On January 27, the United States Court of Appeals for the District of Columbia Circuit ruled that the Environmental Protection Agency (EPA) cannot apply the "bubble concept" in determining whether a modification in an existing plant is subject to new source performance standards under §111 of the Clean Air Act.¹ In ASARCO, Inc. v. Environmental Protection Agency,² the court invalidated portions of EPA regulations providing that the new source performance standards, which are generally more stringent than emission standards for existing sources, do not apply if net emissions from the entire plant do the increase as a result of modification of a portion of it. This concept, which is similar to the agency's emission offset method of determining whether to allow new sources in nonattainment areas,³ treats the collection of individual buildings and facilities which make up an existing plant as a single "source" for the purpose of deciding whether a modification has resulted in an increase in emissions.⁴ If the pollutants emitted by a modified component are more than offset by reductions in emissions from components that have been partially or completely taken out of service, for example, the addition will not bring the new source performance standards into play.⁵ "Modification" of an existing source is covered by the bubble concept whereas new construction or "reconstruction" at the plant site is not.⁶

Although the bubble concept was adopted to deal with the special problems of the nonferrous smelter industry, its ramifications are much broader. The definition applies equally to owners or operators of other major industrial plants seeking to avoid the more stringent new source performance standards. Reductions in emissions from portions of the plant could serve to allow such operators to expand production (and increase emissions) at other facilities without having to acquire the more effective but more expensive control technology required to meet those standards.