23 December 2008

The US Environmental Protection Agency (EPA) has notified 25 governors and 23 tribal leaders that certain areas in their states and tribal lands do not meet the agency’s daily standards for fine particle pollution, known as PM2.5 or particles with diameters below 2.5 microns.

211 counties and parts of counties have been designated as PM2.5 nonattainment areas, which do not meet EPA’s standards for fine particle pollution. Affected states and tribes will be required to take steps to reduce the pollution that forms fine particles. EPA made the final designations after considering recommendations from states and public comments received in response to the intended designations issued in August 2008.

In 2006, EPA strengthened the 24-hour fine particle standards from 65 µg/m3 to 35 µg/m3. Nationwide, monitored levels of fine particle pollution fell 11% from 2000 to 2007. Fine particles can either be emitted directly, or they can form in the atmosphere from reactions of sulfur dioxide and nitrogen oxides. One of the contributors to ambient PM2.5 levels are diesel engines, mainly the non-controlled older models without diesel particulate filters.

Source: US EPA (PM2.5 designations)