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EPA Proposes Revised Secondary National Ambient Air Quality Standard for SO2 for 1st Time Since ‘71

Today we’re gonna talk about revisions to air quality standards - specifically the National Ambient Air Quality Standards, known as NAAQS for short.

Little background: The Clean Air Act requires the U.S. Environmental Protection Agency (EPA) to develop a list of air pollutants that threaten the public health or welfare and establish primary and secondary National Ambient Air Quality Standards (NAAQS) for those pollutants. 

“The primary NAAQS are intended to protect human health and the secondary standards are to protect public welfare, meaning essentially, ecosystems, property, and crops,” GASP senior attorney John Baillie explained.

The act further requires that EPA review the NAAQS from time to time and revise them as necessary. Once a NAAQS is established or revised, it generally falls to the states (subject to approval by EPA) to set rules and regulations that are sufficient to attain the standard within their borders.

The primary NAAQS receive the bulk of attention from EPA, environmental advocates, regulators, and industry. Indeed, we just blogged about EPA’s recent decision to revise the primary NAAQS for fine particulate matter. 

Once in a while, however, EPA turns its attention to the secondary NAAQS - and we do mean “in a while.” The existing secondary NAAQS for SO2 was established way back in 1971.

This is happening now: On April 15, 2024, EPA proposed to retain the secondary NAAQS for oxides of nitrogen and particulate matter, while substantially revising the secondary NAAQS for sulfur dioxide (SO2).

Here’s what you need to know:

  • The existing secondary NAAQS for SO2 requires levels in the ambient air not to exceed 500 parts per billion (ppb) during any three-hour period more than once per year

  • The proposed revision to the standard would require that the annual average of SO2 levels in the ambient air not exceed 10-15 ppb, measured over a three-year period.

“Yes, that’s a 97-98 percent reduction from the existing standard’s benchmark measured over a three-year period,” Baillie added.

 What would this mean for our area? 

According to the Allegheny County Health Department’s (ACHD) Annual Air Quality Reports for the years 2020, 2021, and 2022 (the most recent three years for which ACHD has published such reports), it appears that exceedances of the proposed new standard were measured at air quality monitors located in Liberty Borough and North Braddock, while the air monitored at Lawrenceville and South Fayette would attain the proposed revised standard:



2020 (annual average SO2 levels, in ppb)

2021 (annual average SO2 levels, in ppb)

2022 (annual average SO2 levels, in ppb)

3-Year Average






North Braddock










South Fayette





 The Pennsylvania Department of Environmental Protection (DEP) monitors ambient air quality in areas in our region outside of Allegheny County and operates SO2 monitors in Charleroi and Florence in Washington County, Johnstown in Cambria County, and Strongstown in Indiana County. 

Although DEP does not publish a report like ACHD’s disclosing annual average SO2 concentrations measured at those monitors, it seems likely the monitor at Strongstown at least will not attain the revised secondary NAAQS for SO2 – as we blogged in February, areas near Strongstown that are also downwind of the cluster of coal-burning power plants in Indiana and Westmoreland Counties do not attain the primary NAAQS for SO2. 

“The failure to attain the primary SO2 standard with its 75 ppb benchmark suggests that attainment of the secondary standard, with its lower 10-15 ppb benchmark, will be problematic,” Baillie said.

Accordingly, it appears that both ACHD and DEP will be required to take steps to bring all areas under their jurisdiction into attainment of the revised secondary NAAQS for SO2. 

Assuming that EPA eventually finalizes the revised secondary NAAQS for SO2, we will follow the efforts by ACHD and DEP to develop plans to attain the revised standard and keep you posted on them here.

Editor’s Note: In the meantime, you can read EPA’s proposal to revise the secondary NAAQS for SO2 (and retain the existing secondary standards for oxides of nitrogen and particulate matter) here

EPA is accepting comments on its proposal through June 14. You can submit comments by any of the following means - you should reference Docket ID No. EPA-HQ-OAR-2014-1028 on any comments that you submit.

  • Federal eRulemaking Portal:

  • Email:

  • U.S. Mail:  addressed to U.S. Environmental Protection Agency, EPA Docket Center, Air and Radiation Docket, Mail Code 28221T, 1200 Pennsylvania Avenue NW, Washington, DC 20460.


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