United States Environmental Protection Agency Environmental Monitoring Systems Laboratory Research Triangle Park NC 27711 Research and Development EPA/600/S4-85/036 Apr. 1987 Project Summary A Sensitivity Analysis of the Enhanced Simulation of Human Air Pollution Exposure (SHAPE) Model Jacob Thomas, David Mage, Lance Wallace, and Wayne Ott A sensitivity analysis was undertaken of the Simulation of Human Air Pollution Exposure (SHAPE) model, which in- corporates an enhanced version of the Coburn-Forster-Kane (CFK) physiologi- cal model for predicting a person's blood carboxyhemoglobin (COHb) as a func- tion of time in response to carbon monoxide (CO) exposure. The SHAPE model simulates the physical activities of a sample of people in an urban area, exposing them to pollutant concentra- tions from appropriate microenviron- ments as they move through time and space in a 24-hour period. The CFK model dynamically calculates their blood COHb from their CO exposures, and their ventilation rate which is altered in response to their level of activity. To conduct this sensitivity analysis, the SHAPE program was run many times using different combinations of values for its parameters, thus allowing the contribution to COHb of each of many variables to be examined. The following phenomena were found to have signifi- cant effect, over and above the influence of the ambient CO on the predicted frequency distribution of the maximum COHb levels of the population: (1) CO exposure contributed by the highway microenvironment; (2) altitude of the city; (3) the CFK physiological param- eters (e.g., Haldane constant and en- dogenous CO production). In contrast, if it is assumed that a person using an indoor parking garage spends less than 10 minutes there, then use of a parking garage has very little effect on the COHb frequency distribution of the population. For low-level CO exposure, use of either the linear or nonlinear form of the CFK model yields essentially the same results. This Prelect Summary was developed by ERA'S Environmental Monitoring Systems Laboratory, Research Triangle Park, NC, to announce key findings of the research project that Is fully docu- mented In a separate report of the same title (see Project Report ordering In- formation at back). Introduction The Simulation of Human Air Pollution Exposure (SHAPE) model estimates human exposure to carbon monoxide (CO). It develops an "activity pattern profile" for an individual by allocating each minute of the day into any of 14 microenviron- ments (home, car, office, parking garage, etc.). By knowing where the person is and approximately what he or she is doing, another part of the program "exposes" the person to CO concentrations appro- priate for that microenvironment. Random number generators provide minute-by- minute concentration levels roughly equivalent to those found in the actual microenvironment, based on empirical field studies. For each person, these sequential microenvironmental exposures are brought together to form an "exposure profile" providing CO concentration and estimates of the carboxyhemoglobin (COHb) of the blood as functions of time over the 24-hour period. The previous version of the SHAPE program used a simplified Coburn CO- COHb model, a first-order linear differ- ------- ential equation with three constant coef- ficients. This formulation did not permit. physiological and other subcomponent parameters (for example, blood volume, ventilation rate, altitude) to be varied by themselves, because the latter param- eters were lumped together into the three constants. The research summarized in the full report covers enhancements to the SHAPE model and a sensitivity analysis of the enhanced model. The goal of this analysis is to determine how changes in a variety of fundamental parameters (highway CO concentrations, CO concentrations inside a parking ga- rage, time patterns of ambient CO con- centrations, and various Coburn physio- logical parameters) affect the following three dependent variables for the population: • Maximum 1 -hour average exposure (M1AE) • Maximum moving 8-hour average exposure (MM8AE) • Maximum 1 -hour blood carboxy- hemoglobin(MICOHb) In each case, we are interested in the population's frequency distributions of these variables and the impact of a change in each parameter on these frequency distributions. In general, we examined changes in frequency distributions that result while one parameter is allowed to vary and all the others are held constant. In some cases, two parameters were varied at the same time. Our simulation sample consisted of 400 non-smoking people employed in a large urban area. Their simulated com- mute habits are based on data from the U.S. Census Bureau's "Nationwide Per- sonal Transportation Study", (1973). Modifications to SHAPE The following enhancements have been made to the SHAPE computer program: • All individual Coburn parameters appear explicitly. • Both linear and the more accurate nonlinear versions of the Coburn model can be used to compute blood COHb. • Once the age and sex of the person have been established, height is determined by random sampling from distributions based on data from the National Health and Nutrition Ex- amination Survey (HANES) con- ducted by the National Center for Health Statistics. • Weight is determined from empirical height-weight equations, also based on sex and age. • Blood volume is determined from height and weight, and hemoglobin content is randomly sampled from the HANES data, based on sex. • Some of the person's lung functions (alveolar ventilation and diffusion rates, for example) are allowed to vary throughout the day and are determined from the activities he or she performs, based on metabolic rate assumptions for each physical activity level. • For each person, the program runs through an extra 24-hour exposure sequence on the previous day in order to set the initial conditions for the day of interest. Once these enhancements were made, the program was run many times with different combinations of values. Ambient data for three different dates were used, each with relatively high CO levels se- lected from an earlier sensitivity analysis. In this study, only subsamples of an urban population were simulated for their exposure experience to CO. The sub- sample consisted of nonsmoking em- ployed commuters during a week day. As such, the distributions generated of maxi- mum 1 hour CO exposure, maximum 8 hour exposure and maximum 1 hour COHb are seen to have small standard deviations and less than 10% coefficients of variation. Conclusions The findings of the study are among the following: • Changes in CO levels inside auto- mobiles exert a statistically signifi- cant effect on computed blood COHb of the population. • Altitude exerts a statistically signifi- cant effect on computed blood COHb. • Changes in the levels of CO in indoor parking garages have little effect on total CO exposure and blood COHb of the population due to the brief time (10 minutes) assumed spent there. • For low-level CO exposure, the linear approximation of the solution to the Coburn equation does not differ significantly from the nonlinear solution. This sensitivity analysis has identified the relative significance of a variety of parameters in generating estimates of blood COHb and has provided insight into the manner in which the parameters affect person-to-person variability of blood COHb. It also has identified the likely contribution of various human activities — driving a car, parking in a garage — to population exposure distributions and human dose distributions. Jacob Thomas is with General Software Corporation. Landover, MD 20785; the EPA authors,David Mage (also the EPA Project Officer, see below) and Lanco Wallace, are with Environmental Monitoring Systems Laboratory, Research Triangle Park, NC 27711; and the EPA author, Wayne Ott is with Office of Acid Deposition, Environmental Monitoring and Quality Assurance. Washington, DC 20460. Tho complete report, entitled "A Sensitivity Analysis of the Enhanced Simulation Lf Human Air Pollution Exposure (SHAPE) Model ." 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