A science advisory committee is challenging EPA's decision on tightening the air quality standard for particulate matter, saying the proposal falls far short of what is necessary to protect public health from microscopic particles produced by burning fuel in vehicles, power plants, and factories. During a Feb. 3 teleconference, Rogene Henderson, chair of the Clean Air Scientific Advisory Committee (CASAC), said the panel would draft a letter to EPA Administrator Stephen L. Johnson reiterating and clarifying its recommendation that the agency tighten the standard limiting long-term exposure to fine particles less than 2.5 ??m in diameter. Henderson, senior biochemist and toxicologist at the Lovelace Respiratory Research Institute, Albuquerque, N.M., acknowledged, "We are in uncharted waters here since this is the first time that CASAC has replied to an administrator's proposed ruling." In May 2005, the 22-member advisory committee recommended that EPA tighten the standards for both short-term and long-term exposure to fine particles. But on Dec. 20, the agency proposed lowering the standard for a 24-hour period from 65 µg/m3 to 35 µg/m3 while leaving the existing annual standard unchanged at 15 µg/m3. CASAC had recommended tightening the annual standard to either 13 or 14 µg/m3 and setting the 24-hour standard between 30 and 35 µg/m3. EPA is under a court order to issue a final particulate matter standard by Sept. 27.