In a brief filed February 19, 2013, EPA stated that substantial uncertainties prevented revisions to the nitrogen dioxide and sulfur dioxide secondary NAAQS in 2012. Environmentalists filed suit in the U.S. Court of Appeals for the D.C. Circuit against EPA in late 2012 challenging EPA's decision to retain 1971 NAAQS for nitrogen dioxide and sulfur dioxide rather than update the standards (Center for Biological Diversity, et al. v. EPA). On April 3, 2012, EPA announced it would retain existing standards and plan a five-year pilot program to collect data to help in future development of a multi-pollutant standard.
In its brief, EPA stated that substantial scientific uncertainties regarding the nitrogen dioxide and sulfur dioxide standards prevented a reasoned decision about whether a revised standard would be stringent enough or too stringent. This uncertainty is centered on the impacts of these pollutants on aquatic acidification. EPA stated that the Clean Air Act does not require EPA to revise an air quality standard when faced with this level of uncertainty. Oral arguments have not yet been scheduled for this lawsuit.