Air pollution

Sulfur Dioxide

Clean Air Student Council

What is Sulfur dioxide?


Sulfur dioxide is the chemical compound with the formula SO2. It is a toxic gas responsible for the smell of burnt matches. Sulfur dioxide has a pungent smell. Sulfur dioxide is found on Earth and exists in very small concentrations and in the atmosphere at about 1 ppm.


Where does it come from?


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. Sulfur dioxide is also produced as a by-product of copper extraction. Sulfur dioxide is primarily produced for sulfuric acid manufacture. Most sulfur dioxide is produced by the combustion of elemental sulfur. Some sulfur dioxide is also produced by roasting pyrite and other sulfide ores in air.


What negative impacts does it have?


Health effects

Sulfur dioxide affects the respiratory system, particularly lung function, and can irritate the eyes.

Sulfur dioxide irritates the respiratory tract and increases the risk of tract infections. It causes coughing, mucus secretion and aggravates conditions such as asthma and chronic bronchitis. Therefore, sulfur dioxide is a significant air pollutant and poses severe impacts on human health. 

People sensitive to sulfur dioxide include:

  • People with lung diseases, such as asthma, chronic bronchitis, and emphysema will generally have more serious health effects at higher SO2 levels.
  • Children are at higher risk from SO2 exposure because their lungs are still developing. They are also more likely to have asthma, which can get worse with SO2 exposure.
  • Older adults may be more affected by SO2 exposure, possibly because they are more likely to have pre-existing lung or cardiovascular disease.
  • Active people of all ages who exercise or work outdoors have higher exposure to sulfur dioxide than people who are less active.


Environmental effects

When sulfur dioxide combines with water and air, it forms sulfuric acid, which is the main component of acid rain. Acid rain can:

  • cause deforestation
  • acidify waterways to the detriment of aquatic life
  • corrode building materials and paints.

In addition, the concentration of sulfur dioxide in the atmosphere can influence the habitat suitability for plant communities, as well as animal life. For example, at high concentrations, sulfur oxides can harm trees and plants by damaging foliage and decreasing growth. Sulfur dioxide emissions are a precursor to atmospheric particulates as well.1 Moreover, sulfur dioxide and other sulfur oxides can react with other compounds in the atmosphere to form fine particles that reduce visibility (haze). 


Other statistics:

As of 2006, China was the world’s largest sulfur dioxide polluter, with 2005 emissions estimated to be 25,490,000 short tons (23.1 Mt). This amount represents a 27% increase since 2000, and is roughly comparable with U.S. emissions in 1980.


Air quality standard

The recommended air quality standards for sulfur dioxide are:

  • 0.20 parts per million (ppm) for a 1-hour exposure period
  • 0.08ppm for a 24-hour exposure period
  • 0.02ppm for an annual exposure period.

These standards are designed to protect sensitive individuals, such as children and asthmatics.