Cite as: 531 U. S. 457 (2001)
Opinion of the Court
to be considered in determining whether there is ambiguity" (emphasis deleted)).1
The same defect inheres in respondents' next two arguments: that while the Administrator's judgment about what is requisite to protect the public health must be "based on [the] criteria" documents developed under § 108(a)(2), see § 109(b)(1), it need not be based solely on those criteria; and that those criteria themselves, while they must include "effects on public health or welfare which may be expected from the presence of such pollutant in the ambient air," are not necessarily limited to those effects. Even if we were to concede those premises, we still would not conclude that one of the unenumerated factors that the agency can consider in developing and applying the criteria is cost of implementation. That factor is both so indirectly related to public health and so full of potential for canceling the conclusions drawn from direct health effects that it would surely have been expressly mentioned in §§ 108 and 109 had Congress meant it to be considered. Yet while those provisions describe in detail how the health effects of pollutants in the ambient air are to be calculated and given effect, see § 108(a)(2), they say not a word about costs.
Respondents point, finally, to a number of provisions in the CAA that do require attainment cost data to be generated. Section 108(b)(1), for example, instructs the Administrator to "issue to the States," simultaneously with the criteria documents, "information on air pollution control techniques, which information shall include data relating to the cost of installation and operation." 42 U. S. C. § 7408(b)(l). And
1 None of the sections of the CAA in which the District of Columbia Circuit has found authority for the EPA to consider costs shares § 109(b)(1)'s prominence in the overall statutory scheme. See, e. g., Michigan v. EPA, 213 F. 3d 663, 678-679 (CADC 2000); George E. Warren Corp. v. EPA, 159 F. 3d 616, 623-624 (CADC 1998); Natural Resources Defense Council, Inc. v. EPA, 824 F. 2d 1146, 1154-1163 (CADC 1987) (en banc).
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