EPA Will Consider Whether To Propose Ultrafine Particle Air Quality Standard

Recent comments by EPA officials suggest that the agency will consider whether it should, for the first time, set a standard for "ultrafine" particles when it reviews its particulate matter national ambient air quality standard ("NAAQS") under the Clean Air Act.  The agency establishes NAAQS for certain listed 'criteria pollutants' that it judges to cause or contribute to air pollution that may reasonably be anticipated to endanger public health or welfare.  NAAQS have been established for six such pollutants: sulfur dioxide, carbon monoxide, nitrous dioxide, lead, ozone, and particulate matter ("PM").

The NAAQS for PM are currently divided into two subcategories: one for PM10, particles smaller than 10 micrometers in diameter (or about four ten-thousandths of an inch), and another for PM2.5, particles smaller than 2.5 micrometers in diameter.  (EPA also refers to these, respectively, as "inhalable coarse particles" and "fine particles.")  EPA regulates PM due to