What is Sulfur Dioxide? (SO2)  (also sulphur dioxide)

It is a toxic, colourless gas or vapour with the characteristics of a strong, irritating and foul odour [1]. [2] Sulfur dioxide is non-flammable and reacts easily with other substances to form harmful compounds, such as sulfuric acid, sulfurous acid and sulfate particles. [3]

Sulfur dioxide is a by-product from activities related with the burning of fossil fuels (coal, oil) such as at power plants or from copper smelting. In nature, it can be released to the air, for example, from volcanic eruptions. [4]

What are the main Uses? [2]

  • As a fruit preserving agent and as a food preservative or additive.
  • In the fermentation stage of wine making.
  • For bleaching textile fibres.
  • In the manufacture of paper.
  • As a disinfectant in breweries and food factories.
  • As a fumigant for grains, grapes and citrus fruits.

Where is Sulfur dioxide found?  [2]

  • Industrial: Coal burning power plants; industrial processes such as wood pulping, paper manufacture, petroleum and metal refining and metal smelting, particularly from sulfide containing ores, e.g. lead, silver and zinc ores all emit sulfur dioxide to air.
  • Other: Textile bleaching and food preserving facilities and wineries, fumigation activities all emit sulfur dioxide to air.
  • Natural: Geothermal events, including hot springs and volcanic activity; sulfur dioxide is produced from the natural decay of vegetation on land, in wetlands and in oceans all emit sulfur dioxide to air.
  • Transport: Vehicle exhaust.
  • Consumer goods: Some solvents, DE chlorination agents, peroxides and disinfection products.

How much of an issue is sulfur dioxide in Australia? [9]

The quantities of sulfur dioxide in air is at satisfactory low levels in most Australian towns and cities. The maximum concentrations of sulfur dioxide in the air are found around oil refineries, chemical processing industries, mineral ore processing plants and power stations. 

Which are the most common Ways of Exposure? [4]

Contact to sulfur dioxide mainly occurs by breathing air that contains it. Contact may also result from skin contact to sulfur dioxide. The persons frequently exposed to sulfur dioxide are workers in plants where it occurs as a by-product, such as in the copper smelting industry and in the processing or burning of coal or oil. Other exposures occur in the manufacture of sulfuric acid, paper, food preservatives, and fertilisers. These workers could be exposed to levels of sulfur dioxide that are higher than typical outdoor air levels. In these conditions workers breathe in air containing sulfur dioxide, it is absorbed it into the body through the nose and lungs.

What are the most Serious Health Effects? [2,4]

Brief contact to high levels of sulfur dioxide can be life threatening. Exposure to 100 parts of sulfur dioxide per million parts of air (ppm) is considered immediately dangerous to life. An incident of a healthy non-smoking miners who breathed sulfur dioxide released as a result of an explosion in an underground copper mine developed burning of the nose and throat, breathing difficulties, and severe airway obstructions. If contact occurs to the eyes from liquid sulfur dioxide, (from, for example an industrial accident) can cause severe burns, resulting in vision loss. On the skin it produces burns. Other health effects include headache, general discomfort and anxiety. Those with impaired heart or lung function and asthmatics are at increased risk.

Prolonged contact to low levels of sulfur dioxide can also affect your health. Repeated or continued exposure to moderate concentrations can cause inflammation of the respiratory tract, wheezing and lung damage. Lung function changes have been observed in some workers exposed to 0.4–3.0 ppm sulfur dioxide for 20 years or more. However, these workers were also exposed to other chemicals, making it difficult to attribute their health effects to sulfur dioxide exposure alone. Additionally, exercising asthmatics are sensitive to the respiratory effects of low concentrations (0.25 ppm) of sulfur dioxide.

Engineering Controls

Ventilation: Provide good ventilation and/or local exhaust to prevent accumulation of concentrations above exposure limits.

How are gases or vapours removed from Airflow? [8]

Adsorption: The contaminant is entrapped to other materials such as activated alumina, activated carbon and silica gel.

Absorption: The removal of soluble reactive gases from the air stream by incorporating into the bulk volume of an appropriate liquid.

Catalytic conversion: In this process, a contaminant is converted to a chemical form not considered to be hazardous in the presence of a catalyst. Catalysts are substances that increase the rate of a chemical reaction without being affected by the chemical reaction.

Thermal Oxidation (Combustion): The combustion process (also called incineration) converts volatile organic compounds (VOCs) to carbon dioxide and water vapour by burning them. It is a very effective means of eliminating VOCs. Typical applications for incineration devices include odour control, reduction in reactive hydrocarbon emissions, and reduction of explosion hazards.

 US Regulation [2,6,7]

OSHA Permissible Exposure Limit (PEL) – General Industry
29 CFR 1910.1000 Table Z-1
5 ppm
(13 mg/m3) TWA
HE14 Upper respiratory irritation, nosebleeds
OSHA PEL – Construction Industry
29 CFR 1926.55 Appendix A
5 ppm
(13 mg/m3) TWA
HE14 Upper respiratory irritation, nosebleeds
OSHA PEL - Shipyard Employment
29 CFR 1915.1000 Table Z-Shipyards
5 ppm
(13 mg/m3) TWA
HE14 Upper respiratory irritation, nosebleeds
National Institute for Occupational Safety and Health (NIOSH) Recommended Exposure Limit (REL)

2 ppm
(5 mg/m3) TWA

5 ppm
(13 mg/m3) STEL

HE4 Blindness
HE9 Bronchoconstriction
HE11 Breathing difficulties
HE14 Eye and respiratory irritation, eye and skin burning
American Conference of Governmental Industrial Hygienists (ACGIH) Threshold Limit Value (TLV)(2009)

0.25 ppm
(0.65 mg/m3) STEL


HE9 Bronchoconstriction, exacerbation of asthma
HE10 Decreased lung function, chronic respiratory symptoms
HE11 Lower respiratory irritation and symptoms
HE14 Upper respiratory irritation

2 ppm
(5 mg/m3) TWA

5 ppm
(10 mg/m3) STEL

  • International Agency for Research on Cancer (IARC) carcinogenic classification:Class 3 (not classifiable as to its carcinogenicity to humans)
  • Agency for Toxic Substances and Disease Registry (ATSDR) Inhalation Minimal Risk Level (MRL): 0.01 ppm (acute)
  • NIOSH Immediately Dangerous to Life or Health (IDLH) concentration:100 ppm


 National Ambient Air Quality Standards and Goals:

  • Averaging period 1 hour: Maximum 0.2 parts per million, maximum allowable exceedences: 1 day a year.

  • Averaging period 1 day: Maximum 0.08 parts per million, maximum allowable exceedences: 1 day a year.

  • Averaging period 1 year: Maximum 0.02 parts per million, maximum allowable exceedences: none.

Safe Work Australia:

  • Time Weighted Average Concentration of 2ppm or 5.2 mg/maveraged over a 40 hour work week.
  • Short-term Exposure Limit of 5ppm or 13 mg/m3 time weighted average over 15 minutes


  1. http://en.wikipedia.org/wiki/Sulfur_dioxide
  2. http://www.npi.gov.au/resource/sulfur-dioxide
  3. http://www.environment.gov.au/resource/sulfur-dioxide-so2
  4. http://www.atsdr.cdc.gov/phs/phs.asp?id=251&tid=46
  5. http://avogadro.chem.iastate.edu/msds/sulfur_dioxide.pdf
  6. https://www.osha.gov/dts/chemicalsampling/data/CH_268500.html
  7. http://www.safeworkaustralia.gov.au/sites/SWA/about/Publications/Documents/639/Workplace_Exposure_Standards_for_Airborne_Contaminants.pdf
  8. http://www.ccohs.ca/oshanswers/prevention/ventilation/aircleaning.html
  9. http://www.environment.gov.au/protection/publications/factsheet-sulfur-dioxide-so2