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 2000 came from smelters and utilities, especially electrical generation. Other industrial sources include iron and steel mills, petroleum refineries, and pulp and paper mills. Small sources include residential, commercial and industrial space heating.

Ontario Sulphur Dioxide Emissions by Sector (Emissions From Human Activity, 2000 Estimates)
Pie Chart	Category	Percent
	Smelters	42%
	Utilities	27%
	Area Sources	10%
	Refineries	10%
	Other Processes	7%
	Primary Metals	4%

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	Sulphur Dioxide (SO2)
Very Good	0 - 15	No health effects are expected in healthy people.
Good	16 - 31	Damages some vegetation in combination with ozone.
Moderate	32 - 49	Damages some vegetation.
Poor	50 - 99	Odour; increasing vegetation damage.
Very Poor	100 or over	Increasing sensitivity for asthmatics and people with bronchitis.

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