Air Pollutants – Ozone

Ozone (O3) is a gas. The molecules of the gas are made up of three oxygen atoms.

Ozone as a pollutant

Ozone is not emitted directly from man-made sources in any significant quantities. In the lower layers of the atmosphere, or troposphere, oxides of nitrogen and volatile organic compounds, derived mainly from man-made sources, react in sunlight to form ozone. Ozone occurs more in summer than winter and the levels are usually higher in rural areas than in cities. In summer when there are high temperatures, calm winds and plenty of sunlight, this chemical reaction can lead to very high levels of ozone being recorded. Production of ozone can also be stimulated by carbon monoxide, methane and other VOCs that arise from plants, trees and other natural sources.

img:OzoneThe reactions that take place to form ozone do not happen instantaneously, but over several hours or even days. Once formed, ozone may persist for several days, especially for example downwind of urban areas. Ozone measured at a particular location may have arisen from primary pollutants emitted many hundreds or even thousands of kilometres away. Sometimes on sunny calm days, ozone can be the major component of summertime smog.

Ozone is a substance that has adverse effects on our health. There is consistent evidence for associations between daily deaths and admissions to hospital with daily average concentrations of ozone. There is evidence from USA studies that shows long-term exposure to raised ozone concentrations leads to lower levels of lung function and may impair development of lung function.

Ozone can also damage plants including crops – peas, beans, spinach and wheat are especially sensitive. Some estimates have put the costs of reduced crop yields in Europe, caused by ozone, up to £4.8 billion.

Good Ozone

Small concentrations of ozone occur naturally in the stratosphere, which is part of the Earth’s upper atmosphere. At that level, ozone helps to protect life on Earth by absorbing ultraviolet radiation from the sun, particularly UVB radiation that can cause skin cancer and cataracts, damage crops, and destroy some types of marine life.

Ozone is created in the stratosphere when ultraviolet light from the sun splits an oxygen molecule into two single oxygen atoms. Each of those oxygen atoms can then bind with an oxygen molecule to form an ozone molecule. Depletion of stratospheric ozone poses serious health risks for humans and environmental hazards for the planet, and many nations have banned or limited the use of chemicals that contribute to ozone depletion.

How is ozone measured?


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