The Clean Air Act tasks the EPA with regulating “any air pollutant.” However, briefs for the various petitioners in the cases and opinions in the lower courts have defined the phrase several different ways. Shortly after Peter Keisler, arguing on behalf of the private parties, begins his argument, Justices Sotomayor and Kagan question him about which definition of this phrase he advocates. After Justice Kagan attempts to extract the petitioners’ definition of the phrase, you will hear Justice Sotomayor express support for the EPA by questioning why the Court should not simply defer to the EPA’s interpretation of the phrase when the other “side can’t even come to one interpretation.”

Note: In this clip, the Justices and Keisler use the abbreviation “NAAQS pollutants” and “PSD.” NAAQS refers to the National Ambient Air Quality Standards, which, according to the EPA, establishes maximum pollution concentration levels to protect public health and welfare from harmful levels of certain pollutants. PSD refers to the EPA’s Prevention of Significant Deterioration program, under which the EPA regulates greenhouse gases.