Sulfur dioxide in Washington's air

We monitor air and track emissions from industrial sources to ensure sulfur dioxide levels meet health-based air quality standards.

A model of a sulfur dioxide molecule.

Sulfur dioxide

Sulfur dioxide (SO2) is a colorless gas with a strong smell. It gets into the air when fuel that contains sulfur is burned. Major sources in Washington are:

  • Industrial facilities such as fossil fuel power plants, aluminum smelters, oil refineries, and pulp mills
  • Ships and locomotives

EPA set National Ambient Air Quality Standards (NAAQS) for SO2, which specify levels of the pollutant in outdoor air that can impact public health, or harm the environment. There are two current NAAQS:

SO2 pollution in Washington

Thanks to improved pollution controls on industrial facilities and cleaner transportation fuels, Washington's SO2 levels have declined dramatically in recent decades.

Following EPA adoption of a new standard in 2010, Ecology evaluated SO2 ambient levels in 36 out of 39 counties and found that all of these areas comply with the standard.

The three remaining counties awaiting designation are Chelan, Douglas and Whatcom. At this time, we have completed air quality data collection and analysis for Douglas and Chelan counties. To review our findings and to comment on our recommendation on how EPA should classify these counties, please visit  the  Sulfur Dioxide Air Quality Designation web page.

Monitoring SO2 in the air

We monitor SO2 at six locations:

Health and environmental effects

SO2 irritates the respiratory system and causes inflammation and tightening of airways. Children, older adults, and people with asthma are at increased risk for health effects after SO2 exposure. Sulfate-containing particles can contribute to haze that reduces visibility.

Trees and plants can be harmed by sulfur dioxide in high concentrations. It can contribute to acid rain that harms sensitive ecosystems. When it reacts with fine particles in the air, it reduces visibility, causing a haze.

What you can do

There are some things you can do to protect yourself from SO2. You can:

  • Limit outdoor activities during times of high air pollution.
  • Close windows and vents during periods of high air pollution.
  • Consult with your health care provider about a personal mask and air purification device.