Sulphur dioxide

Sulphur dioxide (SO2) is a colorless gas that smells like burnt matches. It can be chemically transformed into acidic pollutants such as sulpfuric acid and sulpfates (sulphates are a major component of fine particles). SO2 is generally a byproduct of industrial processes and burning of fossil fuels. Ore smelting, coal-fired power generators and natural gas processing are the main contributors. Sulphur dioxide is also the main cause of acid rain, which can damage crops, forests and whole ecosystems.

The PSL Assessment Report for PM10 specifies that PM10 can be released directly into the atmosphere or formed secondarily in the atmosphere from precursors as a result of physical or chemical transformations and identifies SO2 as one of the principal precursors to PM10, which is toxic and constitutes a danger in Canada to human life or health. An Order adding Sulphur Dioxide (SO2) to the List of Toxic Substances of CEPA 1999 was published in Part II of the Canada Gazette of July 2, 2003.

SO2 was also considered in the PSL Assessments for Releases from Primary and Secondary Copper Smelters and Copper Refineries and Releases from Primary and Secondary Zinc Smelters and Zinc Refineries. In these assessments, the risk due to SO2 released from copper smelters/ refineries and zinc plants was analysed based on both direct exposure to SO2 and on associated acidic deposition. Results for direct exposure indicate that there is a risk to vegetation over varying areas near both copper smelters/refineries and zinc plants, to a maximum distance of about 10 km. For acidic deposition, it was determined that copper smelters contributed up to 8% (relative to all anthropogenic and natural sources) of the SO2 resulting in acidic deposition at the four eastern Canadian receptor areas considered. It has been proven that exposure to SO2 in ambient air is associated with severe adverse respiratory effects in humans, and affects sensitive individuals with pre-existing respiratory conditions such as asthma or bronchitis. Also, as a precursor to respirable particulate matter (PM10), it has the potential for induction of cardio respiratory health effects.

CAS (Chemical Abstract Service) registry number: 7446-09-5

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