Since 1995, EPA has followed a policy that any air emissions source that emits one or more hazardous air pollutants (“HAPs”) above major source emissions thresholds is always considered a major source of HAPs. This is so even if the source subsequently decreases its HAP emissions below major source thresholds. This “once in, always in” policy is not derived from the Clean Air Act, but is only a policy position. EPA recently reversed this policy in a guidance memorandum from William L. Wehrum, the Assistant Administrator for Air and Radiation. In issuing the memo, Wehrum said that “[t]he guidance is based on a plain language reading of the [Clean Air Act] that is in line with EPA’s guidance for other provisions of the Act.”