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EPA Looking to Revise Official Definition of ‘Ambient Air’

Gale Hoffnagle | December 18, 2018

Last month the EPA unveiled a proposal to update its definition of “ambient air,” a move that could dramatically affect how air quality modeling is performed around facilities that emit pollutants. The agency is accepting comments on its proposal through December 21.


All pollutant emitting sources must meet the EPA’s National Ambient Air Quality Standards (NAAQS) at the plant boundary but not inside the plant property. Thus, there needs to be a definition of “ambient air.” The regulatory definition of “ambient air” is “that portion of the atmosphere external to buildings, to which the general public has access.”

EPA policy concerning the areas on a plant site exempt from being classed as ambient air was set out in 1980 as “the exemption from ambient air is available only for the atmosphere over land owned and controlled by the source and to which public access is precluded by a fence or other physical barriers.”

As a result, almost all decisions over the last four decades have been based on whether a physical fence is present at a site. When sources need to demonstrate compliance with the NAAQS model receptors are placed along the fence line. Model receptors are not placed inside the fence line because that is not ambient air.


EPA’s proposed changes to exclusions from ambient air clarify that permitting authorities have substantially more leeway than requiring a physical fence in defining these “barriers” to public access. The proposal mentions steep slopes and marsh land as examples of natural barriers to access that can be considered.

The proposal also recognizes that sources have many more options of controlling access to their property than they did in 1980. Electronic monitoring systems, security patrols and drones are specifically mentioned in the draft policy. That flexibility explained in the proposal could also allow sources to simply post areas as “Private Property” or “No Trespassing” zones. The policy notes that signage may be a sufficient barrier as long as it can be demonstrated to be an effective deterrent to public access.


Because most industrial sources typically have their highest modeled concentrations of pollutants close to the facility, the modeled concentration “at the fence line” has often been the one around which regulatory decisions must be made. If the location of the nearest “ambient air” is further from the facility – near the kinds of natural barriers mentioned earlier – maximum modeled concentrations could be lower, making it easier for sources to comply with NAAQS.

In the past, modelers have argued – mostly unsuccessfully – that other barriers should also be considered in the definition of ambient air, such as railroad tracks, major highways and bodies of water. It is notable that the proposed policy change does not mention these kinds of barriers, therefore the air associated or impacted by these types of barriers may still be considered “ambient air”.

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Gale Hoffnagle

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