Air Quality Health Index classroom kit, grades 5 and 6, health: chapter 10

Background Information for Teachers

Air Quality, Smog, Pollution, and Our Health

What is air quality?

Air is made up of different gases (78% nitrogen, 21% oxygen, 0.09% argon, 0.03% carbon dioxide, and the remaining 0.07% is a mixture of water vapour and other trace components). Air quality describes the level of air pollutants in that air. Air pollutants can become dangerous to human health if people are sensitive to elevated levels of pollutants or are exposed to them for extended periods. To reduce risks, people need to know when pollutants are present and in what concentrations.

What is smog?

Most pollution that we know about is usually invisible, meaning we can’t look at the air and determine how much pollution is in it. However, sometimes pollution concentrations can be so high that you can actually see it in the air. If you live in a large city, often, in the summer, the pollution in the form of smog can be seen hovering over the skyline. Smog, whether visible or invisible, is a mixture of different pollutants  that can be seen as a brownish yellow or greyish white haze in the air. The two key components of smog are particulate matter and ground-level ozone.

Smog blanketing a city.

What is particulate matter?

Sometimes very, very tiny solid or liquid particles are suspended in the air, and these are referred to as particulate matter. These include dust, dirt, soot, smoke, and tiny particles of chemical pollutants. This kind of particulate pollution comes from power plants, trash incinerators, motor vehicles, construction activities, forest, and natural dust blown around on the wind. In large cities where there are a lot of vehicles, the particulate matter can be worse than in rural areas.

What is ground-level ozone?

Ozone, like oxygen, is a colourless gas that cannot be seen in the air. High in the atmosphere, ozone forms a barrier to harmful solar radiation. Ground-level ozone, however, is formed from other pollutants already in the air when they mix with sunshine, and so ozone concentrations are normally higher in the summer. Ozone is harmful to people, animals, plants, and other materials.

How is air quality measured?

Environment Canada scientists assess air quality by collecting and analyzing air samples taken from near ground level. Pollutant levels are affected by such factors as emissions sources, weather conditions and topography. Environment Canada scientists have developed complex computer models that now provide air quality forecasts for major centres in Canada.

Adult using an inhaler outside.

How does air pollution affect health?

Your lungs inhale all things in the air around you, including particulate matter and ground-level ozone. If you are sensitive to high pollution levels, you may experience symptoms that are unpleasant or even dangerous. How do you know if you are sensitive? People with diabetes, lung disease (such as asthma, chronic bronchitis, emphysema, lung cancer) or heart disease (such as angina, a history of heart attacks, congestive heart failure, arrhythmia or irregular heartbeat) are more sensitive to air pollution than the average Canadian.

Seniors, too, are at higher risk because of weakening of the heart, lungs and immune system, and increased likelihood of health problems such as heart and lung disease.

Children are also more vulnerable to air pollution: they have less-developed respiratory and defense systems. Because of their size, they inhale more air per kilogram of body weight than adults. Children also spend more time outdoors being physically active, which can increase their exposure to air pollution.

Finally, people participating in sports or strenuous work outdoors breathe more deeply and rapidly, allowing more air pollution to enter their lungs. They may experience symptoms like eye, nose or throat irritation, cough, or difficulty breathing when air pollution levels are high.

What is the Air Quality Health Index?

The Air Quality Health Index (AQHI) is a scale designed to help Canadians understand what the quality of the air around us means to our health. It is a new tool developed by health and environmental professionals to communicate the risk to health posed by air pollution. Everyone is affected by air pollution differently, so some of us are at a higher risk than others.

The AQHI is designed to help us make decisions to protect our health and the environment by:

  • Limiting short-term exposure to air pollution
  • Adjusting our activity during episodes of increased air pollution and encouraging physical activity on days when the index is lower
  • Reducing our personal contribution to air pollution

The index provides specific advice for people who are especially vulnerable to the effects of air pollution as well as the general public.

Family sitting together outdoors in the backyard.

The AQHI is measured on a scale ranging from 1 to 10+:

1-3 = “Low” health risk
4-6 = “Moderate” health risk
7-10 = “High” health risk
Above 10 = “Very high” health risk

AQHI and the weather

The greatest potential for high-risk AQHI days occurs when several weather conditions come together resulting in a deterioration of air quality.

Wind speed plays a role in diluting pollutants. Generally, strong wind disperse pollutants, whereas light winds generally result in stagnant conditions allowing pollutants to build up over an area.

Inversion or “stagnant” conditions are commonly associated with maj or air pollution episodes. Under normal conditions, the air near the surface is warmer. The warmer air rises and mixes with the cooler air above. This condition is known as “unstable”. Inversions can develop when a warmer, less dense air mass moves over a cooler, denser air mass creating a temperature inversion where the air  is  now cooler  closer  to the surface. Pollutants are unable to mix vertically and will stay pooled near the ground due to these "stable" conditions. Inversions can persist for hours or days.

Topography can create conditions that allow the trapping of pollutants. At night, cold air tends to drain downhill, settling into low-lying basins and valleys. Unable to rise, the cool air settles and accumulates in these valleys, trapping air pollutants.

Long-range transport or transboundary transport of air pollution is a significant problem in Canada. Winds coming from the United States (south) and industrialized areas of Ontario and Quebec can result in higher levels of air pollutants in neighbouring Canadian cities.

Clear, cloudless skies allow more sunlight or UV radiation to penetrate the Earth’s surface. Higher intensity of sunlight allows for more photochemical reactions to  occur, producing high levels of ground-level ozone, which is one of the pollutants measured in the AQHI.


Environment Canada, (2006). Sky Watchers Guide to Weather. Toronto, ON: Environment Canada, Ontario Region. (Supplement 1, Air Pollution, Smog and Our Air Quality).

Environment Canada, (2008-08-18). Air Quality Health Index Frequently Asked Questions. Retrieved June 24, 2009, from Environment Canada, Air Quality Health Index (AQHI).


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