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 Volatile Organic Compounds Emissions by Sector (Emissions from Point/Area/Transportation Sources, 2006 Estimates)
Category	Percent
Other Transportation	24%
Printing/Surface Coating	19%
General Solvent Use	18%
Road Vehicles	14%
Other Industrial Processes	14%
Residential	8%
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.

Approximately 38 per cent of the VOCs emitted in Ontario in 2006 came from the transportation sectors. Printing/surface coating and general solvent use were the second and third largest sources of VOC emissions, accounting for approximately 19 per cent and 18 per cent, respectively. Click here for the sources of NO_x. Significant amounts of ozone and its precursors 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	Pollutant Concentration Breakpoints (ppb)	Ozone(O3)
Very Good	0 - 15	0 - 23	No health effects are expected in healthy people.
Good	16 - 31	24 - 50	No health effects are expected in healthy people.
Moderate	32 - 49	51 - 80*	Respiratory irritation in sensitive people during vigorous exercise; people with heart/lung disorders at some risk; damage to very sensitive plants.
Poor	50 - 99	81 - 149	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	150 or over	Serious respiratory effects, even during light physical activity; people with heart/lung disorders at high risk; more vegetation damage.

* Ontario 1-hour AAQC for O₃ = 80 ppb

Note: ppb = parts per billion.