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, oil, or diesel is burned. Sulfur dioxide also converts in the atmosphere to sulfates, a major part of fine particle pollution in the eastern U.S.

What Are the Sources of SO2 Emissions?

Manmade sources in the U.S. emitted more than 6.4 million tons of sulfur dioxide in the most recent reports.1 The largest sources of sulfur dioxide emissions are electricity generation, industrial boilers, and other industrial processes such as petroleum refining and metal processing. Diesel engines are another major source, including old buses and trucks, locomotives, ships, and off-road diesel equipment.

What Causes High Concentrations of SO2?

Coal-fired power plants remain one of the biggest sources of sulfur dioxide in the U.S., particularly in the eastern states. The plume from a coal-fired power plant touches down at ground level during high wind conditions or gets trapped by inversions in the atmosphere. High levels can happen during start-up, shutdown, upsets, and malfunctions of pollution control equipment.

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

People who live and work nearby these large sources get the highest exposure to SO2. After SO2 gets into the air, it changes chemically into sulfate particles, which can blow hundreds of miles away.

What Are the Health Effects of Sulfur Dioxide Air Pollution?

Sulfur dioxide causes a range of harmful effects on the lungs, as the EPA's most recent review of the science concluded:

  • Wheezing, shortness of breath and chest tightness and other problems, especially during exercise or physical activity.
  • Continued 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.
  • Rapid breathing during exercise helps SO2 reach the lower respiratory tract, as does breathing through the mouth.
  • Increased risk of hospital admissions or emergency room visits, especially among children, older adults and people with asthma.2
  1. U.S. Environmental Protection Agency. Data from the National Emissions Inventory, 2011. Accessed August 23, 2016.

  2. U.S. EPA. Integrated Science Assessment for Sulfur Oxides - Health Criteria. EPA/600/R-08/047F, September 2008.

Page last updated: February 12, 2020

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