What is ozone?

Most of us have heard of the ozone (O3) layer which occurs naturally in the upper atmosphere and protects the earth from the sun’s rays. However, ground-level ozone is not something we are so familiar with, and is actually harmful to health.

Almost no ozone is directly emitted by human activities, but it is normally formed when other pollutants, including nitrogen dioxide, react in sunlight. Therefore, on sunny days in polluted air concentrations of ozone can increase leading, in severe cases, to summertime "smog".

The south-east of England often has the highest concentrations of ozone in the UK, mostly because it is close to European pollution sources. Ozone pollution tends to be highest in the countryside and in suburbs, away from central London. This is because certain pollutants that are more prevalent in urban areas are able to "mop up" ground level ozone thus reducing its concentration.

High levels of ozone can irritate and inflame the lungs. It can also irritate the eyes, nose and throat, which can lead to cough and chest discomfort. Inflammation and narrowing of the airways leads to increased sensitivity to cold air and exercise. In sensitive individuals such as asthmatics, ozone pollution episodes can make breathing difficulties worse.

Ozone forming chemicals can remain in the atmosphere for many days and are often transported over long distances. It is for this reason that a reduction in ozone levels can only be achieved through European or even global action. Studies have shown that European ozone levels have increased rapidly since Victorian times, although recent peak concentrations in 2003 and 2006 were around half those recorded in the summer of 1976.

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