On May 27, 2014 the D.C. Circuit upheld the United States Environmental Protection Agency's (EPA) decision to retain its existing air quality standards for nitrogen dioxide (NO2) and sulfur dioxide (SO2) which address the direct effects of exposure to gaseous oxides of nitrogen and sulfur (i.e. what is popularly known as "acid rain") on vegetation. In 2012, EPA determined that while the existing standards for these two pollutants are not adequate to protect against adverse effects on water bodies from acid rain, they should not be revised due to doubts about the accuracy of the underlying scientific studies. See Secondary National Ambient Air Quality Standards for Oxides of Nitrogen and Sulphur, 77 Fed. Reg. 20,218, 20,226 (Apr. 3, 2012). In Center for Biological Diversity v. Environmental Protection Agency, the D.C. Circuit deferred to EPA's determination, further noting that promulgating a rule based on uncertain science would itself be arbitrary and capricious.
EPA has been regulating NO2 and SO2 emissions, not together but separately, in a variety of ways since 1971, as required by the 1970 amendments to the Clean Air Act (CAA). Under the CAA, the agency must regulate any airborne pollutant which, in the EPA's judgment, "may reasonably be anticipated to endanger public health or welfare." 42 U.S.C. § 7408(a)(1)(A). EPA must establish national ambient air quality standards to protect public health and public welfare for each such "criteria pollutant." Id. § 7409(a)(1)(A). The standards must be revisited every five years and revised as necessary. The challenged rule was issued as required by a consent decree.
In determining not to alter the existing standards, EPA reasoned that doubts about the accuracy of the underlying science prevented the agency from identifying with sufficient certainty a standard that would be, as the Clean Air Act requires, "requisite to protect the public welfare." 42 U.S.C. § 7409(b)(2). The Center for Biological Diversity filed a petition for judicial review on the issue of whether EPA's decision to defer adopting a new standard at this time, pending further scientific study, violated the CAA.
In deferring to EPA's determination, the Court reasoned that if the available information was insufficient to permit a reasoned judgment about whether any proposed standard would be "requisite to protect the public welfare," promulgating that standard would have been arbitrary and capricious. The Court went on to find that it "is ridiculous to suppose that the Clean Air Act required EPA to promulgate a secondary standard that would immediately violate the Act." The Court further noted its obligation to defer to the agencies in highly technical or scientific matters, citing to "decades of decisions" along those lines.
Center for Biological Diversity v. Environmental Protection Agency, No. 12-1238 (D.C. Cir.).