A three-judge panel of the U.S. Court of Appeals for the District of Columbia last week upheld the Environmental Protection Agency’s (EPA’s) primary National Ambient Air Quality Standards (NAAQS) for fine particulate matter (PM2.5), dispensing to the agency its third major legal victory on air pollution in a month.

The EPA in December 2012 issued a final rule that strengthens its NAAQS for PM2.5 to 12.0 micrograms per cubic meter (μg/m3), compared to the previous 1997-set annual standard of 15.0 μg/m3. The new level is slightly lower than the lowest concentrations reported as causing adverse health effects in studies analyzed by the agency. The rule, however, retains the 2006-issued 24-hour primary fine particle standard of 35 μg/m3.

The National Association of Manufacturers and other industry groups challenged the rule, arguing that the EPA’s revisions to the level and compliance with the level (determined within the averaging time) were unreasonable. They also posited that the EPA applied inconsistent peer-review standards and afforded more weight to certain studies finding associations between PM2.5 exposure and adverse health effects, and that the agency acted unreasonably by promulgating a final rule without first publicizing some critical implementation documents.

But on May 9, in a succinct 11-page opinion, the D.C. Circuit found that the petitioners had not identified “any way in which the EPA jumped the rails of reasonableness in examining the science. EPA offered reasoned explanations for how it approached and weighed the evidence, and why the scientific evidence supported revision of the NAAQS,” it says.

For example, in response to the petitioners’ contention that the EPA acted unreasonably by eliminating spatial averaging—which allows certain areas to demonstrate compliance with emission standards by averaging results from multiple monitoring sites within that area—the court found that the EPA “reasonably explained” its decision not to employ spatial averaging. “As the agency stated, spatial averaging would enable some portions of a compliance area—particularly those areas where sensitive individuals are likely to live—to exceed the NAAQS for periods of time,” the decision notes.

The case is National Association of Manufacturers v. EPA (No. 13-1069). 

It marks the agency’s third legal victory on air pollution in a month. On April 29, the U.S. Supreme Court upheld the EPA’s authority to regulate power plant emissions across state lines under the Cross State Air Pollution Rule. On April 15, the D.C. Circuit upheld the Mercury and Air Toxics Standards, finding that the agency is not required to take costs into account when it promulgates rules that are “appropriate and necessary” to address hazards to public health.

Sonal Patel, associate editor (@POWERmagazine, @sonalcpatel)