Sulphur Dioxide (SO2)

What is sulphur dioxide?

SO2 is a colourless gas. It smells like burnt matches. It can be oxidized to sulphur trioxide, which in the presence of water vapour is readily transformed to sulphuric acid mist. SO2 can be oxidized to form acid aerosols. SO2 is a precursor to sulphates, which are one of the main components of respirable particles in the atmosphere.

What are the sources of SO2?

Approximately 66% of the SO2 emitted in Ontario in 2012 came from smelters and utilities. Other SO2 industrial processes each accounted for approximately 26% of SO2 emissions. Lesser sources of SO2 include transportation and miscellaneous/residential.

Ontario Sulphur Dioxide Emissions by Sector - 2012 Estimates

Chart: Ontario Sulphur Dioxide Emissions by Sector - 2012

Ontario Sulphur Dioxide Emissions by Sector - 2012
Category Percent
Smelters 62%
Other SO2 Industrial Sources 26%
Transportation 5%
Utilities 4%
Miscellaneous/Residential 3%

Note: 2012 is the latest complete inventory. Emissions may be revised with updated source/sector information or emission estimation methodologies as they become available.

What are the effects of SO2?

Health effects caused by exposure to high levels of SO2 include breathing problems, respiratory illness, changes in the lung's defences, and worsening respiratory and cardiovascular disease. People with asthma or chronic lung or heart disease are the most sensitive to SO2. It also damages trees and crops. SO2, along with nitrogen oxides, are the main precursors of acid rain. This contributes to the acidification of lakes and streams, accelerated corrosion of buildings and reduced visibility. SO2 also causes formation of microscopic acid aerosols, which have serious health implications as well as contributing to climate change.

The Ontario Ambient Air Quality Criteria (AAQC) for 1-hour average SO2 concentrations is 250 parts per billion (ppb), which has been incorporated into Ontario’s Air Quality Health Index to better protect Ontarians. For more information on how the Air Quality Health Index has been modified for reporting in Ontario, please visit the Frequently Asked Questions.