An official website of the United States government.

We've made some changes to EPA.gov. If the information you are looking for is not here, you may be able to find it on the EPA Web Archive or the January 19, 2017 Web Snapshot.

Sulfur Dioxide (SO2) Pollution

Secondary National Ambient Air Quality Standards (NAAQS) for Nitrogen Dioxide (NO2) and Sulfur Dioxide (SO2)

Rule Summary

In 2012, EPA decided to retain the current secondary NAAQS for NO2 and SO2. The secondary NO2 standard is an annual arithmetic mean of 53 ppb. The secondary SO2 standard is 500 ppb averaged over three hours, not to be exceeded more than once per year.

Rule History

04/03/2012 – Final Decision to Retain the Secondary NAAQS for NO2 and SO2

08/01/2011 – Proposed Decision to Retain the Secondary NAAQS for NO2 and SO2

Additional Resources

Fact Sheets and Additional Information Regarding the 2012 Decision to Retain the Secondary NAAQS for NO2 and SO2

Related Rules

Primary National Ambient Air Quality Standard for Sulfur Dioxide

Primary National Ambient Air Quality Standards  for Nitrogen Dioxide

Scientific and Technical Documents Supporting the secondary NAAQS for SO2 and NO2