Sulfur Dioxide Basics

What is SO2 and how does it get in the air?

What is SO2?

EPA’s national ambient air quality standards for SO2 are designed to protect against exposure to the entire group of sulfur oxides (SOx).  SO2 is the component of greatest concern and is used as the indicator for the larger group of gaseous sulfur oxides (SOx).  Other gaseous SOx (such as SO3) are found in the atmosphere at concentrations much lower than SO2. 

Control measures that reduce SO2 can generally be expected to reduce people’s exposures to all gaseous SOx.  This may have the important co-benefit of reducing the formation of particulate sulfur pollutants, such as fine sulfate particles.

Emissions that lead to high concentrations of SO2 generally also lead to the formation of other SOx. The largest sources of SO2 emissions are from fossil fuel combustion at power plants andother industrial facilities. 

How does SO2 get in the air?

The largest source of SO2 in the atmosphere is the burning of fossil fuels by power plants and other industrial facilities. Smaller sources of SO2 emissions include: industrial processes such as extracting metal from ore; natural sources such as volcanoes; and locomotives, ships and other vehicles and heavy equipment that burn fuel with a high sulfur content.    

What are the harmful effects of SO2?

SO2 can affect both health and the environment.

What are the health effects of SO2?

Short-term exposures to SO2 can harm the human respiratory system and make breathing difficult. People with asthma, particularly children, are sensitive to these effects of SO2.

SO2 emissions that lead to high concentrations of SO2 in the air generally also lead to the formation of other sulfur oxides (SOx). SOx can react with other compounds in the atmosphere to form small particles. These particles contribute to particulate matter (PM) pollution. Small particles may penetrate deeply into the lungs and in sufficient quantity can contribute to health problems.

What are the environmental effects of SO2 and other sulfur oxides?

At high concentrations, gaseous SOx can harm trees and plants by damaging foliage and decreasing growth.  

SO2 and other sulfur oxides can contribute to acid rain which can harm sensitive ecosystems. 

Visibility

SO2 and other sulfur oxides can react with other compounds in the atmosphere to form fine particles that reduce visibility (haze) in parts of the United States, including many of our treasured national parks and wilderness areas. 

Deposition of particles can also stain and damage stone and other materials, including culturally important objects such as statues and monuments.

What is being done to reduce SO2 pollution?

EPA’s national and regional rules to reduce emissions of SO2 and pollutants that form sulfur oxides (SOx) will help state and local governments meet the Agency’s national air quality standards. 
EPA identifies areas where the air quality does not meet EPA SO2 standards. For these areas, state, local, and tribal governments develop plans to reduce the amount of SO2 in the air.

Sulfur Dioxide (SO2) Pollution

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Last updated on January 10, 2025