabric and filled with ,100,% polyester ,fjberfi|L._T,he,,carpeting, drapes, and bed were replaced after each experimental run. Real-time temperature, humidity, and barometric pressure measurements were collected during all experimental runs. Temperature and relative humidity were controlled; barometric pressure was moni- tored but not controlled. Air mixing was achieved using two commercially avail- able box fans. Air exchange rates within the test chamber were controlled for ap- proximately 1 exchange per hour, and monitored using the tracer gas decay method. Each experimental run began by simul- taneously releasing PDCB vapor and a tracer gas, sulfur hexafluoride (SF6) into the test chamber at constant emission rates. The flow of SF6 into the test cham- ber from a compressed gas cylinder was controlled by a fine-metering valve, and adjusted to maintain the air concentration inside the test chamber to below 500 ppb. PDCB vapor was generated in the test chamber by sublimation of pure PDCB crystals placed in a flat metal pan that was positioned at the air inlet. A continuous sample of test chamber air was collected at the outlet. After both compounds were maintained at a steady- state equilibrium for a 6-hr period, injec- tion was stopped. Air monitoring for PDCB and SF6, using a gas chromatograph (GC), began prior to the introduction of both compounds into the test chamber and was continued throughout the air concentra- tion buildup period, a 6-hr steady-state equilibrium period, and the period of de- cay (after removal of the sources)rThe air monitoring ended when concentrations of both compounds fell below their respec- tive detection limits. The air concentrations of SFe and PDCB measured at the time the sources were removed were the initial concentrations (C0) used in the decay model (Eq. 2) to approximate a decay curve which is rep- resentative of decay caused solely by dilution. The concentrations of SF6 and PDCB measured from the time the sources were removed were plotted against time, and curves were fitted to the data. The empirical decay curves for each chemical were compared to the theoretical decay curves predicted by Eq. 2. Given that Eq. 2 provided an adequate fit to the SF6 data, sink effects were indicated by an observed rate of decay for PDCB which deviated from that in Eq. 2. This is ex- plained by the fact that other sources than the PDCB concentration at time t = 0 (C0) are contributing to PDCB concentrations (Ct) in the test chamber air during the decay period. This would result in the .. measured air concentrations exceeding those predicted by Eq. 2. The most likely source is PDCB absorbed into surfaces during the experiment, which subsequently desorbs and reenters the air. No other source was identified. The data used in modeling the decay of SPe and PDCB concentrations over time were restricted to the data starting with the first data point obtained just prior to the withdrawal of the gas sources from the test chamber (see data in Appendix G). Further, the data were truncated at the end of each run. Data following the first occurrence when the SF6 concentra- tion fell below 5 ppb were discarded from the statistical analysis. A 1 -ppm cutoff for runs 1 through 6 and a 0.3-ppm cutoff for runs 7 through 12 were used for PDCB concentrations. These figures correspond to the limits of quantitation for the two compounds. ------- Data from each run were analyzed sepa- rately. A series of decay models were fitted to the SF6 and PDCB concentration data over time. These models included first- and second-order exponential decay models for both compounds. A segmented model, consisting of a second-order expo- nential decay curve followed by a first- order linear equation, was also fitted to the PDCB concentration. Each model was forced through C0, the concentration cal- culated at time t=0 when the source was removed. The equations considered take the general forms: First-order exponential decay: Cone = C0 exp(-At) (3) Second-order exponential decay: ' """ Cone = C0 exp(-Bf + Ct2) ' " ' (4) Segmented second-order exponential decay and first-order linear: Cone = C0 exp (~Dt + Et2) If t < T, the time of model transition, Cone = F + Gt otherwise , (5) where A, B, C, D, E, F, G, and T are model parameters (coefficients) to be esti- mated from the data. The coefficient A in Eq. 3, obtained for SF6, provides an esti- mate of the air exchange rate within the test chamber. To model Eqs. 3 or 4 above, the concentrations were first log-trans- formed and the regression parameters es- timated on the log-scale. To use a stan- dard linear regression analysis procedure, the response variable modeled versus time was [log(conc) - log(C0)], selecting the no-intercept model option. The regression coefficients B and C were directly ob- tained from that regression analysis. For each individual run, the sink effect was estimated as a function of time, the initial PDGB concentration at time t = 0 (C0), the air exchange rate (A) of SF6 during the same run, and the parameter estimates (B and C or D, E, F, G, and T) of the PDCB decay function. If Eq.4 above best described the PDCB concentration decay curve, then the sink effect, at time t, expressed in ppm, was estimated by the following difference: Estimated sink effect = C0 exp(-Bt + Ct2) - C0 exp(-At). If Eq. 5 best described the PDCB con- centration decay curve, then the sink ef- fect, at time t, was estimated as follows: Estimated sink effect = C0exp(-Dt+Et2)-C0exp(-At) if t < T, Estimated sink effect = F + Gt - C0 exp(-At) otherwise. Results and Discussion Sink effects were observed in all ex- perimental runs; that is, PDCB air concen- trations measured after the source was removed exceeded that predicted by the simple dilution model. The most likely source of excess PDCB was from the chemical's adsorption to the walls, drapes, carpeting, bed, and comforter during the experiments and subsequent desorption and reentry into the air. No other possible source was identified. The maximum differences between PDCB air concentrations predicted from the models fitted to the calculated con- centrations and those of the air concen- trations predicted from Eq. 2 are as fol- lows: Empty test chamber 0.9 ppm Test chamber with carpet 1.6 ppm Test chamber with carpet and drapes 7.9 ppm Test chamber with carpet and bed 2.6 ppm The PDCB decay models from each run were also used to predict the number of air exchanges needed to reduce PDCB air concentrations from a high to a low level. As an example, the models fitted to the PDCB data in the presence of sinks and the model described by Eq. 2 were used to calculate the number of air ex- changes per hour (ACH) required to re- duce the air concentration of PDCB from 14ppm to 2ppm (Tablel). Under the as- sumption of no sink effect (Eq. 2), it would require 1.95 ACH to reduce the PDCB concentration from 14 to 2ppm, regard- less of chamber configuration. (The num- ber 1.95 is obtained by solving the equa- tion, 2 = 14e'At, for At.) The average number of air exchanges necessary to reduce the PDCB air concentration within the empty test chamber from 14 to 2 ppm, as calculated from the decay models, is 2.50 ACH. It would require an average of 3.21 ACH for the test chamber with car- peting, 6.64 ACH for the test chamber with carpeting and drapes, and 3.75 ACH for the test chamber with carpeting and a bed. Summary and Conclusions The project report presents the data from experimental runs using a chemical, p-dichlorobenzene, in a room-size test chamber. Triplicate experimental runs were conducted in the test chamber using four configurations: empty test chamber; test chamber outfitted with wall-to-wall carpet; test chamber with carpet and drapes; and test chamber with carpet and a full-size bed with a comforter. Sulfur hexafluoride was used to deter- mine the air exchange rates of each run within the chamber and to demonstrate the actual decay of the chemical in the absence of sink effects under the environ- mental conditions set for each run. The empirical decay curve for SFe, obtained from fitting SF6 concentrations over time after the source of the gas was withdrawn from the chamber, coincided with the theo- retical decay curve predicted by the simple dilution model (Eq. 2). Sink effects were observed when the estimated rate of decay of PDCB deviated from the rate predicted'by Eq. 2, using the air exchange rate determined by SF6. This effect can be explained by the presence of sources other than C0, the concentra- tion of PDCB in the air at the beginning of the decay period, which are contributing to PDCB concentration (C() in the cham- ber during the decay period. This results in measured PDCB air concentrations that exceed the air concentrations predicted by Eq. 2 (without sink effects). The sink effect was found in all experimental runs. The most likely source was PDCB adsorbed to chamber surfaces during the experiments, with PDCB subsequently de- sorbing and reentering the air. The mag- nitude of the sink effect was influenced by furnishings added to the chamber. In the presence of sinks, a higher num- ber of air exchanges was required to re- duce PDCB air concentrations within the test chamber to background levels after the source of the contaminant was with- drawn. As an example, the models fitted to the PDCB data in the presence of sinks and the model described by Eq. 2 were used to calculate the number of air ex- changes required to reduce the air con- centration of PDCB from 14 ppm to 2 ppm. The first-order decay model (Eq. 2) would significantly underestimate the num- ber of air exchanges necessary to reduce air concentrations to background levels. As shown in this study, the sorption of PDCB onto chamber surfaces and its sub- sequent desorption was significant rela- tive to the PDCB levels in the room air. Therefore, sink effect terms should be in- corporated into predictive exposure mod- els. Additional modeling of the data base generated during this study should be un- dertaken. Fitting other models to these data, based on some type of equilibrium adsorption phenomenon or other physi- cally-based concept, should be attempted. •U.S. Government Printing Office: 1992— 646-080/60140 ------- Table 1. Number of Air Exchanges Needed to Reduce PDCB Concentrations from 14 to 2 ppm Run no. Time to 14 ppm (V Time to 2 ppm (h) Time difference (h) Number of air exchanges w/o sink effect Number of air exchanges w/sink effect Empty tost chamber 1 0.65 3.13 2 0.28 2.13 3 0.36 2.41 Test chamber with carpet 4 0.47 3.25 5 0.31 2.86 6 0.62 3.73 Test chamber with carpet and drapes 7 0.63 5.99 8 0.05 4.91 9 1.00 6.40 Test chamber with carpet and bed 10 0.62 4.07 12 0.26 2.67 2.48 1.85 2.06 2.78 2.55 3.11 5.36 4.87 5.40 3.45 2.41 1.95 1.95 1.95 Avg. = Std = CV(%) 1.95 1.95 1.95 Avg. = Std = CV(%) 1.95 1.95 1.95 Avg. = Std = CV(%) 1.95 1.95 Avg. = Std = CV(%) 2.51 2.53 2.45 2.50 0.04 1.53% 3.27 3.22 3.14 3.21 0.07 2,14% 6.84 6.03 7.05 6.64 0.54 8.13% 3.99 3.51 3.75 0.34 9.05% The information in this document has been funded wholly or in part by the United States Environmental Protection Agency under Contract 68-DO-0137 to Midwest Research Institute. It has been subjected to the Agency's peer and administrative review, and it has been approved for pub- lication as an EPA document. Mention of trade names or commercial products does not constitute endorsement or recommendation for use. The EPA authors are James Quakenboss, Environmental Monitoring and Systems Laboratory, Las Vegas, NV 89193-3478, and Janet Remmers, Office of Pollution Prevention and Toxics, Washington, DC 20460. James McHugh andKarin Bauer are With the Midwest Research Institute, Kansas City, MO 64110. Joseph V. Behar is the EPA Project Officer (see below). The complete report, entitled 'Tests of Indoor Air Quality Sinks," (Order No. PB92- 218346/AS; Cost: $43.00; subject to change) will be available only from: National Technical Information Service 5285 Port Royal Road Springfield, VA 22161 Telephone: 703-487-4650 , , ............ ...,.,.„.... 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