The U.S. Environmental Protection Agency (EPA) has proposed a rule that would set annual health-based standards for fine particulate matter (PM2.5) in a range between 12 micrograms per cubic meter of air (μg/m3) and 13 μg/m3. This would be lower than the current level of 15 μg/m3. The agency proposes to retain the existing 24-hour standard of 35 μg/m3. The proposal would update the national ambient air quality standards (NAAQS) for PM2.5, including soot, and would add a separate fine particle standard to improve visibility and retain existing secondary standards for PM2.5 and PM10. For coarse particles, the proposal would retain the existing 24-hour standard of 150 μg/m3.  

The Clean Air Act requires EPA to review the particle pollution standards every five years. The proposal is a result of that review and responds to several federal court decisions. EPA will hold public hearings on the proposed rule in July 2012 and accept comments for nine weeks after publication in the Federal Register.