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 (NOx) and volatile organic compounds (VOCs) in the presence of sunlight. High levels typically occur from May to September, between noon and early evening. Additionally, significant amounts of ozone and its precursors are carried into Ontario from the U.S.

What are the effects of ozone?

O3 irritates the respiratory tract and eyes. Exposure to high levels 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 Ontario Ambient Air Quality Criteria (AAQC) for 1-hour average ozone concentrations is 80 parts per billion (ppb), which has been incorporated into Ontario’s Air Quality Health Index to better protect Ontarians.