Sulfur dioxide

Sulfur dioxide (SO2) is a colorless gas with a pungent odor that is used as a preservative and antioxidant in food and beverages. It is also used in the production of sulfuric acid, paper, and textiles. In the atmosphere, sulfur dioxide reacts with other chemicals to form sulfuric acid, which contributes to acid rain and can have adverse effects on the environment and human health. Prolonged exposure to high concentrations of sulfur dioxide can cause respiratory problems and other health issues. It is important to handle sulfur dioxide with care and to follow safety precautions when working with it.


Sulfur dioxide has a variety of uses, including:

  1. Precursor to sulfuric acid: Sulfur dioxide is an intermediate in the production of sulfuric acid and is converted to sulfur trioxide and then to oleum. This is called the contact process and several billion kilograms are produced annually for this purpose.
  2. Food preservative: Sulfur dioxide is sometimes used as a preservative for dried fruits and other food products, such as sulfured molasses, due to its antimicrobial properties and ability to prevent oxidation.
  3. Winemaking: Sulfur dioxide is widely used in winemaking as an antibiotic and antioxidant, protecting wine from spoilage by bacteria and oxidation.
  4. Reducing agent: Sulfur dioxide can act as a reducing agent, decolorizing substances in the presence of water. It is also used to treat chlorinated wastewater prior to release, reducing free and combined chlorine to chloride.
  5. Bleaching: Sulfur dioxide is used as a reducing bleach for papers and delicate materials such as clothes.
  6. Sanitation: In wineries, a mixture of sulfur dioxide, water, and citric acid is commonly used to clean and sanitize equipment.
  7. Pulp and paper production: Sulfur dioxide is used in the production of wood pulp and paper, as well as in the bleaching process of wood pulp.
  8. Other industrial applications: Sulfur dioxide is used in various industrial applications, such as in the manufacture of detergents, fertilizers, and rubber.



Sulfur dioxide is routinely encountered through incidental exposure, such as the smoke from matches, coal, and sulfur-containing fuels. It is mildly toxic and can be hazardous in high concentrations. Long-term exposure to low concentrations can also be problematic. A systematic review concluded that exposure to sulfur dioxide is associated with preterm birth.

U.S. regulations

The American Conference of Governmental Industrial Hygienists reduced the short-term exposure limit to 0.25 parts per million (ppm) in 2008. The Occupational Safety and Health Administration (OSHA) set the permissible exposure limit (PEL) at 5 ppm (13 mg/m3) time-weighted average. The National Institute for Occupational Safety and Health (NIOSH) set the immediate danger to life and health (IDLH) level at 100 ppm. The Environmental Protection Agency (EPA) revised the primary sulfur dioxide National Ambient Air Quality Standards (NAAQS) in 2010, establishing a one-hour standard at a level of 75 parts per billion (ppb).


The Center for Science in the Public Interest in the US lists the two food preservatives, sulfur dioxide and sodium bisulfite, as safe for human consumption except for certain asthmatic individuals who may be sensitive to them, especially in large amounts. Sensitivity to sulfiting agents, including sulfur dioxide, can manifest as potentially life-threatening difficulty breathing within minutes of ingestion. Sulphites may also cause symptoms in non-asthmatic individuals, including dermatitis, urticaria, flushing, hypotension, abdominal pain and diarrhea, and even life-threatening anaphylaxis.

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