concurring:
I concur in Judge Wright’s opinion for the court.
The flexibility of a concurring opinion 1 permits some observations on the flexibility that may be available to the agency.
Conditions in the nonferrous smelting industry apparently prompted the approach reflected in the challenged regulations. The cost of controlling pollution emitted by existing smelters is great. The industry has argued that more efficient pollution control can be achieved by reducing the pollutants emitted by other facilities in the smelting process (a less expensive undertaking) to compensate for any increase in pollution attributable to modified smelters. The agency’s response has been to permit this tactic in the case of all modifications of existing facilities, but not in the case of newly constructed facilities (where the cost of building in appropriate pollution controls may be more modest).
As the majority opinion points out, the manner in which the statute defines the terms “new source” and “stationary source” does not provide leeway for the agency to assign different meanings to “stationary source” depending on whether construction or modification is in question.
However, the statute does not totally prohibit the agency from making appropriate distinctions based on the realistic differences between new construction and modification. While costs may not be considered in determining whether a facility will be subject to an NSPS, as the bubble concept would have done, they may be considered in determining the level at which a standard should be set, and how it should be formulated.
The statute permits the Administrator to “distinguish among classes, types, and sizes' within categories of new sources for the purpose of establishing [NSPS’s].”2 I believe the terms “classes” and “types” are broad enough to permit the Administrator to set more liberal standards for modified facilities than for newly constructed facilities that perform the same function, if he determines that such a distinction is appropriate in a given case. This approach permits custom tailoring, and thus stands in contrast to the challenged regulations.
The challenged regulations potentially immunize all modified facilities — and, as Judge Wright points out, would contravene the policy that contemplated that modification would bring about improvements — and would provide this escape without regard to the costliness of conforming to performance standards for a particular category of modified facilities. The record is devoid of support for the proposition that pollution control for modified facilities will always involve costs disproportionate to the resultant benefits. However, the flexibility to distinguish between classes of new sources may serve to authorize a differential in the stan*89dards applicable to new and modified equipment in those eases where warranted by cost differences and cost-benefit analysis. This approach would not permit the Administrator to immunize a modified facility (one type of new source) from regulation under a performance standard,3 but would permit an alternative course that promotes the underlying statutory concept of progressively bringing all pollution sources within the constraint of performance standards.
. United States v. Ammidown, 162 U.S.App.D.C. 28, at 38, 497 F.2d 615, 625 (1973) (concurring opinion of Leventhal, J.).
. 42 U.S.C. § 1857c-6(b)(2).
. The statute requires the Administrator to promulgate standards of performance for all new sources that he determines may contribute to air pollution.