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HAPEM 4 - Hazardous Air Pollutant Exposure Model

Information provided for informational purposes onlyNote: EPA no longer updates this information, but it may be useful as a reference or resource.

To develop nationwide, screening-level estimates of long-term inhalation exposures to air toxics, EPA is using the Hazardous Air Pollutant Exposure Model, version 4 (HAPEM4). The model bases its estimates on ambient concentrations output from the ASPEN model for the 33 air toxics and diesel PM. As with the ASPEN modeling, the scope of this national modeling effort is the contiguous United States (i.e., excluding Alaska and Hawaii for this initial assessment), Puerto Rico and the Virgin Islands.

Following widely used methodologies for exposure modeling, HAPEM4 can estimate toxic air pollutant inhalation exposures by examining the "activity patterns" of population groups in various microenvironments, and by simulating their movements between home and work locations. "Activity pattern" data in an inhalation exposure assessment, are important in that they depict both the actual physical activity (and an associated inhalation exertion level) as well as the physical location and time of the day in which the activity takes place (e.g., sleeping at home at midnight, jogging in the park at 8:00 am, driving in a car at 5:00 pm). The HAPEM4 model extracts activity data from the Comprehensive Human Activity Database (CHAD), an extensive database developed by EPA's Office of Research and Development. The CHAD activity data, which represent actual daily diary summaries for over 22,000 people nationwide, are coded by age, gender, and race. Thus, the HAPEM4 model can be used to develop exposure estimates for the general population as well as for specific demographic subgroups. Microenvironments (MEs) represent specific indoor and outdoor locations where people are expected to spend their time (e.g., home, work, car, train, restaurants, backyard, park). The toxic air pollutant concentrations in these various MEs are estimated by applying predetermined, pollutant specific, concentration relationships (ME factors) to ambient air quality levels. ME factors for HAPEM4 are currently being compiled for the 33 air toxics and diesel PM based on available data. This exposure model estimates annual average inhalation exposure concentrations at the census tract level, but the results will only be presented down to the county level.

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