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EPA Proposes to Retain National Ambient Air Quality Standards for Sulfur Dioxide

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WASHINGTON  - Today, after carefully reviewing the most recently available scientific evidence and risk and exposure information as well as consulting with the Agency’s independent science advisors, U.S. Environmental Protection Agency (EPA) Administrator Scott Pruitt is proposing to retain, without revision, the existing primary (health-based) National Ambient Air Quality Standard (NAAQS) for sulfur dioxide (SO2). EPA and its Clean Air Scientific Advisory Committee (CASAC), agree that this existing standard continues to provide health protection for Americans, including respiratory effects following short-term exposures to SO2 in ambient air.

“The U.S. is making tremendous progress reducing SO2 concentrations, and the proposal to retain the existing standard is based upon the best available science and the recommendations of our independent science advisors,” said EPA Administrator Scott Pruitt. “EPA recently issued a memo to reform the process by which we set NAAQS in order to ensure we continue the dramatic improvement in air quality across the nation.”

EPA’s CASAC concurred with the Agency “that the current scientific literature does not support revision of the primary NAAQS for SO2.” EPA is soliciting comment on the basic elements of the current NAAQS, including whether there are appropriate alternative approaches that provide comparable public health protection.

As a result of Clean Air Act programs and efforts by state, local, and tribal governments as well as technological improvements, SO2 concentrations in the U.S. have fallen by more than 70 percent since 2000 and more than 50 percent since 2010.


The Clean Air Act requires EPA to set NAAQS for “criteria pollutants.” Currently, sulfur dioxide and five other major pollutants are listed as criteria pollutants. The law also requires EPA to periodically review the relevant scientific information and primary (health-based) standards. If appropriate, EPA revises the standards to ensure they provide requisite protection for public health, allowing for an adequate margin of safety.

In addition, CASAC is separately reviewing welfare-based secondary NAAQS for sulfur oxides, along with nitrogen oxides, and particulate matter.

Earlier this month, Administrator Pruitt announced reforms to the process for setting NAAQS.

EPA will accept comments for 45 days after the proposed decision is published in the Federal Register. The Agency plans to issue a final decision no later than January 28, 2019, consistent with the court-ordered schedule for completion of this review.

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