EPA’s Clean Air Scientific Advisory Committee (CASAC) has now completed its review of the agency’s draft Policy Assessment (PA) for the SO2 NAAQS, and supports the EPA staff recommendation that the current scientific literature does not support revision of the current primary (health based) SO2 NAAQS.

In a draft letter recently released, CASAC notes that “key uncertainties” have emerged since the prior SO2 review, particularly with regard to “at-risk” subgroups such as children who are: obese; of African-American ethnicity; severely asthmatic; and/or live in high density areas near sources of exposure. The Committee believes that while many uncertainties remain in quantifying the sizes of the risks for these groups, they should nonetheless be considered in ensuring that the standard provides an adequate margin of safety. CASAC also recommends that efforts should be made to gather the data necessary to ensure that protection of these groups can be considered with less uncertainty in future reviews of the standard.

According to the draft letter, the Committee believes it possible that the current 75 ppb level may not provide an adequate margin of safety in these groups. However, because there is considerable uncertainty in quantifying the sizes of these higher risk subpopulations and the effect of SO2 on them, the Committee did not recommend reconsideration of the level at this time. CASAC strongly recommends that future assessments better quantify the numbers of individuals expected to be affected at the current (or proposed alternative) standard in these groups so that a more informed judgment about the margin of safety in high risk subgroups can be made. In particular, the Committee suggests that EPA express the size of the at-risk population both in percentage form (which is currently done) and also with numerical estimates, providing the number of people expected to be at risk, given the margin of safety.

The Committee recommended a few changes in the draft PA and stated that with those changes it need not review another draft. We expect that CASAC will finalize its draft letter soon and that EPA will then move to finalize the PA and propose to retain the current standard. We doubt that the current Administration will seize on the “margin of safety” points in the CASAC letter to propose a revised standard (as some prior administrations might have done). However, those issues are likely to be a primary focus of the next review of the SO2 standard.

Apart from SO2, this letter is significant because it is the first official CASAC action under the newly appointed Chair, Anthony Cox, and the other new members appointed by this Administration. It therefore appears that the reported “backlog” of NAAQS reviews caused by CASAC appointment delays will now begin to break.

EPA recently issued new Guidance for determining whether a proposed new or modified source of air pollutants would cause a significant increase in emissions, requiring a major source permit. See Memorandum from Administrator Pruitt to Regional Administrators re: Project Emissions Accounting Under the New Source Review Preconstruction Permitting Program (March 13,2018).

To date, EPA has employed a two-step process for evaluating projected emissions for major source status. Under step 1, the agency determines whether the project by itself – including emission controls – would cause a significant increase in emissions. Under step 2, the agency then looks at other “contemporaneous” projects at the facility – defined as those constructed within the past two years – to determine whether all of the projects combined would result in a “significant net increase” in emissions from the entire facility. If so the facility will require a major source permit.

With a few brief exceptions, the agency throughout its history has not allowed consideration of emissions decreases in step 1. The emissions baseline for the new project is set at zero, and offsets for emissions decreases are not considered until step 2. The practical effect has been to require all major sources to model both steps, even though the modeling in step 1 might show a decrease at that point, precluding the need to move to step 2 (e.g., the project includes new emission controls and also involves shutting down other emissions sources within the plant to produce an emissions decrease).

The new Guidance, effective immediately, revises EPA’s approach to allow consideration of net emissions decreases in step 1. The agency now believes that the prior approach had the practical effect of preventing some projects from proceeding and significantly delaying others, even though those projects would not have resulted in a significant emissions increase. The agency also recognizes that the increased efficiency of new production technologies can result in emission reductions even while expanding production. In such situations, the complexities associated with modeling multi-year contemporaneous netting under Step 2 at a large facility have dissuaded some meritorious projects. The new interpretation is designed to correct this, and the agency believes it is consistent with the relevant language of the Clean Air Act and current NSR regulations.