Frequently Asked Questions

Is Ontario's air quality getting better or worse?

Ontario's air quality has improved steadily since 1988. We have good air quality approximately 90 per cent of the time.

Where is the best air quality and lowest incidences of smog in the province?

Generally, air quality improves as you travel northward and eastward across the province, however, the formation and transport of smog is strongly dependent on meteorological conditions. 

Summer smog episodes in Ontario are often a part of a regional weather condition that prevails over much of northeastern North America.  Elevated levels of ozone and fine particulate matter are typically due to weather patterns that affect the lower Great Lakes region.  Such weather patterns are invariably associated with slow-moving high pressure cells across the region and result in the long-range transport of smog pollutants from neighbouring U.S. industrial and urbanized states during the flow of warm air from the southwest to the northeast.

When looking for a place to live, it is important to keep local sources in mind.  For instance, it is best to stay away from areas with a lot of industry and major roadways.  The impacts of emissions from vehicles on a highway or any roadway depend on a number of factors, including:  the distance from the highway; traffic volume; traffic congestion (i.e. free flowing or congested); and predominant wind directions. 

Generally speaking, air concentration impacts from a highway decrease significantly with distance from the roadway.  Typically, moving 100 metres from the edge of the road can result in a decrease in pollutant concentrations of 60-80 per cent.  In addition, trees can filter the air, so a well-treed area can have better air quality than one without trees.

What is smog?

Smog is a general term used to describe a mixture of air pollutants, dominated by ground-level ozone and fine particulate matter. Ozone is created when nitrogen oxides and volatile organic compounds combine in the presence of sunlight – making ozone primarily a summer phenomenon, occuring mostly in southern Ontario. Fine particulate matter is primarily formed from chemical reactions in the atmosphere and through fuel combustion. It can elevate smog levels during all months of the year.

Where does smog come from?

The contaminants that create smog are released during the combustion of fossil fuels in our vehicles, power plants, factory boilers and homes. They are also released by industrial processes, the evaporation of liquid fuels and the use of solvents and other volatile products such as oil-based paints. Smog-causing contaminants are released during forest fires, and emitted by natural sources such as: trees, bogs, and volcanic activity. Most of Ontario's smog problems are caused by a combination of local emissions and pollutants carried by the wind from pollution sources in the United States. More than half of our smog problem comes from south of the border.

How is smog forecast?

The Ministry of the Environment meteorologists combine real-time information on pollutant levels with data on weather patterns, topographical conditions and emission sources to predict impending smog problems. The data are obtained by a network of 40 air monitoring stations across the province. Over the last four years the ministry has invested more than $6 million in upgrading Ontario's air monitoring network, making it the most modern and best equipped in North America.

What is the Air Quality Index?

The Air Quality Index (AQI) is an indicator of our air quality, based on pollutants that have adverse effects on human health and the environment. The pollutants are ozone, fine particulate matter, nitrogen dioxide, carbon monoxide, sulphur dioxide and total reduced sulphur compounds.

If the AQI falls below 32, the air quality is considered good or very good. An AQI reading between 32 and 49 indicates moderate air quality, and an AQI reading from 50 to 99 indicates poor air quality. A reading over 99 indicates very poor air quality.

How is the AQI calculated?

The AQI is a calculated index that provides Ontarians with air quality information in near real-time.  This index has its own concentration breakpoints for the descriptive categories (i.e. very good, good, moderate, poor, and very poor).  It may be difficult to compare AQI calculations from jurisdiction to jurisdiction across North America because they are not standardized and the formulas involved are complex.

The relationship between AQI and the individual pollutant concentrations are shown below.  For each category, a linear relationship is assumed between index values and the concentrations of the sub-index pollutant.  At the end of each hour, the concentration of each pollutant measured at individual AQI sites is converted into a number ranging from zero upwards using a common scale or index. The calculated number for each pollutant is referred to as a sub-index. At a given site, the highest sub-index becomes the AQI reading for that hour at that location.

Ozone (O3)
AQI Category [O3] (ppb) AQI Equation
Very Good 0 to 23 0.6520 × [O3] + 0
Good 24 to 50 0.5800 × [O3] + 2.154
Moderate 51 to 80 0.5900 × [O3] + 2.1
Poor 81 to 149 0.7200 × [O3] - 8.37
Very Poor >149 0.7200 × [O3] - 8.37


Fine Particulate Matter (PM2.5)
AQI Category [PM2.5] 3-hour
average (µg/m3)
AQI Equation
Very Good <12 1.364 × [PM2.5] + 0
Good 12 to 22 1.500 × [PM2.5] - 2.000
Moderate 23 to 45 0.7727 × [PM2.5] + 14.228
Poor 46 to 90 1.113 × [PM2.5] - 1.298
Very Poor >90 1.100 × [PM2.5] + 0


Sulphur Dioxide (SO2)
AQI Category [SO2] (ppb) AQI Equation
Very Good 0 to 79 0.1899 × [SO2] + 0
Good 80 to 169 0.1685 × [SO2] + 2.520
Moderate 170 to 250 0.2125 × [SO2] – 4.125
Poor 251 to 1999 0.0280 × [SO2] + 42.97
Very Poor >1999 0.0500 × [SO2] + 0


Nitrogen Dioxide (NO2)
AQI Category [NO2] (ppb) AQI Equation
Very Good 0 to 50 0.300 × [NO2] + 0
Good 51 to 110 0.2543 × [NO2] + 3.00
Moderate 111 to 200 0.191 × [NO2] + 10.79
Poor 201 to 524 0.1517 × [NO2] + 19.5
Very Poor >524 0.1903 × [NO2] + 0


Carbon Monoxide (CO)
AQI Category [CO] (ppm) AQI Equation
Very Good 0 to 12.49 1.2500 × [CO] + 0
Good 12.50 to 22.49 1.6700 × [CO] - 5.67
Moderate 22.50 to 30.49 2.4300 × [CO] - 23.86
Poor 30.50 to 49.49 2.7200 × [CO] - 34.39
Very Poor >49.49 2.0000 × [CO] + 0


Total Reduced Sulphur (TRS)
AQI Category [TRS] (ppb) AQI Equation
Very Good 0 to 5.49 3.0000 × [TRS] + 0
Good 5.50 to 10.49 3.7500 × [TRS] - 6.50
Moderate 10.50 to 27.49 1.0625 × [TRS] + 20.31
Poor 27.50 to 999.49 0.05046 × [TRS] + 48.59
Very Poor >999.49 0.1000 × [TRS] + 0


  • parts per billion (ppb)
  • micrograms per cubic metre (µg/m3)
  • parts per million (ppm)

How have we improved our smog forecasts?

As of May 1, 2000, as part of its new Air Quality Ontario initiative, the ministry provided earlier, more effective notification of poor air quality than ever before.

When air quality and weather conditions are likely to produce smog, the ministry takes steps to inform the public, and warn those most vulnerable to the health impacts of poor air quality. It provides two levels of alert:

  • A Smog Watch is issued when there is a 50 per cent chance that elevated smog levels are forecast to occur within the next three days.
  • A Smog Advisory is issued when there is a strong likelihood that elevated smog levels are forecast to occur within the next 24 hours, or it can be issued immediately if widespread, poor AQI readings occur, and weather conditions conducive to the persistence of such levels are expected to continue for several hours.

When the weather changes, resulting in cleaner air, the ministry issues an advisory termination notice.

In May 2001, the ministry adopted the policy of issuing a smog advisory immediately if widespread, poor AQI readings occur, and weather conditions conducive to the persistence of such levels are expected to continue for several hours.

As of August 2002, the ministry added fine particulate matter (PM2.5) to its AQI. This new sub-index provides Ontarians with more information on air quality so they can make informed decisions to protect their health and help improve the air we all share.

Is it possible to have poor air quality even if a smog watch or smog advisory has not been called?

Yes. Since smog is closely tied to the weather, it is impossible to be 100 per cent accurate 100 per cent of the time. For example, a weather system could arrive in Ontario before the predicted time, or could change direction.

So poor air quality could occur without a smog advisory being called. It is also possible that poor air quality will not materialize even though a smog advisory has been called.

How can I get air quality information?

  1. On this website: This site provides Air Quality Index (AQI) readings, ambient air pollution data and air quality forecasts, as well as information on actions that can be taken when a smog alert is issued. You can also subscribe to the ministry's smog alert network, and receive an automatic email whenever the ministry issues a smog watch or smog advisory.
  2. By telephone: You can get AQI readings from recorded telephone messages by dialing 1-800-387-7768 (toll-free) or 416-246-0411 in Toronto. To obtain AQI readings in French, dial 1-800-221-8852.
  3. Via radio and television: The MOE has worked for many years with news media across Ontario to inform the public about smog conditions. This will continue to be a crucial method for communicating information about smog and actions that can be taken to reduce smog-causing emissions. This smog season you can expect your local radio and television stations to add smog watches and smog advisories to their weather forecasts.

Where can I obtain hourly pollutant concentration data?

The Ontario Ministry of the Environment’s (MOE) Air Quality Index web site provides users with access to hourly pollutant concentration data from MOE’s ambient sites.The data output, which includes station and pollutant information, is available in both .HTML and/or .CSV format(s) which may be imported directly into Excel or any other spreadsheet application. Currently, online data is available from 2000 to 2014. The tool may be accessed at:

How is PM2.5 monitored in Ontario?

In 2002, Ontario was the first province in Canada to introduce monitoring of PM2.5 to the Air Quality Index. The ministry reported real-time PM2.5 with the Thermo Scientific TEOM 1400AB/SES until December 31, 2012. Continuous PM2.5 monitoring technologies have evolved dramatically over the last decade. The network is now reporting real-time PM2.5 concentrations using Thermo Scientific SHARP 5030, an approved Class III Federal Equivalent Method designated by the United States Environmental Protection Agency in 2009. Ontario evaluated the SHARP monitor starting in 2009, and adopted this method in 2012 in concurrence with Environment Canada. In 2013, the SHARP monitors were deployed across Ontario's AQI network.

What is the effect of the SHARP PM2.5 monitor on reported PM2.5 concentrations?

The new SHARP monitor is able to detect additional components of PM2.5, especially during cold weather. As a result of this improvement in monitoring technology, there is potential of reporting higher PM2.5 concentrations during the winter months. This is a reflection of more accurate measurements and does not necessarily mean that Ontario's air quality is changing. The air is the same; only the monitoring method has changed.

Why did Ontario upgrade the PM2.5 monitoring network?

Ontario strives to be a leader in air quality reporting and continues to benefit from one of the most comprehensive air monitoring systems in North America. The network upgrade was funded by Environment Canada under a national initiative to standardize PM2.5 monitoring methods across Canada and ensure data comparability. The objective is to have all jurisdictions operating federally approved PM2.5 monitors by 2013.

What is the purpose of smog alerts?

The purpose of these alerts is to inform people with breathing difficulties to avoid unnecessary exposure to smog, to inform major pollution sources that they should consider, if possible, reducing their emissions, and to solicit everyone's help in lessening the problem by curtailing activities that produce smog.

What should I do if a smog alert is called?

Here are some actions you can take to help protect the environment and your own health:

At home:

  • Conserve electricity year-round by adjusting the heat or air conditioner and turning off lights you are not using.
  • Avoid letting your car, or any other engine, idle for long periods.
  • Reduce your use of gasoline-powered equipment.
  • Avoid mowing the lawn when air quality is poor.
  • Don't use oil-based products such as paints, solvents or cleaners if you can avoid them. They contain volatile organic compounds (VOCs), which contribute to smog.
  • Avoid or reduce strenuous physical outdoor activities when smog levels are high, especially during the late afternoon. Do not exert yourself outdoors.
  • If possible, stay indoors in a cool, air-conditioned environment.
  • Get engine tune-ups and car maintenance checks as advised by the car manufacturer's maintenance schedule.
  • Limit the amount of wood you burn in your fireplace or woodstove. When burning wood, use only the dry, seasoned variety.

At work:

  • If possible, take public transit, or walk to work.
  • If you use a car, don't travel alone; encourage and facilitate car pooling.
  • Avoid traffic congestion.
  • Consider teleconferencing, instead of traveling to meetings.

As always, consult your doctor for specific medical advice on how to cope with poor air quality.

Where can I obtain information on smog and my health?

For information on smog and associated health impacts, please refer to our Smog and Your Health Fact Sheets, located at

What is a smog alert response plan?

When smog levels reach critical levels, steps should be taken immediately to protect the most vulnerable members of society and reduce emissions that could worsen local air quality. A smog alert response plan is a written protocol of actions to be taken by an organization, government agency, company or municipality when poor air quality is forecast. Such a plan should include a mechanism for informing residents or employees of the potential health threat, as well as specific actions that can be taken to reduce the risk.

More information on developing a smog alert response plan and additional suggestions for what you can do to combat smog is available in the ministry publication – Smog Alert Response: A Municipal Guide to Action.

How many smog advisories were issued in previous years?

The ministry monitors air pollution levels and issues smog advisories when there is a strong likelihood that widespread elevated and persistent smog levels are expected.

Here is a summary of smog advisories issued for Ontario by the ministry since 2003:

Smog advisories issued for Ontario by the ministry since 2003
Year Number of Advisories Total Number of Days
2003 7 19
2004 8 20
2005 15 53
2006 6 17
2007 13 39
2008 8 17
2009 3 5
2010 3 12
2011 5 9
2012 12 30
2013 1 2
2014 0 0
2015* 0 0

* as of May 22, 2015.

What is Ontario doing to improve and protect air quality?

Ontario is committed to doing its part to reduce emissions and improve air quality. The Air Quality Ontario initiative is one component of a comprehensive strategy to protect the environment and safeguard public health. Other components of the strategy include:

  • Drive Clean – a program that reduces nitrogen oxides (NOX) and volatile organic compounds (VOCs) through emissions testing of motor vehicles and enforcement through the Smog Patrol;
  • Ontario Regulation 194/05 – Industry Emissions: Nitrogen Oxides (NOX) and Sulphur Dioxides (SO2) – this regulation establishes industry sector emission caps starting in 2006 and defines how these caps will be reduced in future years (2007, 2010 and 2015);
  • Ontario Regulation 419/05: Air Pollution – Local Air Quality – sets air quality standards for toxic substances to protect local communities;
  • Ontario Regulation 397/01: Emissions Trading – caps nitrogen oxides (NOX) and sulphur dioxide (SO2) emissions from the electricity sector;
  • Ontario Regulation 127/01: Airborne Contaminant Discharge – Monitoring and Reporting – focuses public attention on environmental practices, thus compelling high emitters to clean up their act, and OnAir: Ontario’s online registry for reporting to the public;
  • Developing cleaner sources of energy to replace coal-fired generation by early 2009, a mandate to create renewable energy, encouraging energy efficiency and conservation programs, including use of co-generation;
  • Stage 1 Vapour Recovery (Ontario Regulation 455/94) requires gasoline facility operators to install, maintain and operate gasoline vapour recovery systems, and Gasoline Volatility (Ontario Regulation 271/91 as amended by Ontario Regulation 45/97) which limits gasoline vapour pressure during the summer;
  • Environmental Training for Dry Cleaners (Ontario Regulation 323/94);
  • Canadian Council of Ministers of the Environment (CCME) Guideline A-5 for New and Modified Combustion Turbines; and,
  • CCME Guideline A-9 for New Commercial/Industrial Boilers and Heaters.