Through the media, residents often hear about Ontario’s Air Quality Health Index (AQHI) and air-quality alerts issued by the Ontario government. Unlike the U.S. Air Quality Index (AQI), Ontario’s AQHI is a health-based scale from 1 to 10+, calculated using concentrations of ground-level ozone, fine particulate matter (PM2.5), and nitrogen dioxide. The higher the number, the greater the potential health risk.

Air pollution in Ontario comes from both local and long-distance sources. Pollution can travel across provincial and international borders, including from the United States, while weather conditions such as stagnant air can allow pollutants to accumulate. Wildfire smoke has also become an increasingly important source of fine particulate pollution in Ontario and across Canada.

Ground-level ozone and particulate matter are important components of smog. Ground-level ozone forms when nitrogen oxides (NOx) and volatile organic compounds (VOCs) react in sunlight. It is different from the ozone layer in the stratosphere, which protects the Earth from harmful ultraviolet radiation. Particulate matter can come directly from sources such as vehicle exhaust, industrial activity, agriculture, construction, road dust, and wood burning, or it can form in the atmosphere through chemical reactions involving other pollutants.

Air pollution remains an important health concern in Canada and throughout North America. Transportation, industrial activity, electricity generation, residential wood burning, agriculture, and wildfire smoke can all contribute to exposure to air pollutants. The relative importance of these sources varies considerably by location and season.

Evidence shows that both short- and long-term exposure to air pollution can harm human health. Fine particulate matter is associated with cardiovascular and respiratory disease, while long-term exposure is also associated with an increased risk of lung cancer. Ground-level ozone can irritate the respiratory system, worsen asthma symptoms, and make breathing more difficult. Children, older adults, and people with existing heart or lung conditions can be particularly vulnerable to the effects of air pollution.

By Michael Chiu, MWS, CI
Previously published in Ming Pao Daily News – Updated and Revised in 2026