Ground-level Ozone

Ground-level ozone is a colorless and highly irritating gas that forms just above the earth's surface. It is called a "secondary" pollutant because it is produced when two primary pollutants react in sunlight and stagnant air. These two primary pollutants are nitrogen oxides (NOx) and volatile organic compounds (VOC). NOx and VOC come from natural sources as well as human activities.

NOx are nitrogen-oxygen compounds that include the gases nitric oxide and nitrogen dioxide, and are produced mostly by burning fossil fuels. VOC are carbon-containing gases and vapors such as gasoline fumes and solvents (but excluding carbon dioxide, carbon monoxide, methane, and chlorofluorocarbons).

Human activities are responsible for the increases in ground-level ozone in recent years. About 95 per cent of nitrogen oxides from human activity come from the burning of coal, gas and oil in motor vehicles, homes, industries and power plants. VOC come mainly from gasoline combustion and from the evaporation of liquid fuels and solvents. Ozone not only affects human health, it can damage vegetation and decrease the productivity of some crops. It can also injure flowers and shrubs and may contribute to forest decline in some parts of Canada. Ozone can also damage synthetic materials, cause cracks in rubber, accelerate fading of dyes, and speed deterioration of some paints and coatings. As well, it damages cotton, acetate, nylon, polyester and other textiles.

Airborne Particles

Airborne particles include microscopic particles and remain suspended in the air for some time. Particles can be both primary pollutants and secondary pollutants, sent directly into the atmosphere in the form of windblown dust and soil, sea salt spray, pollen and spores. Secondary particles are formed through chemical reactions involving nitrogen oxides, sulfur dioxide, VOCs and ammonia.

Particles give smog most of its color and affect visibility. Depending on the type of particles, the air can appear yellowish-brown, or even white. Like ozone, particles are believed to have adverse effects on vegetation, and on various synthetic and natural surfaces.

Other Pollutants

Nitrogen Dioxide (NO2) is a principal member of the family of nitrogen oxides (NOx ). It is a toxic, irritating gas that results from all combustion processes.

Sulphur dioxide (SO2) is a colorless gas that smells like burnt matches. It can be chemically transformed into acidic pollutants such as sulfuric acid and sulfates (sulfates are a major component of fine particles). The main sources of airborne SO2 are coal-fired power generating stations and non-ferrous ore smelters. Sulfur dioxide is also the main cause of acid rain, which can damage crops, forests and whole ecosystems.

Carbon Monoxide (CO) is a colorless, odorless and tasteless gas that comes primarily from automobile emissions. Ammonia is another pollutant in smog. Most of fine PM is either ammonium sulphate or ammonium nitrate.

Particulate Matter (PM<10)

Airborne particulate matter, known as PM, is one of the major components of smog. PM include microscopic particles in the air. These particles, capable of being inhaled by humans, are divided into two size ranges: PM2.5 and PM<10.

Between the two, "fine" particles less than 2.5 micrometers in size (PM2.5) are responsible for causing the greatest harm to human health. 1/20th the width of a human hair, these fine particles can be inhaled deep into the lungs reaching areas where the cells replenish the blood with oxygen. They can cause breathing and respiratory symptoms, irritation, inflammation and damage to the lungs and premature deaths.

Some PM2.5 are released directly to the atmosphere from industrial smokestacks and automobile tailpipes, but a large percentage is actually formed in the atmosphere from other pollutants such as sulphur dioxide (SO2), nitrogen oxides (NOx) and volatile organic compounds (VOC). Fossil fuel combustion in motor vehicles, power plants and large industries, as well as industrial process and solvent use are major sources of these other pollutants.

Although not as serious a threat to human health as PM2.5, "coarse" particles covering the range from 2.5 to 10 micrometers in diameter (PM<10), are also known to cause adverse health effects. When inhaled, they tend to be deposited in the upper parts of the respiratory system from which they can be eventually expelled back into the throat. Coarse particles generally remain in the form in which they are released into the atmosphere without chemical transformation, eventually settling out under the influence of gravity. While some of these coarse particles are generated naturally by sea salt spray, wind and wave erosion, volcanic dust, windblown soil and pollen, they are also produced by human activities, such as construction, demolition, mining, road dust, tire wear and grinding processes of soil, rocks, or metals.

Health Effects of PM

Numerous studies have linked PM to aggravated cardiac and respiratory (heart and lung) diseases such as asthma, bronchitis and emphysema and to various forms of heart disease. Children and the elderly, as well as people with respiratory disorders such as asthma, are particularly susceptible to health effects caused by PM.

Scientists now believe that there is no "threshold," or safe level, for exposure to PM. Particulate matter is not limited to urban areas. Exposure to PM in Canada is widespread, and it remains a problem in every region of Canada all year round. A correlation has been established between high levels of airborne PM and increases in emergency room visits, hospital admissions and deaths.

PM is also an effective delivery mechanism for other toxic air pollutants, which attach themselves to particulate matter that floats in the air. These toxics are then delivered into the lungs, where they can be absorbed into the blood and tissue.

Environmental Impacts of PM<10

The effects of PM on materials have been investigated for metals, wood, stone, painted surfaces, electronics and fabrics. The deposition of PM on these materials may cause soiling and discoloration, thus reducing their aesthetic appeal. Exposure to PM may also cause physical and chemical degradation of materials through the action of acidic particles.

Particulate matter is also associated with reduced visibility with poor air quality. The presence of particles in the air reduces the distance at which we can see the colour, clarity, and contrast of far away objects because the particles in the atmosphere scatter and absorb light.

The most obvious effect of particulate deposition on vegetation is the physical smothering of the leaf surface. This will reduce light transmission to the plant in turn causing a decrease in photosynthesis. Particle composition may cause both direct chemical effects on the plant and indirect effects through impacts on the soil environment. Particle accumulation on the leaf surface may increase the plant's susceptibility to disease.