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Transportation & Toxic Air Pollutants

Pollutants are generated by a wide variety of sources and enter the air, water, and soil through different media. Toxic air pollutants-also known as Hazardous Air Pollutants or HAPs-are those that are known to cause or suspected of causing cancer or other serious health ailments.

The Clean Air Act Amendments of 1990 listed 188 HAPs and addressed the need to control toxic emissions from transportation. In 2001, EPA issued its first Mobile Source Air Toxics Rule, which identified 21 mobile source air toxic (MSAT) compounds as being hazardous air pollutants that required regulation. A subset of six of these MSAT compounds were identified as having the greatest influence on health and included benzene, 1,3-butadiene, formaldehyde, acrolein, acetaldehyde, and diesel particulate matter (DPM). More recently, EPA issued a second MSAT Rule in February 2007, which generally supported the findings in the first rule and provided additional recommendations of compounds having the greatest impact on health. The rule also identified several engine emission certification standards that must be implemented. Unlike the criteria pollutants, toxics do not have National Ambient Air Quality Standards (NAAQS) making evaluation of their impacts more subjective.

To address stakeholders concerns and requests for MSAT analysis during project development and alternative analysis, FHWA developed the Interim Guidance on Air Toxic Analysis in NEPA Documents. The guidance provides a tiered approach for analyzing MSAT in NEPA documents. Depending on the specific project circumstances, FHWA has identified three levels of analysis: No analysis for projects with no potential for meaningful MSAT effects; Qualitative analysis for projects with low potential MSAT effects; or Quantitative analysis to differentiate alternatives for projects with higher potential MSAT effects.

The FHWA's ongoing work in air toxics includes a research program to better understand and quantify the contribution of mobile sources to air emissions, the establishment of policies for addressing mobile source emissions in environmental reports, and the assessment of scientific literature on health impacts associated with motor vehicle emissions.

Transportation agencies are interested in knowing the emissions burden associated with mobile sources. This understanding is a primary goal of FHWA's research. Critical components of our work are development and evaluation of methods to analyze and forecast future emissions from facilities and the fleets that are useful for decision makers and stakeholders to discern differences between transportation alternatives during the environmental documentation phase of project delivery.

Updated: 1/30/2023
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