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10 Ministry of the Environment Air Quality Report

Ministry of the Environment Air Quality Report

Original source: The Ministry of the Environment (in annual air quality reports)
Distributed by: The Ministry of the Environment
Suggested citation (see Data Citation Notes):
Ministry of the Environment, Air Quality in Ontario [year]

Data Notes

  • Improvements were made to the AQI in August 2002 when Ontario added PM2.5 to its AQI.1 When comparing 2003 average AQI and/or number of days AQI was >=32 to previous years, changes in the AQI may have led to increased average AQI values.
  • Data are available from Ministry of Environment annual air quality reports2 as well as from the air quality web site.3
  • Air quality measures are dependent on the location of the air monitors. Some air monitors might have been moved within a particular town or city. Refer to MOE documentation.
  • Adverse health effects from air pollution are well documented.4,5,6 Air pollution can cause health problems for all people but particularly the elderly, children, and those with chronic respiratory or heart problems. Air pollution causes premature mortality, respiratory hospital admissions and cardiac hospital admissions. The Ontario Medical Association estimated that 1,900 premature deaths, 9,800 hospital admissions and 13,000 emergency room visits were the result of air pollution among Ontarians in the year 2000.7
  • AQI: The AQI is based on six pollutants that have adverse effects on human health and the environment: ozone (O3), fine particulate matter (PM2.5), nitrogen dioxide (NO2), carbon monoxide (CO), sulphur dioxide (SO2), and total reduced sulphur (TRS) compounds. At the end of each hour, the concentration of each pollutant measured at a particular site is converted into a number ranging from 1 upwards using a common scale or index. The calculated number for each pollutant is called a sub-index. At a given site, the highest sub-index for any given hour becomes the AQI reading. The index is a relative scale, in that, the lower the index, the better the air quality. If the AQI value is below 32, the air quality is categorized as good. For AQI values in the 32-49 range (moderate category), there may be some adverse effects on very sensitive people. For index values in the 50-99 range (poor category), the air quality may have adverse effects on sensitive members of human and animal populations, and may cause significant damage to vegetation and property. With an AQI value of 100 or more (very poor category), the air quality may have adverse effects for a large proportion of those exposed. If an AQI station records an AQI above 49, the local medical officer of health is informed. The AQI is highly influenced by ozone.
  • In Aug 2002, suspended particles (PM10) was replaced with fine particulate matter (PM2.5). This will likely lead to an increase in the number of times Ontario’s air quality hits the “poor” category and the number of Smog Advisories/Watches in Ontario. Calculation methods for the ozone sub-index changed in 1995 to reflect new scientific findings.
  • AQI readings, smog forecasts and other air quality information are available at .
  • If poor AQI readings (>49) are predicted to be sustained over a period of time and over a wide area, then a Smog Alert is issued for that area. A Smog Watch is called when there is a 50 percent chance that widespread, elevated and persistent smog levels are forecast within the next three days. A Smog Advisory is called when there is a strong likelihood that widespread elevated and persistent smog levels are forecast within the next 24 hours. If widespread, elevated smog levels occur without warning and weather conditions conducive to the persistence of such levels are forecast to continue for several hours, then a smog advisory is issued immediately.
  • The MOE reports the number of AQI exceedence days (AQI > 49) in its annual reports. Data for each day, from which the number of days of moderately poor air quality can be determined, are available from the Air Quality Ontario web site.
  • Ozone: Ground-level ozone is a gas formed when nitrogen oxides and volatile organic compounds (VOCs) react in the presence of sunlight. Ozone levels are strongly influenced by the weather - levels are highest on hot days. Some effects of ozone include inflammation of the lung, decreased lung function and other respiratory symptoms, causing coughing wheezing and chest tightness. Existing heart and lung conditions may be aggravated. There appears to be no threshold for adverse effects - effects can be seen at 10 ppb, with clear effects apparent at ozone concentrations of 20 ppb. The O3 one-hour average Ambient Air Quality Criteria (AAQC) is 80 ppb.
  • Carbon monoxide: Carbon monoxide (CO) is a colourless, odourless, tasteless gas that is poisonous at high concentrations. CO is produced by the incomplete burning of fossil fuels; the main sources are from the transportation sector. General health effects include impairment of visual perception, work capacity, learning ability and performance of complex tasks. The CO one-hour average AAQC is 30 ppm and eight-hour average AAQC is 13 ppm. These criteria have not been exceeded since 1991.
  • Nitrogen dioxide: Nitrogen dioxide (NO2) is a reddish-brown gas with a pungent and irritating odour that can cause increased sensitivity for people with asthma and bronchitis. Main sources are automobiles, thermal power plants and incineration. Nitrogen oxides are some of the building blocks of smog. The NO2 one-hour average AAQC is 200 ppb and 24-hour average AAQC is 100 ppb.
  • Sulphur dioxide: Sulphur dioxide (SO2) is a colourless gas with a strong odour similar to burnt matches. It causes breathing discomfort, respiratory illness, and aggravation of existing respiratory and cardiovascular disease. People with asthma, chronic lung or heart disease are most sensitive to SO2. The SO2 AAQC for the one-hour average is 250 ppb, 100 ppb for the 24-hour average and 20 ppb for the one-year average. The annual AAQC has not been exceeded since 1972.
  • Particulate Matter: Particulate matter (PM) is a mixture of solid particles and liquid droplets found in the air. PM is divided into inhalable particles (PM10) with size less than 10 microns and respirable particle (PM2.5) with size less than 2.5 microns. Inhalable particles can aggravate respiratory conditions such as asthma. Respirable particles are linked with serious health effects including decreased lung function, increased hospital admissions, increased respiratory symptoms and disease, and premature death. The 24-hour average AAQC for PM10 is 50µg/m3 and the proposed standard for PM2.5 is 30µg/m3.
  • Coefficient of Haze (COH): This measure is used inter-changeably with the PM10 and/or suspended particles terminology. The AAQC for the COH is a coefficient of 1.0 for 1-hour averages and a coefficient of 0.5 for annual means.
  • Total Reduced Sulphur (TRS) compounds: TRS are a class of compounds that produce offensive odours similar to rotten eggs. Common sources of TRS compounds are the steel industry, and pulp and paper mills. TRS compounds are not normally considered a health hazard except at very high concentrations. The AAQC for 24-hour maximum levels is 27 ppb.
  • Health effects can occur for some pollutants at levels below the Ambient Air Quality Criteria (AAQC), particularly ozone and particulates, which may not have a “safe” threshold.
  • Because values depend on the location of the monitor, the AQI and pollutant levels cannot be used to assess the air quality of a large area such as a health unit. Values can change dramatically with even a small change in a monitor’s location. Trends over time can be assessed only if the location has not changed.
  • Not all health units have air monitors within their jurisdictions. Not all monitors measure all pollutants.

References and Resources

  1. Ministry of the Environment. Green Facts: Ontario’s Air Quality Index, May 2005. URL:
  2. Ministry of the Environment Air Quality Reports. URL:
  3. Ministry of the Environment Air Quality Web Site. URL:
  4. Burnett RT, Cakmak S, Brook JR. The Effect of the Urban Ambient Air Pollution Mix on Daily Mortality Rates in 11 Canadian Cities. CJPH 1998;89(3):152-156.
  5. Toronto Public Health, Air Pollution Burden of Illness in Toronto, 2004. URL:
  6. Colburn KA, Johnson PRS. Policy Forum: Air Pollution Concerns Not Changed by S-PLUS Flaw. Science Jan 31,2003;299:665-666.
  7. Ontario Medical Association. Ontario’s Air: Years of Stagnation. June 2001.
This page last updated: April 4, 2006
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