In urban areas last year, there were 17 days of moderate or higher air pollution on average per site, compared to 30 days in 1999 and 23 in 1998. The main causes of moderate or higher air pollution at urban sites are ozone, particulate matter and sulphur dioxide.
Vehicle emissions contribute significantly to ground level ozone production.
Ozone occurs naturally in the atmosphere and plays a critical function in the stratosphere by protecting life on Earth from the sun's harmful ultraviolet rays. NOx and SOx are produced by natural sources such as forest fires and by motor vehicles' exhaust and industrial emissions. The UK established National Air Quality Standards in 1993. The standards represent defined levels which avoid significant risks to health. As levels increase above the standard, the likelihood of effects on health increases.