Interstate pollution is air pollution that comes from an upwind source that travels across a state's border and affects air quality in downwind states. It's a common problem in many parts of the country, and it's a challenge that the federal government must address.
The Clean Air Act's "good neighbor" provision requires EPA and states to address interstate pollution that affects downwind states' ability to attain and maintain National Ambient Air Quality Standards (NAAQS). In order to meet this requirement, a state must address upwind emissions in its State Implementation Plan.
To evaluate whether a State's upwind emissions significantly contribute to nonattainment or interfere with maintenance of the NAAQS, EPA has developed a 4-step interstate transport framework under the Clean Air Act. Under this approach, a State's upwind emissions are evaluated in light of multiple factors, including air quality and cost considerations.
Step 1 of the 4-step framework uses a screening metric to evaluate whether emissions from upwind sources will significantly contribute to nonattainment or interfere significantly with maintenance of the NAAQS at downwind receptors. For ozone, this screening metric is 1 percent of the NAAQS, as required by EPA's CSAPR Update and corresponding memoranda.
In evaluating the state's contribution to ozone nonattainment and maintenance under CAA section 110(a)(2)(D)(i)(I), it is essential that a consistent framework be applied. The EPA's selection of 1 percent is a consistent metric to identify those upwind states that should have responsibility for addressing the downwind nonattainment and maintenance problems to which they collectively contribute.
