Ground level ozone

Figure 1. Ozone (O3) consists of 3 oxygen atoms bonded together.[1]

Ground level ozone is a highly reactive secondary pollutant produced from primary pollutants - specifically hydrocarbons and nitrogen oxides - when exposed to sunlight. It is irritating to human life from a visual, scent, and health perspective, as it is a major component of photochemical smog.[2] It is what gives photochemical smog its unpleasant odor, and can cause many problems for healthy lung functioning, especially in young children.


Ground level ozone is created by chemical reactions of volatile organic compounds and nitrogen oxides, and is stimulated by the sun and high temperatures.[3] This makes its production within warm, densely populated, urban cities a common occurrence. However, since ozone takes some time to form, it won't necessarily stay within these cities. The pollutants may blow down-wind before ozone begins to form, therefore small towns and rural populations may also experience high levels of ozone.[4]

Although emissions of ozone-producing substances have decreased in recent years (see data visualization below) many urban areas do not yet meet air quality standards.[3]

The pollutants responsible for the formation of ozone are produced by hydrocarbon combustion in vehicles, factories and power plants. Within cities however, vehicles can make up nearly 90% of these pollutants.[3] Nitrogen oxides and VOCs are produced in the morning by cars travelling to work, and ozone is produced shortly after, typically in the afternoon. Since pollutants may flow down-wind from where they are produced, the ozone can find its way around the globe and be produced in safer quantities. However if there is an inversion layer present over the area of production, then the pollutants can be "trapped" over populated cities and form to unsafe levels in the form of smog.[3]

Figure 2. Ozone formation can be seen on the right, in which it is produced from photolyzed nitrogen dioxide.[5]

Health effects

Due to its reactivity and close proximity to life, ground level ozone can have large effects. It may cause breathing problems, coughing and irritation to the eyes, nose and throat. Its exposure to people with asthma, bronchitis, emphysema and heart disease may amplify and aggravate their conditions even further. It can also reduce the body's defense system and make cold and flu more likely. Last of all, lung function may be affected adversely.[2]

Its effects do not stop at human health however, as plants and trees may be damaged by it. Its involvement in smog reduces visibility, and it can also damage rubber, fabrics and paints.[2]

Visit the EPA to learn more about ground level ozone.

Data Visualization

The line graph below is used to show how levels of various emissions have been improving for Canada as a whole, particularly with efforts to scrub flue gases clean with various air pollution control devices. Because ozone is a secondary pollutant it's emissions can not be tracked directly, however, tracking the primary pollutants that create ozone gives some clue as to what's happening with ozone. NOx and VOCs have been decreasing, which are the two primary pollutants responsible for ozone formation. Please see the page on detailed pollution data for more extensive information on the Canadian release of these pollutants.


  1. "Ozone-CRC-MW-3D-vdW" by Ben Mills - Own work. Licensed under Public Domain via Wikimedia Commons -
  2. 2.0 2.1 2.2 G. Tyler Miller, Jr. and D. Hackett, "Table 20-2 Major Outdoor Air Pollutants," in Living in the Environment, 2nd ed. USA: Nelson , 2011, ch.20, sec.2, pp.464
  3. 3.0 3.1 3.2 3.3 R. A. Hinrichs and M. Kleinbach, "Nitrogen oxides, photochemical smog and ozone," in Energy: Its Use and the Environment, 5th ed. Toronto, Ont. Canada: Brooks/Cole, 2013, ch.8, sec.C, pp.250-252
  4. US EPA. (Accessed July 27, 2015). Basic Information [Online], Available:
  5. Adapted from Living In The Environment: G. Tyler Miller, Jr. and D. Hackett, "Photochemical and Industrial Smog," in Living in the Environment, 2nd ed. USA: Nelson , 2011, ch.20, sec.3, pp.465-471.

Authors and Editors

Jordan Hanania, Kailyn Stenhouse, Jason Donev
Last updated: November 13, 2015
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