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.


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