The Environmental Protection Agency (EPA) proposes to retain the current National Ambient Air Quality Standards (NAAQS) for fine particulate matter (particles with a diameter of ≤2.5 μm [PM_2.5]) — that is, levels not exceeding an annual average of 12 μg per cubic meter and a 24-hour average of 35 μg per cubic meter.¹ The current NAAQS were set in 2012 on the basis of a scientific review that was largely completed in 2010.² At that time, available epidemiologic evidence, supported by toxicologic evidence and a risk assessment conducted by EPA staff, indicated that annual exposure to PM_2.5 caused premature death at ambient concentrations as low as 11 μg per cubic meter. However, on the basis of more recent evidence, as described below, exposure to ambient PM_2.5 at the levels of the current standards is estimated by the EPA to be responsible for tens of thousands of premature deaths in the United States each year.