This is a result of local pollution, especially near busy roads, combined with pollution blown over from Europe and the Sahara.
While most people will not be affected by this, some individuals, particularly those with existing heart or lung conditions, may experience increased symptoms.
Pollution levels are expected to decrease tomorrow and return to a low level by Thursday.
Air pollution levels are affected by weather and at certain times of the year may reach higher levels for short periods of time.
We encourage people to take sensible precautions based on the levels of air pollution in their region and their health, such as reducing or avoiding strenuous activity and ensuring they have access to their usual medication, such as asthma inhalers.
Find out about current and forecasted air quality levels in your region via the following link: http://uk-air.defra.gov.uk or by calling Defra’s freephone helpline on 0800 556677. Both offer health advice to the general public and to those who may be particularly sensitive to air pollution. You can subscribe through the website to receive email bulletins for pollution forecasts and the latest pollution information, or follow @DefraUKAir on Twitter.
Information on the health effects of air pollution from the Committee on the Medical Effects of Air Pollutants (COMEAP) www.comeap.org.uk