The U.S. Environmental Protection Agency (USEPA) recently proposed a Revised Policy on Exclusions from “Ambient Air” (Draft Policy) to clarify the exclusion of certain areas from the scope of “ambient air” under the Clean Air Act.  The term “ambient air” is broadly defined by USEPA as “that portion of the atmosphere, external to buildings, to which the general public has access.”  40 CFR §50.1(e).  Stationary sources are often required to model facility impacts on “ambient air” when seeking a permit under the Prevention of Significant Determination (PSD) and some state-level programs.  This proposed policy change could have the effect of increasing the area not considered to be “ambient air” for air modeling purposes, thus making it easier to meet the National Ambient Air Quality Standards (NAAQS) in some instances.   

In 1980, USEPA wrote guidance stating that a source could exclude an area from “ambient air” if “(1) the area . . . is owned or controlled by the source, and (2) access to the area by the public is precluded by means of a fence or other physical barriers.”  In response to some stakeholders’ concerns, the current USEPA has analyzed the terms in the regulatory definition of ambient air and identified three core conceptual elements: (1) “access,” (2) “general public,” and (3) “external to buildings.”  The agency then assessed how it has been applying each of these terms under the existing ambient air policy and concludes that the access element warrants revision.

The access element of the ambient air policy encompasses two concepts – (1) physical or practical access, and (2) legal access.  In response to stakeholder requests and after considering past case-specific determinations, USEPA now believes that physical barriers are not the only means of satisfying the access element of the exclusion from ambient air. 

Accordingly, the revised ambient air policy replaces “a fence or other physical barriers” with “measures, which many include physical barriers” to preclude public access to the land.  Examples of various measures given in the guidance include areas subject to video surveillance and monitoring, clear signage, routine security patrols, drones, and potential future technologies provided that they “provide reasonable assurance that the general public will not have access.”  The stated goal of this change is to take into account advances in technology since the 1980 policy was published and provide sources with additional flexibility to determine what areas must be modeled for air quality analyses. 

USEPA instructs air agencies to evaluate the effectiveness of a “measure” in precluding public access based on the specific circumstances.  This evaluation should address relevant factors, such as the nature of the measure used, facility location, type and size of facility and property to be excluded, surrounding area, and other factors affecting the extent to which a person would be likely or able to trespass upon or otherwise have access to the facility’s land.  However, since the agency’s proposed revision is in guidance rather than a change in regulation, sources should still consider how their local permitting authorities interpret the term ambient air.

In the Draft Policy, the agency also evaluates the second aspect of the access element – legal access.  This concerns whether the public is legally precluded from entering onto the property by its owner.  The Draft Policy makes no change to the agency’s typical analysis of this portion of the access element.

To review the draft guidance, visit USEPA’s website at: