Smog is a specific type of air pollution. It is a combination of harmful pollutants (often appearing relatively low to the ground as a yellow-brown haze) that are introduced into the atmosphere by both natural and human induced processes. It was first described over 5 decades ago as a mixture of smoke and fog, hence the name "smog"—but today it has a more specific definition and composition.
Smog is made up of many chemicals including nitrogen oxides (NOx), sulphur dioxide (SOx), carbon monoxide (CO), and volatile organic compounds (VOCs), but the two main components of smog are particulate matter (PM) and ground-level ozone (O3).
Normal smog (often called London-type smog) is mainly a product of burning large amounts of high sulfur coal. Whereas photochemical smog is a more modern phenomena commonly produced by vehicle emissions in contact with sunlight—mostly from burning gasoline and diesel. Photochemical smog forms in warm, densely populated cities with many vehicles. Visit the page here to learn more about photochemical smog.
This page does not include the chemical formulas in the formation of smog—to see them, along with more detail about smog, visit the UC Davis ChemWiki.
Smog forms when pollutants are released into the air. The pollutants are formed both naturally and by humans, however, the human-induced pollutants are of most concern due to the magnitude of pollutants produced by the burning and extraction of fossil fuels, which are known to cause extreme health effects. The location of smog formation is also of great concern, especially for human health, as a good portion of it is produced within cities where large portions of the population live.
One of the primary constituents, ozone, is created through chemical reactions between sunlight and certain pollutants. The other primary constituent, particulate matter, can also form through chemical reactions but is introduced to the atmosphere through other means as well. Wind may disperse these particles among land, thereby decreasing the amount within a given area. In addition, rainfall may wash these pollutants out of the local atmosphere, however, this can result in other unwanted events like acid rain. When smog encounters an inversion layer (caused by warm areas in the upper atmosphere) it can stay over a region for an extended period of time—exposing people to its effects for longer. Visit the inversion layer page to see how this phenomena occurs.
The health effects from smog and its components can be severe, and depend on many variables. Smog is harmful when inhaled, with the severity of it depending on the amount inhaled, the types of pollutants contained in it, as well as the individual's age, weight, activity level, and well-being. However studies show that any exposure to these pollutants is harmful, with extended exposure and higher doses obviously causing the most damage.
Types of effects smog has include:
This effects can be seen in Figure 3 below, with more serious and rare effects at the top, and the more common yet milder effects towards the bottom.
Visit the Government of Canada for information of smog in Canada, and how to reduce your risk of its adverse effects.
Ozone can damage plant cells and inhibit their growth, reducing the amount of carbon dioxide that they take in during photosynthesis. This results in reduced agricultural yields in all sorts of crops. It can also affect synthetic materials like rubber, cotton and other materials, causing deterioration and even disintegration.
Particulate matter smothers plants limiting their exposure to the sun and inhibiting their ability to take in carbon dioxide, therefore, decreasing its photosynthesis capabilities. The chemical composition of the PM may also have an effect on the plants and their surrounding soil. Particulate matter causes the presence of haze in the air, thus reducing the clarity and colour of what can be seen. Humidity of the surrounding air also plays a role in its hazy effect. According to the U.S. Environmental Protection Agency, visibility is reduced from 144 kilometers to around 30 kilometers in certain parts of the United States.
Visit Environment Canada for more details.