On February 26, 2019, EPA announced its decision to retain the current sulfur dioxide (SO₂) National Ambient Air Quality Standard (NAAQS). The current primary SO₂ NAAQS  of 75 parts per billion (ppb) averaged over one hour was established in 2010. The 2010 ruling was significant because it established a short-term, one-hour standard and revoked the prior 24-hour and annual standards. 

The Clean Air Act requires EPA to set NAAQS for six criteria pollutants: carbon monoxide, lead, nitrogen dioxide, ozone, particulate matter, and sulfur dioxide. The statute also requires it to review and revise the NAAQS every five years. After EPA did not review the SO₂ NAAQS within five years of the last determination, environmental groups sued it to force a review.  As a result, EPA was under a consent decree to decide whether to retain or revise the SO₂ NAAQS by February 25.

Several industry groups asserted that the current SO₂ NAAQS was too tough and argued that EPA should weaken the standard. They recommended that EPA raise the NAAQS to 110 – 150 ppb, but environmental groups argued the standard was not protective enough of human health and should be lowered to 50 ppb. Ultimately, EPA rejected both arguments and determined the current standard “is requisite to protect public health, with an adequate margin of safety, from effects of SO₂ in ambient air.”  It noted that this was consistent with the April 2018 recommendation of the Clean Air Scientific Advisory Committee, an independent advisory board.  EPA also supported its decision by reiterating that SO₂ levels in the United States had dropped by more than 85 percent between 1990 and 2017 and more than 60 percent since 2010. 

As a result of this decision, revisions to state implementation plans will not be triggered, and existing sources will not face potential imposition of requirements for installation of  additional controls for SO₂. However, the one-hour standard remains a potential barrier to permitting new projects with significant SO₂ emissions