Recommended Practices for NEPA Air Toxics Analysis of Highway Projects

The Clean Air Act Amendments of 1990 designated 188 air toxic pollutants as Hazardous Air Pollutants (HAPs). Certain of these HAPs are characteristic of motor vehicles and have become known as mobile source air toxics (MSAT). Public concern about evaluating MSATs in the National Environmental Policy Act (NEPA) process has prompted transportation agencies to begin addressing air toxic emissions as part of their environmental review procedures. This study developed best-practices guidance for MSAT assessments under NEPA. Current approaches used by transportation agencies to assess and communicate MSAT emissions and health impacts were compiled and reviewed. A tiered methodology was developed which guides the analyst in identifying the appropriate level of analysis for a transportation project, using typically available information and potential level of exposure based on project characteristics. Five potential levels of analysis are suggested based on both technical and policy considerations. Details are presented on how to conduct the MSAT assessment as well as on the amount of information that should be included at each level of analysis. Recommendations are provided on how best to communicate the findings of the MSAT assessment as part of environmental documents and in public review.