What is smog?

Smog is a form of air pollution. The word "smog" is a combination of the words "smoke" and "fog." Smog is a mixture of many pollutants, mainly ground level ozone and fine particulate matter. Sources of smog include:

  • barbecues
  • coal-fired power generation stations
  • construction
  • factories
  • gasoline and diesel powered vehicles
  • lawnmowers
  • oil-based paints, solvents and cleaners
  • pesticides
  • pollutants carried by the wind

Smog comes from local sources but can also drift from places as far as the United States. Since smog travels with the wind, both urban and rural areas can experience smoggy days.

About ozone

An important component of smog is ground-level ozone. Ozone that is formed naturally in the atmosphere (the "good" ozone) protects life from the sun's damaging ultraviolet rays. At ground level, however, ozone acts as an invisible air pollutant that is harmful to humans, animals, plants, and man-made materials. Ground-level ozone is the "bad" ozone. It is created when gases such as nitrogen oxides react with volatile organic compounds (VOCs) when they are combined with sunlight and heat. This is why smog is more of a problem on hot summer days. Nitrogen oxides are produced by burning fossil fuels such as coal, oil, gas, and diesel in motor vehicles, industries, power plants, and homes. VOCs include carbon-containing gases that are created when gasoline and other oil-based solvents are burned. Studies show that every major Canadian urban centre has levels of ground-level ozone that are high enough to pose health risks.

Ozone is not only a problem for people; it damages vegetation and degrades man-made materials. It is a powerful greenhouse gas and contributes to climate change.

Particles in the air

The other component of smog is fine particulate matter. Fine airborne particles are usually 10 micrometres in diameter or smaller and are a mixture of solid particles and liquid droplets, usually soot and acids, that can also be described as acid water droplets or acid aerosols. The particles are microscopic and remain suspended in the air for some time. Particulate matter decreases visibility and contributes to the brownish-yellow colour that is characteristic of smog. It can be generated naturally by dust, sea salt spray, or windblown soil and pollen, but studies show that particulate matter generated from human activities is more harmful. Industrial and car emissions, road dust, and the processes of demolition and construction all generate fine airborne particles.


How can smog harm my health?

Overall, smog is harmful to both the respiratory (lungs) and cardiovascular (heart) systems. It aggravates heart problems, bronchitis, asthma, and other lung problems. Smog reduces lung function even in healthy people. Even at low levels, ground level ozone and fine particulate matter are harmful. There are no "safe" levels of smog.

Ground-level ozone irritates the eyes, nose, and throat. When it is inhaled, it can dry out and inflame the protective membranes of the nose and throat. This can make it more difficult for the body to fight against an infection. Inflamed breathing passages can also decrease the lung's working capacity. Symptoms can include:

  • shortness of breath
  • wheezing
  • coughing
  • pain when breathing

When the amount of ground-level ozone increases, so do the number of emergency room visits and hospital admissions. It can even cause premature death.

The smaller the particle, the deeper it can be inhaled, which makes fine airborne particles dangerous. Larger particles usually settle in the mouth and nose, while fine particles can get stuck in the lungs. Once in the lungs, the particles can decrease the lungs' working capacity and aggravate respiratory symptoms such as wheezing, coughing and shortness of breath. When there is more particulate matter in the air, death rates are higher.

When people inhale ozone and particulates from the air, their arteries tighten, which reduces the blood flow (and oxygen supply) to the heart.

Who is most at risk?

Although smog affects each and every one of us, it is especially harmful to:

  • children: Children's lungs are still developing. Children have smaller airways and breathe more rapidly than adults. They also tend to spend more time outdoors in the summer. All these factors increase their risk of inhaling more polluted air.
  • people with existing lung conditions, such as asthma sufferers: People with asthma already have poor lung function to begin with. Ground-level ozone and particulates can cause additional inflammation in the lungs that can aggravate symptoms or trigger an asthma attack. People with respiratory illnesses such as chronic obstructive pulmonary disease (which includes chronic bronchitis and emphysema) or lung cancer are also sensitive to smog and air pollution.
  • people with existing heart conditions: Smog is particularly dangerous for people who suffer from heart disease, congestive heart failure, heart rhythm problems, or hardened arteries. People with diabetes are also more sensitive because they are more likely to have heart disease.
  • seniors: Seniors are at higher risk not only because of their age and their generally weaker heart, lungs, and immune system, but also because seniors are more likely to have a pre-existing health condition. Smog is particularly dangerous for seniors who suffer from cardiovascular disease and lung problems such as asthma and chronic obstructive pulmonary disease. Seniors who smoke or are especially active outdoors need to be especially cautious.
  • people who work or exercise outdoors: While exercising, you breathe harder and mostly through the mouth, so the air doesn't get the filtering provided by the nose, which means more polluted air gets through. You might feel you are doing your body good by jogging down a congested urban street, but you are actually putting your lungs at a greater risk.

Other people who need to be careful on smoggy days include people with allergies, pregnant women, and smokers.

Warning signs that smog may be causing you harm:

  • breathing difficulties (especially during exercising), including shortness of breath
  • increased mucus production in the nose and throat
  • chest tightness
  • cough or throat irritation
  • eye irritation
  • feeling unusually tired
  • headache
  • light-headedness
  • low energy
  • wheezing


How can I tell if it's smoggy?

What is the AQI?

The Air Quality Index, or AQI, is a rating of how clean the air is. It is issued by the Ministry of the Environment, which monitors our air quality every day. The AQI evaluates the following air pollutants:

  • carbon monoxide
  • nitrogen dioxide
  • ozone
  • fine particulate matter (particles that can be inhaled, which are less than 2.5 micrometres in diameter)
  • sulphur dioxide
  • total reduced sulphur compounds
  • total suspended particles

The AQI scale ranges from 0 to 120. Higher numbers mean higher health risks. A reading of 0-15 is considered as very good, 16-31 as good, 32-49 as moderate, 50-99 as poor, and 100 or over as very poor. Two kinds of Smog Alerts are issued. A Smog Watch is issued when there is a 50% chance of a smog day within the next 3 days. When the AQI is expected to get higher than 50 in the next 24 hours, a Smog Advisory is issued. To learn about AQI levels in your community, contact your provincial ministry of health or go to Environment Canada's website.

Environment Canada has introduced another index of air quality called the Air Quality Health Index (AQHI). Although not available for all cities across Canada, the AQHI is a tool that was created to help people understand what the air quality means to their health. It is calculated based on the health risk level from the pollutants ozone, particulate matter, and nitrogen oxide. It is reported on a scale of 1 to 10+. An AQHI of 1-3 indicates low health risk, of 4-6 indicates moderate health risk, of 7-10 indicates high health risk, and of 10+ indicates very high health risk. The AQHI also provides information on how you can modify your activity level when air quality is poor and on how you can improve air quality. For more information about AQHI, go to Environment Canada's website.

Can I tell if it is smoggy by looking outside?

Even without access to a weather report, you can easily get an idea of the air quality. Generally, the hotter the weather is, the worse the ozone smog is likely to be. Similarly, the hazier it is, the higher the concentrations of smog particles. Smog alerts often occur on sunny days without wind or rain (water washes some of the pollution out of the atmosphere). Although smog alerts occur mostly from early May to late September, they can occur at any time of the year, including the winter season.

Since air pollution can cause harm at concentrations well below smog warning levels, it is important to take action if you experience any warning signs of smog-induced health effects.

Look out for these signs:

  • breathing difficulties (especially during exercise), including shortness of breath
  • increased mucus production in the nose and throat
  • chest tightness
  • cough or throat irritation
  • eye irritation
  • light-headedness
  • low energy
  • wheezing

How can I protect myself against smog?

You may think that there is nothing you can do about the air you breathe. However, there are easy ways to protect yourself against smog and air pollution.

How to protect yourself:

  • Be aware. Listen to smog forecasts and consider ways to modify daily activities accordingly (e.g., car pool, take transit, work from home, group errands together). Check the Air Quality Health Index in your community, remembering that "smog season" is generally from May to September.
  • Limit your exposure. On days when ozone levels are high, you have a greater likelihood of being harmed by smog if you stay outdoors for longer periods and if you perform strenuous activity.
  • Avoid using gas-powered engines, pesticides, and oil-based paints. These all contribute to lower air quality.
  • Stay hydrated. Hot temperature and high humidity often correspond with high smog levels. This makes it important to stay in the shade and drink lots of fluids to stay hydrated, especially before, during, and after exercise.
  • Consider exercising indoors in a smoke-free, air-conditioned environment. Avoid exercise outdoors, especially from mid-morning to early evening, when smog levels are higher.
  • Drive less. Carpool, ride a bike, walk, or use transit. If we all do our part, we can improve the air quality for everyone.
  • Protect those at higher risk of health problems. Pay special attention to children, seniors, those with lung problems (such as people with asthma), those with heart problems, and those who work outdoors.