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Air Quality

This web site provides information on FHWA's air quality programs, including transportation conformity, air toxics, and the Congestion Mitigation and Air Quality Improvement (CMAQ) program.

Transportation Conformity

Transportation conformity ("conformity") is a way to ensure that Federal funding and approval goes to those transportation activities that are consistent with air quality goals. Conformity applies to transportation plans, transportation improvement programs (TIPs), and projects funded or approved by the Federal Highway Administration (FHWA) or the Federal Transit Administration (FTA) in areas that do not meet or previously have not met air quality standards for ozone, carbon monoxide, particulate matter, or nitrogen dioxide. These areas are known as "nonattainment areas" or "maintenance areas," respectively. Regulations governing transportation conformity are found in Title 40 of the Code of Federal Regulations (40 CFR Parts 51 and 93).

Air Toxics

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 CMAQ program was created under the Intermodal Surface Transportation Efficiency Act (ISTEA) of 1991, and reauthorized under every successive Transportation Bill up to and including the Fixing America's Surface Transportation (FAST) Act in 2015. Administered by FHWA, the CMAQ program provides funding to areas that face the challenge of attaining or maintaining the National Ambient Air Quality Standards (NAAQS) for ozone, carbon monoxide, and/or particulate matter. In addition, States that have no nonattainment or maintenance areas - facing much less of a clean air challenge - still receive a minimum apportionment of CMAQ funding. FHWA released Interim Program Guidance under MAP-21 on November 12, 2013. While project eligibilities remain largely the same under FAST Act, the legislation places increased emphasis on diesel engine retrofits including construction equipment, port-related landside non-road or on- road equipment and alternative fuel infrastructure in designated alternative fuel corridors. See FHWA's FAST Act CMAQ website.

Updated: 8/7/2020
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