Matching Dispersion Model Hazardous Chemical Concentrations with Health Effects Needs in Projects Supporting Litigation

Atmospheric dispersion models are able to calculate high-resolution distributions, in space and time, of hazardous chemical concentrations for use in assessing health effects of accidental releases. Health effects experts are able to use the dispersion model outputs to assess the effects of exposure to the specific chemicals. For extremely hazardous chemicals such as chlorine or hydrogen fluoride, a few seconds of exposure to concentrations above 10,000 ppm can be deadly. On the other hand, for radiological chemicals, it is the dosage (accumulated over time of exposure) that is important, and that might require dispersion modeling over months. There are also formal regulatory definitions of needed averaging times. For example, Acute Exposure Guidelines Limits (AEGLs) are defined by the EPA for specific chemicals for averaging times from 10 minutes to 8 hours.

In addition, there are different spatial domain needs for calculating health effects. Are we interested only in a few specific persons or groups’ locations? Do we only need to know the maximum distance from the source where a certain concentration limit is exceeded? Is there interest in the integrated population dosage for, say, a specific city or county?

In the above comments, the concentration in the air is important, implying an inhalation hazard. Sometimes deposition to the ground or to skin is a more important hazard.

Clearly it is important to have an early discussion between the dispersion modelers and the health assessment experts. This is sometimes difficult because of the tendency in litigation projects to separate the experts as much as possible.

This paper will provide some examples, without getting into specific details.