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Risk Assessment and Modeling -
Human Exposure Model (HEM)
Risk Assessment and Modeling
- General Information. The Human Exposure Model (HEM) is used primarily for performing risk assessments for major point
sources (usually producers or large users of specified chemicals) of air toxics. The HEM only addresses the inhalation
pathway of exposure, and is designed to predict risks associated with emitted chemicals in the ambient air (i.e., in the
vicinity of an emitting facility but beyond the facility's property boundary). The HEM provides ambient air concentrations,
as surrogates for lifetime exposure, for use with unit risk estimates and inhalation reference concentrations to produce
estimates of cancer risk and noncancer hazard, respectively, for the air toxics modeled.
The HEM contains (1) an atmospheric dispersion model, the Industrial Source Complex Model , with included meteorological data: and (2) U.S. Bureau of Census population data at the Census block level. The model utilizes 2000 Census data (Note: the model and it's datasets have been populated for the 50 states, DC, PR, and the VI only). Each source in HEM must be specifically located by latitude and longitude, and its release parameters must be described. These include stack height, exit velocity, emission rate, etc. Based on the inputs for source parameters and the meteorological data, the model estimates the magnitude and distribution of ambient air concentrations in the vicinity of each source. The model is generally used to estimate these concentrations within a radial distance of 50 kilometers (30.8 miles) from the source. Exposure estimates generated by HEM are the ambient air concentrations predicted by the model, in micrograms per cubic meter. These exposure estimates are actually surrogates, as important exposure variables (e.g., duration, human activity patterns, residential occupancy period, etc.) are not explicitly addressed.
The HEM is available in two versions: HEM-Screen and HEM-3.
HEM-Screen can generate chronic cancer risk and hazard estimates for multiple facilities nationwide in one run. This model uses a simplified version of the Industrial Source Complex Model (Long-term), Version 2 (ISCLT2), dispersion model. Several simplifications and assumptions are built in to HEM-Screen, and user-supplied data requirements are relatively low. For these reasons, HEM-Screen may be more appropriate for lower-tier or screening-level assessments involving a large number of facilities.
HEM-3 generates chronic cancer risk and chronic and acute hazard estimates for one facility at a time. This model uses either the Industrial Source Complex Model (Short-term), Version 3 (ISCST3), or the AERMOD dispersion model. Data requirements are somewhat higher for HEM-3 compared to HEM-Screen; however, the results are typically more refined because ISCST3 provides several additional dispersion modeling options.
- Download Model
- User's Guide Those planning to use HEM are encouraged to carefully review the User's Guides. They describe technical
information about the models and the steps involved in running them.
HEM-Screen User's Guide (PDF) (1.1 MB) HEM-3 User's Guide (PDF) (379 KB) - Peer Review and Publications. The ISC model has undergone review and evaluation as part of the regulatory models process.
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Appendix A (labeled Appendix W in CFR) of Guideline on
Air Quality Models provides a summary description of the ISC model.
The SCRAM web site provides documentation of ISC, version 2, as well
as the current version 3 of ISC.
- Other Supporting Documents.