127.12 Air Quality

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If a proposed transportation project is located in a Clean Air Act (CAA) nonattainment or maintenance area (i.e., the St. Louis area is nonattainment for ozone and particulate matter in Missouri) the project cannot create a new National Ambient Air Quality Standards (NAAQS) violation, increase the frequency or severity of existing violations, or delay attainment of the air standard. The CAA applies to all federal projects, including those classified as Categorical Exclusions (CEs) under the National Environmental Policy Act (NEPA). The CAA requires the Federal Highway Administration (FHWA) evaluate every regionally significant project or program in a nonattainment area to determine whether it could have a negative impact on air quality.

The NAAQS, often referred to as air quality standards, are federal standards, established through extensive scientific review, that set allowable concentrations and exposure limits for certain pollutants. One set of limits, the primary standards, protect health; another set of limits, the secondary standards, are intended to prevent environmental and property damage (i.e., damage to crops, vegetation, buildings). Air quality standards have been established for six criteria pollutants—ozone (or smog), carbon monoxide, particulate matter, nitrogen dioxide, lead, and sulfur dioxide. The U.S. Environmental Protection Agency (EPA) calls these pollutants criteria air pollutants because the agency has regulated them by first developing health-based criteria for setting permissible levels. A geographic area with monitored levels that meet or do better than the NAAQS primary standard for each of these pollutants is called an attainment area; areas that do not meet the primary standard for any one or more of these pollutants are called nonattainment areas. Transportation contributes to four of the six criteria pollutants: ozone, carbon monoxide, particulate matter, and nitrogen dioxide.

The CAA requires each state to develop a state implementation plan (SIP) that establishes air quality objectives and regulations that the state will follow: EPA must approve each state’s SIP and can take over enforcing the CAA in that state, if the SIP is not acceptable. Transportation conformity, as required by the CAA, ensures that federally funded or approved transportation plans, programs, and projects conform to the air quality objectives established in the SIP. The EPA develops transportation conformity regulations with the U.S. Department of Transportation's (DOT's) input and concurrence. The state DOT is responsible for implementing the conformity regulation in nonattainment and maintenance areas. EPA has a consultative role in the analysis and required findings.

Metropolitan Planning Organizations (MPOs) are responsible for modeling the regional air emissions for each proposed project. East-West Gateway Council of Governments (EWGCOG) is the MPO responsible for the St. Louis non-attainment area. If the projected motor vehicle emissions from the planned transportation system do not exceed the motor vehicle emissions budget established in the SIP, the MPO places the project on the Transportation Improvement Program (TIP) and the Missouri Highways and Transportation Commission (MHTC) incorporates the project in the Statewide Transportation Improvement Program (STIP), allowing it to go forward.

The conformity process is intended to ensure that the projects proposed in long-range transportation plans and TIPs conform to the SIP benchmarks against which progress is measured in meeting regional goals for cleaner air.

127.12.1 Laws and Regulations

  • The Clean Air Act Amendments of 1990 (Title I, Part D, subpart 1, Sec. 176 “Limitation on certain federal assistance”) contain the regulations for air quality in the planning and design of federally aided highways.
  • 42 U.S.C. §§ 7401 et seq., the Clean Air Act of 1970, is the comprehensive federal law regulating air emissions from area, stationary, and mobile sources and authorizing the EPA to establish the air quality standards to protect public health and the environment.
  • The National Environmental Policy Act (NEPA) of 1969 provides broad authority and responsibility for evaluating and mitigating adverse environmental effects, including highway traffic effects on air quality, and directs the federal government to use all practical means and measures to promote the general welfare and foster a healthy environment.

127.12.2 Process

When apprised of a proposed regionally significant highway project or program, MoDOT design environmental staff will determine whether the project is within the jurisdiction of an MPO that EPA has designated as nonattainment of maintenance for any component of the NAAQS. If the project is in such an MPO area and is included in both the TIP and STIP, all conformity regulations have been met. Whether a project meets the NAAQS is usually determined before a project proceeds through the project development milestones. If, however, it is found during the initial project screening stage that the project has not been modeled for conformity, the district will submit the project to the MPO for inclusion in the modeling for the regional air quality emission data. Air quality conformity must be demonstrated before NEPA is concluded for projects requiring an Environmental Assessment (EA) or Environmental Impact Statement (EIS).


If the project is not within an MPO area that is currently in nonattainment or maintenance status, as determine by EPA, or is an area outside the jurisdiction of an MPO, all transportation conformity requirements have been satisfied.

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