The US Court of Appeals for the District of Columbia Circuit ruled Tuesday that the Environmental Protection Agency’s (EPA) actions in enforcing air quality standards across state lines were somewhat lacking. While the court determined that some of the EPA’s steps in evaluating air quality assessment requests under the Clean Air Act’s “Good Neighbor” provision were appropriate, they held that the EPA did not act properly in other steps.

The Clean Air Act calls for a national standard for air quality, but because air quality is inherently complicated due to the distance pollutants can travel, Congress included the Good Neighbor provision. This provision permits states to petition the EPA for a finding that another state’s emissions would travel and pollute the air in the downwind state to an extent that would prevent them from meeting air quality emission standards.

In this case, Maryland and Delaware filed such petitions to request that the EPA impose more limits on upwind states that were allegedly impacting their ability to achieve air quality standards. The EPA denied the petitions after applying a four-step evaluation framework. Delaware’s petition did not pass step one, as the EPA found that Delaware had not sufficiently demonstrated a current or future air quality problem. The EPA denied Maryland’s petition at the third step because Maryland had not sufficiently identified further cost-effective emission reductions.

Maryland had argued that implementing non-catalytic controls instead of catalytic controls at various sources would provide a cost-effective alternative that would reduce emissions. The court held that the EPA had not sufficiently explained why non-catalytic controls were not cost-effective enough to allow Maryland to have passed this step, thus granting Maryland’s petition for review in part and sending the issue back to the EPA to be reevaluated.