Ground-level Ozone

What is ozone?

O₃ is a colourless, odourless gas at ambient concentrations and is a major component of smog.

What are the sources of ozone?

Ground-level ozone is not emitted directly into the atmosphere. It results from photochemical reactions between oxides of nitrogen (NO_x) and volatile organic compounds (VOCs) in the presence of sunlight. High levels typically occur from May to September, between noon and early evening.

Ontario VOC Emissions by Sector (Emissions From Human Activity, 2000 Estimates
Pie Chart	Category	Percent
	General Solvent Use	24%
	Road Vehicles	19%
	Other Processes	15%
	Residential	13%
	Other Transportation	11%
	Surface Coating	10%
	Miscellaneous Area Sources	8%

The above table and graph shows estimates of Ontario's VOC emissions caused by human activity, by sector. Transportation modes account for approximately 30 per cent of VOC emissions. Owing to the large forested area in Northern Ontario, biogenic emissions of certain VOCs are significant, approximately three times those from sources caused by human activity. Click here for the sources of NO_x. Significant amounts of ozone and ozone forming compounds are carried into Ontario from the U.S. During periods of widespread elevated levels of ozone, it is estimated that more than 50 per cent of Ontario's ground-level ozone comes from the U.S.

What are the effects of ozone?

O₃ irritates the respiratory tract and eyes. Exposure to high levels of O₃ results in chest tightness, coughing and wheezing. People with respiratory and heart problems are at a higher risk. Ozone has been linked to increased hospital admissions and premature death. Ozone causes agricultural crop loss each year in Ontario and noticeable leaf damage in many crops, garden plants and trees.

The following table shows the health effects of different Air Quality Index (AQI) levels caused by ground-level ozone.

Health effects of different Air Quality Index (AQI) levels caused by ground-level ozone
Category	AQI	Ozone(O3)
Very Good	0 - 15	No health effects are expected in healthy people.
Good	16 - 31	No health effects are expected in healthy people.
Moderate	32 - 49	Respiratory irritation in sensitive people during vigorous exercise; people with heart/lung disorders at some risk; damage to very sensitive plants.
Poor	50 - 99	Sensitive people may experience irritation when breathing and possible lung damage when physically active; people with heart/lung disorders at greater risk; damage to some plants.
Very Poor	100 or over	Serious respiratory effects, even during light physical activity; people with heart/lung disorders at high risk; more vegetation damage.

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