On July 23, 2013, the United States Court of Appeals for the District of Columbia Circuit affirmed the U.S. Environmental Protection Agency’s primary National Ambient Air Quality Standards (NAAQS) for ozone, which EPA established in a 2008 rulemaking at 0.075 parts per million (ppm), 0.005 ppm lower than the 1997 0.08 standard. Mississippi v. EPA, No. 08-1200, slip op. (D.C. Cir. July 23, 2013). EPA determined that the 0.075 standard was “requisite to protect the public health” within the meaning of the Clean Air Act (Act). 42 U.S.C. § 7409(b)(1). In the same rulemaking, 73 Fed. Reg. 16,436 (Mar. 27, 2008), EPA also set the secondary ozone standard—which must be “requisite to protect the public welfare,” Id. §7409(b)(2)—to be exactly the same standard as the primary ozone NAAQS.

Historical photo the George Washington Bridge through heavy smog in New Jersey and New York (Source: EPA "40 Years of Images" at www.epa.gov)

Historical photo the George Washington Bridge through heavy smog in New Jersey and New York (Source: EPA “40 Years of Images” at http://www.epa.gov)

The Clean Air Act requires EPA to establish NAAQS for any air pollutant it determines to be  “harmful to public health and the environment.” Ozone, more commonly known as smog, is one of the six compounds for which EPA has established NAAQS. Ground-level ozone is formed by a chemical reaction between volatile organic compounds (VOCs) and oxides of nitrogen (NOx) in the presence of sunlight. As motor vehicle exhaust is a prime source of VOCs and NOx, ozone problems are generally associated large urban areas such as Los Angeles. Excessive exposure to ozone can lead to various adverse environmental and health effects, particularly respiratory disease.

In Mississippi v. EPA, the D.C. Circuit consolidated several appeals of EPA’s rulemaking, some claiming the new ozone standards were too strict, some not strict enough. In a colorfully worded opinion, the Court rejected the arguments of both sides and determined that under the highly deferential “arbitrary and capricious” standard of review, EPA’s decision to set the primary ozone standard at 0.075 ppm was rational and could not be disturbed by the Court. Most notably, the Court held EPA had adequately explained why it chose not to set the standard even lower, as recommended by the Clean Air Scientific Advisory Committee (CASAC) (a recommendation that led former EPA Administrator Lisa Jackson, an Obama appointee, to famously describe the Bush-era 2008 standards as “not legally defensible”).

As to the secondary standard, however, the Court noted that EPA set the standard merely by comparing the level of protection afforded by the primary standard to possible secondary standards and finding the two roughly equivalent. This approach, the Court held, did not satisfy EPA’s obligation under the Clean Air Act to affirmatively determine what level of protection is “requisite to protect the public welfare,” particularly when both EPA staff and CASAC had recommended a cumulative seasonal standard. The Court remanded the secondary standard portion of the rule for further explanation or reconsideration by EPA, temporarily leaving the secondary standard in place pending EPA’s remand proceedings.

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