Several asthmatic citizens of Phoenix, Arizona appealed the Environmental Protection Agency's (EPA) federal implementation plan (FIP) for air quality in Phoenix under the Clean Air Act ("CAA" or the "Act").[1] Plaintiffs alleged that EPA's FIP was illegal because it failed to adopt regulatory and technology-limited controls for sources of airborne particulate matter under ten microns in size (PM-10). The Ninth Circuit held that EPA's interpretation of the Act to exempt de minimis sources of PM-10 was reasonable and entitled to deference.[2]

In 1995, Phoenix was classified as a serious non-attainment area for two national ambient air quality standards (NAAQS) for PM-10 pollution--an annual standard and a 24-hour standard.[3] In 1998, EPA determined that attainment of the PM-10 NAAQS by the statutory deadline of December 31, 2001 was "impracticable."[4] Therefore, the FIP exempted de minimis sources of PM-10 pollution from all regulatory controls under the Act. The plaintiffs challenged the FIP's de minimis exemption as an unreasonable interpretation of the statute. The Ninth Circuit rejected that challenge, holding that EPA's interpretation was reasonable.

In reaching its conclusion, the court noted that the CAA makes no explicit provision for de minimis exemptions; however, EPA's preamble to its regulations provides for excluding insignificant emission sources from further consideration.[5] Although the preamble does not establish thresholds for de minimis sources, the court analyzed the agency's rationale for exempting de minimis levels and its application of those levels to these sources in Phoenix. To qualify as de minimis, a source must contribute less than one microgram per cubic meter to a location of expected annual exceedances and less than five micrograms per cubic meter to a location of expected twenty-four-hour standard exceedances. EPA designated thirteen source categories as de minimis and eventually exempted four sources from regulation: gas on-road motor vehicles, diesel on-road motor vehicles, locomotives, and airport ground support. EPA adopted these de minimis thresholds from the federal new source performance program (NSPS).[6]

The asthmatic citizens challenged EPA's de minimis exemptions on three grounds: 1) that adopting the de minimis exemptions from the NSPS was arbitrary and capricious, 2) that EPA did not specify how much of a public health hazard is caused by the de minimis sources, individually and collectively, and 3) that EPA improperly focused on the statutory deadline. The Ninth Circuit rejected each of these arguments. First, the court held that the de minimis thresholds in both the NSPS and in this case--serious non-attainment--are based on the source's contribution to the pollution in non-attainment, and therefore, EPA's interpretation was permissible. Second, the court reasoned that a source of pollution that has only a de minimis effect on the effort to bring Phoenix into conformity with these NAAQS has only a de minimis effect on the public health; therefore, EPA's interpretation was reasonable.[7] Finally, the Ninth Circuit held that by requiring controls on de minimis sources if NAAQS will be met--and not requiring controls if NAAQS will not be met--by the deadline serves to limit the de minimis exemptions. The court determined that EPA's decision to decline to control the de minimis sources of PM-10 pollution "makes sense" and was not arbitrary or capricious.[8]