Audit, Compliance and Risk Blog

EPA Reaffirms National Sulfur Oxide Standard

Posted by Jon Elliott on Tue, Apr 02, 2019

FreewayThe Environmental Protection Agency (EPA) has completed a long review, and reaffirmed the primary National Ambient Air Quality Standard (NAAQS) for oxides of sulfur (SOX; usually measured as sulfur dioxide (SO2)). This is the first review of the primary SOX NAAQs since 2010 (primary standard – EPA did not review the secondary SOX NAAQS established in 2012).

What is EPA’s Responsibility to Establish and Administer NAAQSs?

The Clean Air Act (CAA) requires EPA to identify and list “conventional” air pollutants that meet the following conditions:

  • Emissions of the pollutant cause or contribute to air pollution, which may reasonably be anticipated to endanger public health or welfare.

  • Presence of the pollutant in ambient air results from “numerous or diverse mobile or stationary sources”.

  • EPA intends to issue air quality criteria for the pollutant.

EPA evaluates atmospheric concentrations that would lead to unacceptable health and environmental impacts, and sets NAAQSs for atmospheric (ambient air) concentrations. EPA sets NAAQSs (primary and secondary standards) for SOX, as well as for carbon monoxide (CO), lead, nitrogen dioxide, ozone, and particulates (measured separately as PM-10 (particles with an aerodynamic diameter less than or equal to a nominal 10 microns) and as PM-2.5 (less than or equal to 2.5 microns)). NAAQSs must “accurately reflect the latest scientific knowledge useful in indicating the kind and extent of all identifiable effects on public health or welfare….” CAA requires EPA to re-evaluate each NAAQS at least every 5 years, but EPA often misses these deadlines. EPA revises its review policies from time to time, most recently when former Administrator Pruitt promulgated “Back-to-Basics” Process for Reviewing [NAAQSs]” in May 2018 (I wrote about this policy here).

What are the National SOX Standards?

EPA’s SOX standards use SO2 concentrations as an indicator for oxides of sulfur, and include a primary standard (which CAA requires to be “requisite to protect the public health” with an adequate margin), and a secondary standard (which CAA requires to be “requisite to protect the public welfare from any known or anticipated adverse effects).” (42 USC sec. 7409). EPA has just reaffirmed the following:

  • Primary standard: 75 parts per billion (ppb), as the 99th percentile of daily maximum 1-hour SO2 concentrations, averaged over 3 years.

  • Secondary standard: 3-hour average standard set at 0.5 ppm, not to be exceeded more than once per year (established in 1971).

In addition, EPA is considering the welfare effects of SOX and the ecological effects of particulate atmospheric transformation products in its separate review of the secondary NAAQS for SOX, NOX, and PM. EPA is also considering the visibility, climate, and materials damage-related welfare effects of particulate sulfur compounds are in its separate review of the secondary NAAQS for PM.

What Now?

Since EPA reaffirmed the existing primary NAAQS, there are unlikely to be any incremental compliance requirements. It’s possible that other rulemakings, such as the secondary NAAQS rulemaking, may produce additional requirements. However, the Trump Administration is loath to increase regulation.

However, readers should note that states can and do promulgate stricter standards. For example, California has stricter primary standards based on one-hour and 24-hour averages, and also administers an ambient air quality standard for “sulfates.”

Self Assessment Checklist

Does the organization conduct activities that emit SOX?

  • If so, are any of these activities characterized as “major sources” that are subject to the most extensive permit requirements?

  • If so, are any of these activities in an area in which ambient concentrations are close to the NAAQS, where stricter standards may have led to stricter compliance requirements?

Where Do I Go For More Information?

Specialty Technical Publishers (STP) provides a variety of single-law and multi-law services, intended to facilitate clients’ understanding of and compliance with requirements. 

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About the Author

Jon Elliott is President of Touchstone Environmental and has been a major contributor to STP’s product range for over 25 years. 

Mr. Elliott has a diverse educational background. In addition to his Juris Doctor (University of California, Boalt Hall School of Law, 1981), he holds a Master of Public Policy (Goldman School of Public Policy [GSPP], UC Berkeley, 1980), and a Bachelor of Science in Mechanical Engineering (Princeton University, 1977).

Mr. Elliott is active in professional and community organizations. In addition, he is a past chairman of the Board of Directors of the GSPP Alumni Association, and past member of the Executive Committee of the State Bar of California's Environmental Law Section (including past chair of its Legislative Committee).

You may contact Mr. Elliott directly at:

photo credit: ITB495 I-69 Construction Indiana via photopin (license)

Tags: Environmental risks, Environmental, EPA, Greenhouse Gas, ghg, SOX, CAA