On February 22, 2017, EPA released a Risk and Exposure Assessment (REA) planning document as part of its review process of the existing one-hour Sulfur Dioxide (SO2) National Ambient Air Quality Standard (NAAQS) to determine whether to retain, weaken or tighten the existing SO2 NAAQS. The document demonstrates that EPA is again going to place heavy emphasis on the short-term public health impacts of exposure to SO2 air emissions. EPA placed a similar emphasis on the short-term public health impacts of SO2 in 2009 when it last reviewed the SO2 NAAQS. This focus on the short-term in 2009 resulted in EPA tightening the standard to 75 parts per billion using a one-hour averaging time to control for short-term exposure that EPA found impacted human health.

The new REA planning document attempts to bolster the emphasis on the short-term with better emissions data and modeling techniques improved as a result of the 2009 review and tightened SO2 NAAQS. The short-term standard required the creation of a new monitoring network and new monitoring techniques. In particular, EPA required monitoring agencies submit five-minute SO2 measurements. The approach for the new REA “will be based on linking the health effects information to population exposure estimates that draw on this improved understanding of 5-minute concentrations of SO2 in the ambient air.” EPA hopes this improved data will help reduce scientific uncertainties that were inherent in the earlier SO2 review.

In addition to the planning REA, EPA issued two drafts of its Integrated Science Assessment (ISA) for its review of the SO2 NAAQS. The second draft ISA weakened several findings of adverse human health effects from the original draft based on concerns from science advisers. In the original draft, EPA classified the relationship between short-term SO2 exposure and certain health effects as “suggestive.” The health effects at issue were short-term cardiovascular effects, reproductive and developmental effects, total mortality from long-term exposure, and cancer from long-term exposure. Members of the Clean Air Scientific Advisory Committee, which counsels EPA on how to set NAAQS, expressed concern this classification resulted in an overstatement of health effects. The second ISA draft now classifies the relationship between short-term SO2 exposure and these health effects as “inadequate,” potentially undermining the case for any strengthening of the NAAQS.

It is unclear when EPA will issue the revised NAAQS. Under the NAAQS’s five-year review cycle, EPA should have issued a new SO2 NAAQS in 2015, but it is several years behind schedule. While EPA did not provide a timeline for when the REA will be completed, the information gained in the REA will eventually help influence whether EPA will seek to change the SO2 NAAQS in the future.