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 PM_2.5, 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 its effect on lung and heart health; it is also regulated for its effect on visibility. “Ultrafine” particles would be defined as those smaller than 0.1 micrometers in diameter.

EPA is required by the Clean Air Act to review the NAAQS every five years; the next review of the PM standards should be completed by the end of 2017. The agency tightened some PM standards in the last round of review in 2012, although that rulemaking is the subject of ongoing litigation. In its new review, EPA may decide to tighten standards within the existing categories, establish a third sub-category of PM, neither, or both. (In the last review, EPA tightened some PM standards but did not create a separate ultrafine category.) All regulated entities whose activities produce PM or its precursors should closely track the public and regulatory debate, which will help predict which of these approaches EPA is likely to take.