Interstate air pollution occurs when emissions from sources in one state travel across state lines and degrade air quality in neighboring states. Ozone, particulate matter, and other pollutants can travel hundreds of miles from their source. This creates a fundamental governance problem: downwind states bear the health costs of pollution they didn't create and can't control. The Clean Air Act addresses this through the Good Neighbor provision, which requires upwind states to prevent their emissions from significantly contributing to air quality problems downwind. When voluntary cooperation fails, EPA can impose federal requirements on upwind states — but this depends on EPA's willingness to enforce the law.