What Is Sulfur Dioxide?

Sulfur dioxide (SO2) is a gaseous air pollutant composed of sulfur and oxygen. SO2 forms when sulfur-containing fuel such as coal, petroleum oil, or diesel is burned. Sulfur dioxide gas can also change chemically into sulfate particles in the atmosphere, a major part of fine particle pollution, which can blow hundreds of miles away. 

What Are the Health Effects of Sulfur Dioxide Pollution?

Sulfur dioxide causes a range of harmful effects on the lungs:

  • Wheezing, shortness of breath and chest tightness and other problems, especially during exercise or physical activity. Rapid breathing during exercise helps SO2 reach the lower respiratory tract, as does breathing through the mouth.
  • Long-term exposure at high levels increases respiratory symptoms and reduces the ability of the lungs to function.
  • Short exposures to peak levels of SO2 in the air can make it difficult for people with asthma to breathe when they are active outdoors.
  • Increased risk of hospital admissions or emergency room visits, especially among children, older adults and people with asthma.

What Are the Sources of Sulfur Dioxide Emissions?

As of 2020, human-made sources in the U.S. emit about 1.8 million short tons of sulfur dioxide per year (down from just over 6 million short tons per year in 2011) mainly from burning fuels. Power plants, commercial and institutional boilers, internal combustion engines, manufacturing, and industrial processes such as petroleum refining and metal processing are the largest sources of emissions, followed by diesel engines in old buses and trucks, locomotives, ships, and off-road equipment such as construction vehicles. Emissions of sulfur dioxide will decline as cleanup of many of these sources continue in future years.

Where Do High SO2 Concentrations Occur?

Coal-fired power plants remain one of the biggest sources of sulfur dioxide in the U.S. Columns of emissions (plumes) such as from chimneys of a coal-fired power plant are moved by wind over long distances before touching down at ground level at far away sites. These plumes could also get trapped at the ground level by unusual weather conditions such as a layer of warmer air occurring higher up in the atmosphere (inversion).

Ports, smelters, and other sources of sulfur dioxide also cause high concentrations of emissions nearby.

People who live and work near these large sources get the highest exposure to SO2.

What Can We Do about it?

SO2 levels have improved over time, thanks to policies requiring cleaner fuels and pollution controls on power plants. The nation achieved major reductions in this pollutant through its successful program to reduce acid rain.

However, it remains a health concern. What’s more, even with pollution controls installed, high levels can occur when a polluting source such as a power plant is starting up or shutting down its operation or if its equipment malfunctions.

Individuals can take steps to protect themselves on days with unhealthy levels of air pollutants and also ask policymakers at all levels of government to continue to require cleanup of air pollution.

Page last updated: October 26, 2023

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