Sulfur dioxide (SO2)

Sulfur dioxide (SO2), a colorless, bad-smelling, toxic gas, is part of a larger group of chemicals referred to as sulfur oxides (SOx). These gases, especially SO2, are emitted by the burning of fossil fuels — coal, oil, and diesel — or other materials that contain sulfur. Sources include power plants, metals processing and smelting facilities, and vehicles. Diesel vehicles and equipment have long been a major source of sulfur dioxide, but recent federal regulations to reduce the sulfur content of diesel fuels have made a significant improvement in emissions from this sector.  Sulfur dioxide is also a natural byproduct of volcanic activity.

Like nitrogen dioxide, sulfur dioxide can create secondary pollutants once released into the air. Secondary pollutants formed with sulfur dioxide include sulfate aerosols, particulate matter, and acid rain.

Health impacts of sulfur dioxide (SO2)

Sulfur dioxide, associated SOx, and secondary pollutants can contribute to respiratory illness by making breathing more difficult, especially for children, the elderly, and those with pre-existing conditions. Longer exposures can aggravate existing heart and lung conditions, as well. Sulfur dioxide and other SOx are partly culpable in the formation of thick haze and smog, which can impair visibility in addition to impacting health.

Beyond human health impacts, sulfur dioxide’s contribution to acid rain can cause direct harm to trees and plants by damaging exposed tissues and, subsequently, decreasing plant growth. Other sensitive ecosystems and waterways are also impacted by acid rain.

For additional information about the health impacts of air pollution in Minnesota, you can check out our most recent air quality report, or visit the Air quality and health webpage. You can also sign up for air quality alerts and forecasts, and check out current air quality.

Air quality standards that help protect us from adverse effects of sulfur dioxide (SO2)

The Clean Air Act regulates sulfur dioxide as a criteria pollutant. The U.S. EPA sets National Ambient Air Quality Standards (NAAQS) for sulfur dioxide, including both primary standards to protect public health and secondary standards to protect the environment.

In 2010 and 2012, the EPA reviewed the science related to the human health and environmental impacts of sulfur dioxide, respectively, and revised the NAAQS to reflect the most up-to-date information. The hourly primary standard sulfur dioxide emissions is currently 75 ppb and the three-hour secondary standard is 500 ppb.

The state of Minnesota is currently in compliance with the national standards for sulfur dioxide. To see the MPCA’s monitoring data for SO2 and other criteria pollutants, explore our Criteria Pollutant Data Explorer.

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