Ozone, the main ingredient of smog, presents a serious air quality problem in many parts of the British isles and Europe. Even at low levels, ozone can cause a number of respiratory effects.

What is ozone?

Ozone is a gas that occurs both in the Earth�s upper atmosphere and at ground level. Ozone can be good or bad, depending on where it is found:

"Good" Ozone. Ozone occurs naturally in the Earth�s upper atmosphere - 10 to 30 kilometres above the Earth�s surface, known as the ozone layer - where it shields us from the sun�s harmful ultraviolet rays. Here, ozone is decreased by chemical reaction with human-induced pollutants.

"Bad" Ozone. In the Earth�s lower atmosphere, near ground level, ozone is formed when pollutants emitted by cars, power plants, industrial boilers, refineries, chemical plants, and other sources react chemically in the presence of sunlight. Ozone pollution is a concern during the summer months when the weather conditions needed to form ground-level ozone - lots of sun, hot temperatures - normally occur.

Are you at risk from ground-level ozone?Several groups of people are particularly sensitive to ozone, especially when they are active outdoors, because physical activity causes people to breathe faster and more deeply.

Active children are the group at highest risk from ozone exposure because they often spend a large part of the summer playing outdoors. Children are also more likely to have asthma, which may be aggravated by ozone exposure.

Active adults of all ages who exercise or work vigorously outdoors have a higher level of exposure to ozone than people who are less active.

People with asthma or other respiratory diseases that make the lungs more vulnerable to the effects of ozone will generally experience health effects earlier and at lower ozone levels than less sensitive individuals.

People with unusual susceptibility to ozone. Scientists don�t yet know why, but some healthy people may experience health effects at more moderate levels of outdoor exertion or at lower ozone levels than the average person.

In general, as concentrations of ground-level ozone increase, more and more people experience health effects, the effects become more serious, and more people are admitted to the hospital for respiratory problems. When ozone levels are very high, everyone should be concerned about ozone exposure.

Air Quality Index

The Air Quality Index, or AQI, is a scale used to report actual levels of ozone and other common pollutants in the air. The higher the AQI value, the greater the health concern. As shown in the table below, the AQI scale has been divided into categories that correspond to different levels of health concern.