United States Environmental Protection Agency Air and Energy Engineering Research Laboratory Research Triangle Park NC 27711 Research and Development EPA/600/S8-91/013 July 1991 v>EPA Project Summary EXPOSURE Version 2, A Computer Model for Analyzing the Effects of Indoor Air Pollutant Sources on Individual Exposure Leslie E. Sparks A model for calculating individual ex- posure to indoor air pollutants from sources is presented. The model is de- signed to calculate exposure due to indi- vidual, as opposed to population, activity patterns and source use. The model uses data on source emissions, room-to-room airflows, air exchange with the outdoors, and indoor sinks to predict concentra- tion/time profiles for all rooms. The con- centration/time profiles are then combined with individual activity pat- terns to estimate exposure. The model allows analysis of the effects of air clean- ers located in either/or both the central air circulating system or individual rooms on indoor air quality (IAQ) and exposure. The model allows simulation of a wide range of sources including long term steady state sources, on/off sources, and decaying sources. Several sources are allowed in each room. The model allows the analysis of the effects of sinks and sink re-emissions on IAQ. The results of test house experiments are compared with model predictions. The agreement between predicted concentration/time profiles and experimental data is good. The average deviation of the predicted value from the experimental value is less than 30% for all experiments, and the maximum deviation is less than 60%. This Project Summary was developed by EPA's Air and Energy Engineering Re- search Laboratory, Research Triangle Park, NC, 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 An indoor air quality (IAQ) computer model, INDOOR, was developed to use emission characteristics of sources to pre- dict in-room pollutant concentrations. INDOOR's predictions for several sources have been verified by experiments con- ducted in an IAQ test house, and emission characteristics have been determined using small environmental test chambers. INDOOR, however, does not allow calcu- lation of individual exposure due to a given source and personal activity pattern. The ability to estimate individual exposure is nec- essary before guidance on exposure reduc- tion can be given. Therefore, a new model, EXPOSURE, that allows analysis of expo- sure due to indoor sources (given pollutant source characteristics and individual activity patterns) was developed. General Mathematical Framework EXPOSURE is a multiroom model based on INDOOR. EXPOSURE allows calcula- tion of pollutant concentrations based on source emission rates, room-to-room air movement, air exchange with the outdoors, and indoor sink behavior. Printed on Recycled Paper ------- Each room is considered to be well mixed. The validity of the well mixed assumption was verified in several experiments in the EPA IAQ test house and by other data. A mass balance for each room gives: dCj Vj - CJIN QJIN - QOUT QJOUT + Sj - RJ 0) where Vj is the volume of the room, Cj is the concentration of room i, CHN is the concen- tration entering the room, QMN is the air flow into the room, CJOUT is the concentration leaving the room, QJOUT is the air flow leaving the room, Si is the source term, and RJ is the removal term. RJ includes pollutant removal by air cleaners and sinks. The well mixed assumption requires that CJOUT equal Cj. Equation (1) can be rewrit- ten as: *r *-N dCj ^ Vj 77- Ca Qa,i + dt -i- H.i-' j-N 2 (CjQjj) + Sj - RJ H,l-i - CjQi,a - (2) where Ca is the concentration outdoors, Qa,i is the air flow from the outdoors into room i, Cj is the concentration in room j, QJ,J is the air flow from room j into room i, Qj,ais the airflow from room i to the outdoors, and QJ,J is the air flow from room i into room j. Equation (2) is one of a set of identical equations that must be solved simultaneously in a multiple room model. EXPOSURE uses afastdiscrete time step algorithm to solve the series of equations. The algorithm is based on the assumption that, for sufficiently small time steps, dt, the source and sink terms and all neighboring concentrations are constant. The exact so- lution to equation (2) can then be used to calculate the concentration at the end of the time step. The exact solution is: Lit P: Ci-Ci(to)e- ^~\ Li (3) where Cj(to) is the concentration at time to, t is some time greater than to, U is: N Qi,a + Qi,h + J Qi,j Vj and Pj is given by: Qa,jCa (5) where Qj,h is the air flow from room i into the HVAC system, Qh.i is the air flow from the HVAC into room i, Cj(t) is the concentration in room j at time t, and Ch is the concentration in the HVAC system. Equation (3) is stable for all time steps and is accurate for sufficiently small time steps. (The size of the time step depends on how rapidly concentrations are changing. In gen- eral a time step of 1 minute is small enough for situations when concentrations are changing rapidly, and time steps of several minutes are adequate when concentrations are near steady state. The model automat- ically adjusts the time step to maintain a balance between speed and accuracy.) Source Terms The model incorporates a wide range of emission characteristics to allow simulation of the range of sources encountered in in- door spaces. Several sources are allowed in each room The model includes a data base of source emission rates for these various sources based on research conducted by the Indoor Air Branch, Air and Energy Engineering Re- search Laboratory, of EPA. The user can add to the data base and can override the data base emission rates. Sink Terms Research in the EPA test house and small chamber laboratory has shown that sinks (i.e., surfaces that remove pollutants from the indoor air) play a major role in determin- ing indoor pollutant concentrations. These sinks may be reversible or irreversible. A reversible sink re-emits the material col- lected in it, and an irreversible sink does not. Sink behavior depends on the pollutant and the nature of the sink. Considerable re- search is necessary to define the behavior of sinks. The sink model used in EXPOSURE is: Rs (6) where Rs is the rate to the sink (mass per unit time), ka is the sink rate constant (length per time), C is the in-room pollutant concen- tration (mass per length cubed), Asink is the area of the sink (length squared), kd is the re-emission or desorption rate constant, Ms is the mass collected in the sink per unit area (mass per length squared), and n is some power between 1 and 2. For typical gaseous organic pollutants of interest in indoor air: ka ranges from about 0.1 to 0.5 m/h; kd ranges from about 0 003 to 0.009; and n ranges from 1 to 1.5. Experiments are under way to provide estimates of ka and kd for a range of pollutants and sink materials. Sinks reduce the peak exposure to pollu- tants slightly but increase the cumulative exposure because of re-emissions. Exposure Because the most common route for ex- posure to indoor air pollutants is via inhala- tion, it is convenient to define inhalation exposure, Ej as: Ej = C(t)bv (7) where C is the pollutant concentration, b is the breathing rate, and v is the volume per breath. The exposure defined by equation (7) is instantaneous; i.e., the exposure at any instant in time, t. The peak exposure is the maximum of the instantaneous exposure versus time curve. The cumulative inhala- tion exposure, EJC, is given by: C(t)bvdt (8) The advantage of defining inhalation ex- posure is that the exposures calculated by the computer can be used without requiring the user to manually calculate the amount breathed. For exposure by mechanisms other than inhalation, the instantaneous exposure, E, to a pollutant at time t is the concentration, C(t), the person is exposed at time t: E = C(t) (9) The cumulative exposure from t1 to \2 is given by: -/ Ml C(t)dt (10) Calculation of exposure requires the pol- lutant concentration, the time exposed to the concentration, and (for inhalation exposure) the breathing rate and the volume per breath. The time exposed to the concentra- tion depends on the individual activity pat- tern. An activity pattern, in the context of the model, is defined by providing the time a person enters and leaves the various rooms of the building, or leaves the building for the outdoors. The model allows up to 10 room changes per day. The model is based on a 24-hour day. The activity patterns (and most source usage patterns) in the model repeat from day to day. The model provides instantaneous expo- sure time plots and cumulative exposure time plots for individual activity patterns. The instantaneous exposure allows identifi- cation of high exposure situations and of the peak exposure. While the model was designed to allow assessment of the impact of indoor air pollu- tion sources and sinks and IAQ control op- tions on individual exposure from specific activities, it can also be used to help estimate population exposures if data on population activity patterns are available. The model can be run for each activity pattern and then ------- the results can be weighted according to the population statistics. Model Verification The model predictions of concentration versus time have been compared to experi- mental data from the EPA IAQ test house. In all cases the agreement between predic- tions and experiment has been excellent. An example from a recent experiment is shown in Figure 1. Exposure Predictions The examples below demonstrate some of the model's capabilities. The first exam- ple is calculation of the exposure to an aero- woo 100 10 . 0.1 6 sq m of floor stained with 311 g Sink term ka = 0.09 m/hr, kd = 0.003 Source term Ro = 17,000 mg/rrf-hr, k = 0.4/hr Air exchange with outdoors = 0.3 ACH Data from EPA IAQ test house Background total hydrocarbon = 0.3 mg/cu m 0 50 100 150 200 250 300 350 400 450 Time (hrs) Figure 1. Total hydrocarbon concentration in living room from wood stain. 10 :r 1 : 0.1 : 0.01- 0.001 Person uses product, leaves after 1 hr, and returns at 10 hrs Inhalation rate 1200 breaths/hr Inhalation volume 0.6 I/breath Person, not using product, stays in house 24 hrs Aerosol release 280 mg VOC in 2 sec Air exchange with outdoors 0.25 ACH All factors based on test house data sol spray product. The activity patterns are for a person who (1) uses the product in a bathroom for 10 minutes, moves to the living room, and then leaves the building after 1 hour; and (2) stays in the building for 24 hours. The instantaneous and cumulative inhalation exposures for the two individuals are given in Figures 2 and 3. Note that, while the initial instantaneous exposure for the person using the product is much higher than for the other person, the cumulative exposure for the person using the product is less. However, the exposure for the person using the product is probably underesti- mated irt this example. The local concentra- tion near the person is somewhat higher for several minutes than the average room con- centration. EXPOSURE can deal with this situation if a pseudo room with a volume of about 5 m3 and an air exchange of 30 m3/h with the rest of the room is defined. For Figure 2, the difference in exposures is not great because the bathroom is relatively small (20 m^. The second example shows the exposure due to wood stain, a "typical wet source." Because of adsorption and re-emission from sinks, the exposure lasts for a considerable time. The cumulative exposures for a per- son spending 24 hours in the building and for a person spending 16 hours in the building (starting 8 hours after the stain is applied) are shown in Figure 4 both with and without a sink. Note the major effect of the sink on the exposure of the person spending part time in the building. The two examples model experiments conducted in the EPA test house. All model input is based on the conditions in the test house at the time of the experiments. The model predictions of concentration versus time for both examples are in excellent agreement with the test house data. Conclusions EXPOSURE is a tool to estimate individual exposure to air pollution from a wide range of indoor sources. The model allows analy- sis of a wide range of activity scenarios in a single model calculation. The effect of miti- gation options on individual exposure can be evaluated by simulating the mitigation option in the model. 10 15 20 25 Time (hrs) Figure 2. Instantaneous inhalation exposure to VOC from aerosol spray product. ------- 2.5 c -§ 4! 7.5- 1 0.5- Person, not using product, stays in house 24 hrs Person uses product, leaves after 1 hr, and returns at 10 hrs Air exchange with outdoors 0.25 ACH Aerosol spray releases 280 mg of VOC in 2 sec Inhalation rate 1200 breaths/hr inhalation volume 0.6 I/breath 0 5 10 15 20 Time (hrs) Figure 3. Cumulative inhalation exposure to VOC from use of aerosol product. 25 4000 3500- 3000- 24 hr/day in building with no sink 6 sq m of floor stained with 311 g Long term emission factor: Ro = 17,000 mg/sq m, k = 0.4/hr Initial emission 3,200 mg/sq m Air exchange with outdoors 0.3 ACH 24 hr/day in building with sink 16 hr/day in building with no sink 16 hr/day in building with sink 1 I I I -S o 0 50 100 150 200 250 300 350 400 450 Time (hrs) Figure 4. Cumulative exposure to VOC emissions from wood stain. &U.S. GOVERNMENT PRINTING OFFICE: 1991 - 548-028/40044 ------- ------- ------- ------- The EPA author, Leslie E. Sparks (also the EPA Project Officer, see below), is with Air and Energy Engineering Research Laboratory, Research Triangle Park, NC 27711. The complete report, entitled "EXPOSURE Version 2, A Computer Model for Analyzing the Effects of Indoor Air Pollutant Sources on Individual Exposure:" Paper Copy (Order No. PB 91-201 095; Cost $23.00, subject to change) Diskette (Order No. PB 91-507 764; Cost $130.00, subject to change) (Cost of diskette includes paper copy) The above items will be available only from: National Technical Information Service 5285 Port Royal Road Springfield, VA 22161 Telephone: 703-487-4650 The EPA Project Officer can be contacted at: Air and Energy Engineering Research Laboratory U.S. Environmental Protection Agency Research Triangle Park, NC 27711 United States Environmental Protection Agency Center for Environmental Research Information Cincinnati, OH 45268 BULK RATE POSTAGE & FEES PAID EPA PERMIT NO. G-35 Official Business Penalty for Private Use $300 EPA/600/S8-91/013 ------- |