EPA has published a final rule setting thresholds for state agencies and other
authorities to use in evaluating whether fine particulate (PM 2.5) emissions from
new or expanded industrial facilities would degrade air quality. 75 Fed. Reg. 64,863
(10/20/10). The thresholds will be used by regulators in areas that comply with
national ambient air quality standards (NAAQS) under the Clean Air Act when
issuing prevention of significant deterioration (PSD) permits to industrial faculties
for PM 2.5 emissions. The new PM 2.5 thresholds will apply in addition to existing
standards for particles smaller than 10 microns (PM-10). The final rule amends 40
C.F.R. Parts 51 and 52 and will be effective on December 20, 2010.

The final rule sets three measurements for state regulators to consider when
issuing PSD permits for PM 2.5 emissions: (i) increments—the maximum
amount of ambient particle pollution a facility can contribute to the air before
quality is considered degraded; (ii) significant impact level—the minimum
threshold, expressed as an ambient pollutant concentration, that would
indicate an emissions source might cause or contribute to a violation of either
the increment or the air quality standard itself; and (iii) significant monitoring
concentration—a level of projected emissions increase based on computer
modeling, above which an industrial plant would have to conduct actual
emissions monitoring and submit the data as part of its permit application.