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Air Quality Program

Sulfur Dioxide (SO2)

Sulfur dioxide is a gas or liquid that results from burning fuel with sulfur in it. Common sources include:

  • Fossil fuel power plants
  • Non-ferrous smelters
  • Kraft pulp production

Health Effects

It increases symptoms in people with asthma and irritates the respiratory system.

National Ambient Air Quality Standard (NAAQS) for Sulfur Dioxide

In 2010, EPA adopted a more protective sulfur dioxide standard of 75 parts per billion averaged over 1 hour. We evaluated the levels of SO2 in the air in Washington’s 36 counties and sent to EPA our recommendation and response to public comment about how each area is meeting the standard. Ecology's recommendation for designation:

  • Lewis and Thurston Counties — attainment, based on modeling of emissions data
  • Other 34 counties — attainment/unclassifiable, based on available air quality data and absence of large SO2 sources or emissions
  • Chelan, Douglas, and Whatcom Counties — coming in 2020, after we collect monitoring data
Technical Table
Pollutant Averaging Time Level Remarks Measurement Method Interpretation Method
Sulfur Dioxide Annual* 0.02 ppmv Not to be exceeded in a calendar year 40 C.F.R. Part 50, Appendix A-1 or A-2 WAC 173-476-130(3)
24-hour* 0.14 ppmv Not to be exceeded more than once per year
3-hour 0.5 ppmv Not to be exceeded more than once per year
1-hour 75 ppbv 99th percentile of 1-hour daily maximum concentrations, averaged over 3 years

* The annual and 24-hour SO2 standards will no longer apply in a specific area one year after EPA has designated an area in attainment for SO2.


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