A recurring discussion during the oral argument was the value of the health benefits created by the regulation. The EPA claimed it was $33-90 billion; industry opponents claimed it was only $4-6 million. The discrepancy comes from “co-benefits.” Power plants are not able to solely limit their emissions of mercury. When they put cleaner technologies into place, they also limit their emissions of other dangerous chemicals. The industry only considers the benefits from the mercury reduction, while the EPA is measuring the full health benefits all reduced emissions.

In this exchange, Chief Justice Roberts—who is expected to be a swing vote in the case—worries the EPA is using its mercury regulation as an “end run” to regulating other dangerous chemicals. The Chief Justice mentions the “HAP program” in his question. “HAP” stands for hazardous air pollutants, and is a reference to the section of the Clean Air Act that allows the EPA to regulate mercury emissions.