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. 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 the Industrial Source Complex Model (Short-term), Version 3 (ISCST3), 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.
