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From time-to-time weather conditions lead to higher than normal levels of air pollution.  In winter, cold, still conditions can lead to a build up of pollutants at ground level; in the summer, hot weather, without much wind, also leads to raised concentrations of pollutants including ozone.  These raised levels of air pollution can be known as 'smog'.  Raised concentrations of air pollutants can affect health.

Ground level ozone is formed when sunlight acts on nitrogen dioxide and other atmospheric substances close to the ground. The pollutants that cause ground level ozone come from a range of sources, including petrol and other fuels.

Some people are more sensitive than others to ozone and may notice an effect on their breathing when concentrations are raised. People with asthma are not necessarily more sensitive and, if affected, can use their 'reliever' inhaler.

In such instances the public is urged to take sensible precautions such as avoiding exercise outdoors in the afternoon to reduce exposure to ozone.

Regular updates on levels of particulate matter, sulphur dioxide, nitrogen dioxide, ozone and carbon monoxide are available at Defra's UK Air Information Resource webpages and from Defra's freephone helpline (0800 556677), which also offers health advice.

Air Quality Measurement and Forecasts

Air pollution is described as 'Low (1-3)', 'Moderate (4-6)', 'High (7-9)' or 'Very High (10)'. The classifications were chosen on the basis of effects on health and are based on medical and scientific research. Full details of the bands for all the pollutants are available on Defra's Air Pollution Health Bandings webpage.

Health advice

When pollution in the UK is classed as 'high' or 'very high', levels of ozone, nitrogen dioxide and particles may be raised. Most people will experience no ill effects. Those suffering from lung diseases, including asthma, particularly the elderly, should be aware that their symptoms might worsen.

They may need to consider modifying their treatment as they usually do when symptoms increase, consulting their doctor if this is not effective.

People who have noticed in the past that their breathing is affected on hot, sunny days should avoid strenuous outdoor activity, particularly in the afternoon.

Children with asthma should be able to take part in games in the usual way, although they may need to increase their use of reliever medicines before participating. There is no need for them to stay away from school.

Those suffering from a heart condition and who notice a change in their symptoms should get medical advice as they normally would.