The Environmental Protection Agency has set National Ambient Air Quality Standards for particulates. One, in effect since 1987, regulates particles smaller than 10 microns in diameter (PM10); the other, promulgated in 1997, would regulate particles smaller than 2.5 microns in diameter (PM2.5) — but court challenges are delaying its implementation. These are of interest to agriculture because dust from tillage and smoke from burning field residues may contribute to pollutant levels. Whether controls might be imposed on agricultural activities depends largely on how each state chooses to meet the standards, however.