EPA Science Advisors Support Current SO2 NAAQS

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.