Drive through Vancouver, fly over Toronto, look at a picture of any major city in the world and you see it: smog. A gray or brown cloud hanging ominously in the air. And it can be deadly.

What is Smog?

H.A. Des Voex first coined the term "smog" in 1905, combining the words "smoke" and "fog." He used it to describe the foggy conditions in urban areas caused by sulphur dioxide emissions from newly-created smokestacks.

» RELATED: The story of smog

Smog is a combination of pollutant gases from:

• industrial manufacturing plants (such as coal-fired power plants, pulp and paper plants, smelting plants and cement plants)
• exhaust particles from trucks and automobiles

How is smog formed?

Smog is made up, in large part, of ozone gas. It's formed in the lower atmosphere just above the Earth's surface when energy from the sun reacts with industrial pollutants and motor vehicle exhaust particles in the air. The pollutants and the exhaust contain two key components to the development of smog:

• Nitrogen oxides – These are produced when fossil fuels like gasoline, natural gas, heating oil and coal are burned. These gases are also produced naturally in the soil and in forest fires and volcanoes.
• Volatile organic compounds (VOC) – These are compounds containing carbon and, in most cases, hydrogen. However, most of these compounds also contain oxygen, nitrogen, chlorine, sulphur or phosphorous. The major artificial source of VOC emissions is motor vehicles, but the evaporation of gasoline, oil-based paints, nail polish remover, barbecue starter, surface coatings, inks and hydrocarbons from the petrochemical industry is also a significant source.

Both nitrogen oxides and volatile organic compounds exist naturally in the earth's atmosphere. However, motor vehicle exhaust and industrial pollutants pump more of these chemicals into the air. Sunlight breaks down these chemicals compounds allowing them to join with oxygen in the air. This joining of broken down nitrogen oxide and volatile organic compound particles with oxygen creates ozone, one of the chemicals in smog.

What is smog made up of?

A variety of chemicals make up smog including:

• Ground-level ozone – ground-level ozone is the same kind of ozone found 15 to 30 kilometres above the earth. When it's that high up, the ozone is referred to as stratospheric ozone and it shields us from the sun's harmful ultraviolet rays. While ground-level ozone does occur naturally, it only does so in small quantities. When a high level of these gases accumulate it becomes air pollution. The haziness and brown colour associated with smog are caused by the accumulation of tiny particles and nitrogen dioxide in stagnant air. At a ground level, these gases can harm people, animals and vegetation.
• Particulate Matter (PM) – particulate matter refers to the very small solid or liquid particles emitted from any activity that burns materials or generates dust. The thick smoke that belches out of automobile exhaust pipes, the white smoke coming from industrial plants and smoke stacks, the clouds of dust picked up by wind – all are sources of particulate matter. The tiny particles can harm crops and vegetation and get caught in people's lungs causing serious respiratory problems.

How does smog affect me?

Smog, caused by ground-level ozone and particulate matter affects the cardio-respiratory system of people. It can cause:

• respiratory distress
• coughing
• asthma
• bronchitis
• increased hospital admissions
• reduced resistance to lung infections and colds
• eye irritation

But make no mistake. Smog can be much more than just a minor inconvenience to one's health. In June 2005, the Ontario Medical Association said smog-related illnesses lead to the premature deaths of 5,800 Ontario residents each year. It also said that toll is expected to almost double in the next 20 years.

The OMA study said smog will send an estimated 17,000 people a year to hospital with breathing problems.

Smog can also have a negative effect on vegetation. Because it prevents sufficient photosynthesis, it can damage leaves and reduce the yield of plants and crops.

What weather conditions lead to smog?

A number of weather conditions contribute to the formation of smog including:

• warm temperatures ranging from the high 20s up
• slow wind speeds
• still, stagnant air

When is smog worst?

Because smog formation depends on temperature and sunlight, summer is the time of year when smog is worst. From May to the end of September is typically when most smog forms. However, it is possible to get winter episodes of smog if you have slow winds and strong air stagnation.

Afternoons and early evenings are the peak periods for smog formation.

Where is smog worst?

Smog is usually worse in densely populated areas and low valley areas where air can become trapped, stagnant and very warm. In Canada, the worst smog is seen in three areas:

• the Lower Fraser Valley in B.C.
• the urban belt running from Windsor to Quebec
• Atlantic Canada (inherits the polluted air of urban centres along the eastern United States)

When is smog dangerous to my health?

Many health scientists believe there is no safe level for smog and that no matter how little smog is in the air, someone will be negatively affected.

What should I do when a smog warning is issued?

When a smog warning is issued medical experts suggest that:

• you reduce physical activity, especially activity outside.
• you restrict the amount of time young children spend outside.
• people with pre-existing respiratory problems should consult their physician about how much time to spend outside and what precautions to take.

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