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EPA Refines “Ambient Air” Policy

On November 9, 2018, EPA released a draft memorandum, “Revised Policy On Exclusions from ‘Ambient Air,’” that expands the number of measures that may be considered by sources and permitting authorities in determining where the “ambient air boundary” is located and hence where compliance with the National Ambient Air Quality Standards (NAAQS) is required.


EPA begins its analysis by quoting the definition of “ambient air” as “that portion of the atmosphere, external to buildings, to which the general public has access.” 40 CFR § 50.1(e) (emphasis added). EPA notes that in 1980 Administrator Costle had provided guidance that “the exemption from ambient air is available only for the atmosphere over land owned or controlled by the source and to which public access is precluded by a fence or other physical barriers” (emphases added). EPA has historically looked to both legal control – the authority to exclude – and practical control – is the public effectively excluded – in setting the ambient air boundary, while recognizing that certain individuals, such as company employees or “business invitees,” are not considered the general public and do not convert areas to which they have access to ambient air. This policy has remained in place for many years, with minor exceptions granted for mostly physical barriers, such as cliffs, rough terrain, and similar barriers.

The memorandum notes that many stakeholders have approached the EPA about relaxing the restriction to physical barriers due to advances in technology and to the increased stringency of the recent 1-hour NAAQS, which have made modeling demonstrations more difficult. The memorandum reviews some cases where EPA had, on a case-by-case basis, used a more relaxed approach in the past and concludes that the governing element in these determinations is public access.

EPA also considered changes in technology, noting:

With advances in technology and greater experience in a variety of ambient air scenarios since the 1980 letter, the EPA believes there are various measures other than fencing or other physical barriers that a facility can employ to serve as an effective deterrent to public access. These measures may include traditional fencing, but may also include video surveillance and monitoring, clear signage, routine security patrols, drones, and other potential future technologies. In many cases, such measures are already being used as effective means to deter or preclude public access to private property, even if not specifically for purposes of an ambient air exclusion.

EPA cautions, however, that agencies should still review proposed measures for effectiveness.

Finally, EPA states its new policy: “Accordingly, the EPA’s revised ambient air policy, consistent with its discretion available under the regulatory definition of ambient air, is that it is appropriate to exclude the atmosphere over land owned or controlled by the stationary source, where the owner or operator of the source employs measures, which may include physical barriers, that are effective in deterring or precluding access to the land by the general public.” Memo, at 6-7 (emphasis in original). EPA states that the new policy is effective immediately in directly administered and delegated states and that it “expects” state and local approved programs to apply the definition consistent with the approved program


This change has been a long-time coming, having been highlighted by EPA at an Air & Waste Management Association conference over a year ago. Nevertheless, the new policy is a welcome recognition that public access may be effectively precluded by more than just the seemingly-required 6-foot chain link fence. It provides a helpful solution for areas, such as entry gates, rail gates, and port areas that are difficult to keep closed due to traffic by recognizing that patrols and cameras may serve as an equivalent deterrent. The policy is also useful for large, remote facilities with little public access by reducing the considerable cost of fencing in areas with little likelihood of public access.

It will be interesting to watch how the EPA, its delegated permitting authorities, and authorized States apply this guidance over time.

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