On December 2, 2019 US EPA issued a memorandum reinterpreting the definition of ambient air found in 40 CFR 50.1(e). EPA’s view of this definition, which dates back to a 1980 letter, had been that the atmosphere over “land owned or controlled by the source and to which public access is precluded by a fence or other physical barriers[.]” is exempt from areas considered ambient air. In its new reading of the definition, EPA is expanding that exemption to allow for the atmosphere over lands that are owned by private sources that are not protected by only fences or physical barriers, but rather, which the source has control over and takes other measures to preclude public access. While this reading does not have the force of law, it will impact how EPA administers its permit programs and measures air quality when sources apply for permits. Importantly, this memo does not apply to states and local jurisdictions, which administer most air permit programs.