Ozone levels in Oxfordshire were the worst in Britain for one day last week.
The Department of Environment's own air quality monitoring system recorded ground level ozone at 78 parts per billion in Oxfordshire on Thursday.
The concentration was the highest in Britain and more than 20 parts per billion higher than city centres in Birmingham, Manchester, Glasgow and Leeds.
Ground level ozone forms when other pollutants such as nitrogen oxide and volatile organic compound gases react with sunlight. It can impair people's ability to breathe and causes shortness of breath, chest pain, wheezing and coughing. The Government's critical level for ozone is 120 parts per billion.
Craig Simmons, of the Oxford Green Party, said: "The level of ozone in Oxfordshire on Thursday could cause problems. The main cause of ground level ozone are exhaust fumes from cars. We have an awful lot of traffic in Oxfordshire and private car ownership is ex- tremely high."
The concentration of ozone had fallen back to 37 parts per billion on Friday.
Story date: Monday 24 May
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