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 69 per cent of the SO₂ emitted in Ontario in 2006 came from smelters and utilities. Downstream petroleum industry and other industrial processes each accounted for approximately 9 per cent of SO₂ emissions. Lesser sources of SO₂ include transportation, cement and concrete, and miscellaneous.

Ontario Sulphur Dioxide Emissions by Sector (Emissions from Point/Area/Transportation Sources, 2006 Estimates)

Category	Percent
Smelters	50%
Utilities	19%
Downstream Petroleum Industry	9%
Other Processes	9%
Cement and Concrete	6%
Transportation	4%
Miscellaneous	3%

Note: 2006 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 following table shows the health effects of different Air Quality Index levels caused by sulphur dioxide.

Health effects of different Air Quality Index (AQI) levels caused by sulphur dioxide
Category	AQI	Pollutant Concentration Breakpoints (ppb)	Sulphur Dioxide (SO₂)
Very Good	0 - 15	0 - 79	No health effects are expected in healthy people.
Good	16 - 31	80 - 169	Damages some vegetation in combination with ozone.
Moderate	32 - 49	170 - 250*	Damages some vegetation.
Poor	50 - 99	251 - 1999	Odour; increasing vegetation damage.
Very Poor	100 or over	2000 or over	Increasing sensitivity for asthmatics and people with bronchitis.