On May 31, 2019 the US Court of Appeals for the District of Columbia Circuit dismissed challenges to 2016 revisions to Clean Air Act regulations governing four monitoring quality assurance requirements for the program to prevent serious deterioration of air quality in areas that meet national standards. These rules can be found at 40 CFR 58 and the changes were made in 81 Fed. Reg. 17,248 (Mar. 28, 2016). The challengers, Sierra Club and Earthjustice in particular, argued that EPA should apply the same requirements to the PSD monitors as it does to monitors ensuring NAAQS compliance and that EPA failed to respond to its comments opposing the proposed rule. The court disagreed and stated that a failure to respond to comments is significant only insofar as it demonstrates that the agency’s decision was not based on a consideration of the relevant factors. The court found that EPA did consider relevant factors in making this rule and, as a result, sided with EPA and dismissed the challenges.