Sulphur Dioxide


Sulphur dioxide is the chemical compound with the formula SO2. It is a toxic gas with a pungent, irritating smell. [1] It is a liquid when under pressure, and it dissolves in water very easily. [2] Sulphur dioxide gas is heavier than air. In water, the solution is a medium strength acid. It reacts violently with ammonia, acrolein, acetylene, alkali metals, chlorine, ethylene oxide, amines, butadiene. It also reacts with water or steam causing a corrosion hazard. Sulphur dioxide attacks many metals including aluminium, iron, steel, brass, copper and nickel in presence of water and is incompatible with halogens. It attacks plastics, rubber and coatings in liquid form. [3] Sulphur dioxide in the air comes mainly from activities such as the burning of coal and oil at power plants or from copper smelting. In nature, sulphur dioxide can be released to the air from volcanic eruptions. [2]

Uses [1]

  • Precursor to sulphuric acid: Sulphur dioxide is an intermediate in the production of sulphuric acid. Sulphur dioxide is converted to sulphur trioxide, and then to oleum, which is made into sulphuric acid via a method called the contact process.
  • As a preservative: Sulphur dioxide is sometimes used as a preservative for dried apricots, dried figs, and other dried fruits owing to its antimicrobial properties, and it is sometimes called E220 when used in this way. As a preservative, it maintains the colourful appearance of the fruit and prevents rotting. It is also added to sulphured molasses.
  • In winemaking: Sulphur dioxide is an important compound in winemaking. It serves as an antibiotic and antioxidant, protecting wine from spoilage by bacteria and oxidation. Its antimicrobial action also helps to minimise volatile acidity. SO2 is also a very important compound in winery sanitation. Wineries and equipment must be kept clean, and because bleach cannot be used in a winery due the risk of cork taint, a mixture of SO2, water, and citric acid is commonly used to clean and sanitise equipment.
  • As a reducing agent: Sulphur dioxide is a good reductant. In the presence of water, sulphur dioxide is able to decolourise substances. Specifically it is a useful reducing bleach for papers and delicate materials such as clothes. This bleaching effect normally does not last very long. Oxygen in the atmosphere reoxidises the reduced dyes, restoring the colour. In municipal wastewater treatment, sulphur dioxide is used to treat chlorinated wastewater prior to release. Sulphur dioxide reduces free and combined chlorine to chloride.
  • Biochemical and biomedical roles: Sulphur dioxide is toxic in large amounts. It is responsible for blocking nerve signals from the pulmonary stretch receptors (PSRs) and abolishing the Hering–Breuer inflation reflex.
  • As a refrigerant: Being easily condensed and possessing a high heat of evaporation, sulphur dioxide is a candidate material for refrigerants. Prior to the development of CFCs, sulphur dioxide was used as a refrigerant in home refrigerators.
  • As a reagent and solvent in the laboratory: Sulphur dioxide is a versatile inert solvent that has been widely used for dissolving highly oxidising salts. It is also used occasionally as a source of the sulfonyl group in organic synthesis.

Sources of Emission & Routes of Exposure

Sources of Emission [4]

  • Industry sources: Fossil fuel combustion sites particularly coal burning power plants; industrial processes such as wood pulping, paper manufacture, petroleum and metal refining and metal smelting, particularly from sulphide containing ores, e.g. lead, silver and zinc ores all emit sulphur dioxide to air.
  • Diffuse sources: Small textile bleaching and food preserving facilities and wineries, fumigation activities all emit sulphur dioxide to air.
  • Natural sources: Geothermal activity, including hot springs and volcanic activity; sulphur dioxide is produced from the natural decay of vegetation on land, in wetlands and in oceans all emit sulphur dioxide to air.
  • Transport sources: Vehicle exhaust.
  • Consumer products: Some solvents, dechlorination agents, bleaches and fumigation products.

Routes of Exposure [4]

The main route of exposure to sulphur dioxide is via the inhalation of contaminated air. Upon entry, nose, throat and lungs may be affected. Sulphur dioxide can also enter our bodies when we eat or drink food or beverages (wine), which contain sulphur dioxide as a preservative. Sulphur dioxide can enter the body through skin contact.

Health Effects [5]

Acute Effects

•Inhalation: Sulphur dioxide is very toxic and can result in death. It can also cause severe irritation of the nose and throat. At high concentrations it can cause life-threatening accumulation of fluid in the lungs (pulmonary oedema). Symptoms may include coughing, shortness of breath, difficult breathing and tightness in the chest. A single exposure to a high concentration can cause a long-lasting condition like asthma. If this occurs, many things like other chemicals or cold temperatures can easily irritate the airways. Symptoms may include shortness of breath, tightness in the chest and wheezing. {Reactive Airways Dysfunction Syndrome (RADS)}.

•Skin Contact: Sulphur dioxide is corrosive. The gas irritates or burns the skin. Permanent scarring can result. Direct contact with the liquefied gas can chill or freeze the skin (frostbite). Symptoms of mild frostbite include numbness, prickling and itching. Symptoms of more severe frostbite include a burning sensation and stiffness. The skin may become waxy white or yellow. Blistering, tissue death and infection may develop in severe cases.

•Eye Contact: Sulphur dioxide is corrosive. The gas irritates or burns the eyes. Permanent damage including blindness can result. Direct contact with the liquefied gas can freeze the eye. Permanent eye damage or blindness can result.

Chronic Effects

May harm the respiratory system. Can irritate and inflame the airways.

Sulphur dioxide is not known to cause cancer. The International Agency for Research on Cancer (IARC) has classified sulphur dioxide as a Group 3 chemical – Not classifiable as to its carcinogenicity to humans. The American Conference for Governmental Industrial Hygienists (ACGIH) has classified it as: A4 – Not classifiable as a human carcinogen. Sulphur dioxide is a suspect mutagen. May cause genetic damage based on animal information.

Safety [6]

First Aid Measures

Inhalation: If breathed in, move person into fresh air. If not breathing, give artificial respiration. Consult a physician.

Skin Contact: Take off contaminated clothing and shoes immediately. Wash off with soap and plenty of water. Take victim immediately to hospital. Consult a physician.

Eye Contact: Rinse thoroughly with plenty of water for at least 15 minutes and consult a physician.

Ingestion: Do NOT induce vomiting. Never give anything by mouth to an unconscious person. Rinse mouth with water. Consult a physician.

Exposure Controls & Personal Protective Equipment

  • Respiratory protection: Where risk assessment shows air-purifying respirators are appropriate use a full-face respirator with multipurpose combination (US) or type AXBEK (EN 14387) respirator cartridges as a backup to engineering controls. If the respirator is the sole means of protection, use a full-face supplied air respirator. Use respirators and components tested and approved under appropriate government standards such as NIOSH (US) or CEN (EU).
  • Hand protection: Handle with gloves. Gloves must be inspected prior to use. Use proper glove removal technique (without touching glove’s outer surface) to avoid skin contact with this product. Dispose of contaminated gloves after use in accordance with applicable laws and good laboratory practices. Wash and dry hands.
  • Eye protection: Tightly fitting safety goggles. Faceshield (8-inch minimum). Use equipment for eye protection tested and approved under appropriate government standards such as NIOSH (US) or EN 166(EU).
  • Skin and body protection: Complete suit protecting against chemicals, The type of protective equipment must be selected according to the concentration and amount of the dangerous substance at the specific workplace.
  • Hygiene measures: Avoid contact with skin, eyes and clothing. Wash hands before breaks and immediately after handling the product.

Regulation [7,8]

Exposure Limits

United States

  • The Occupational Safety and Health Administration (OSHA) has set a limit of 2 ppm over an 8-hour workday, 40-hour workweek
  • EPA has set an air quality standard of 0.03 ppm for long-term, 1-year average concentrations of sulphur dioxide. Short-term, 24-hour air concentrations should not exceed 0.14 ppm more than once a year.


Safe Work Australia has established the following limits for sulphur dioxide:

  • TWA of 2 ppm
  • TWA of 5.2mg/m3
  • STEL of 5ppm
  • STEL of 13mg/ m3