The Clean Air Act (CAA) requires each state to develop a state implementation plan (SIP), which establishes air quality objectives and regulations that the state will follow.  The Environmental Protection Agency (EPA) must approve each state’s SIP. 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 Federal Highway Administration input and concurrence. 

Federal standards have been established through extensive scientific review, that set allowable concentrations and exposure limits for certain pollutants.  Primary standards are to protect public health.  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

A geographic area with monitored levels that meet or do better than the 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.  MoDOT is responsible for implementing the conformity regulation for transportation actions in attainment and nonattainment areas (e.g., the St. Louis area is a nonattainment area for ozone and particulate matter in Missouri).