Summertime smog levels broke Government health limits at 76 out of 80 Government pollution monitoring sites last week, according to analysis released by Friends of the Earth.

And at 25 sites smog levels broke health limits every day from Monday 4th to Sunday 10th August. At some sites the levels experienced were more than double the health standard.

But city escapees and holidaymakers hoping to leave the smog behind in the city may have been thwarted by the physics of the chemical reaction that produces the smog.

The interaction of nitrogen oxides and hydrocarbons in sunlight form ozone pollution, called summertime smog. This means that levels tend to be highest in summer, when there is more sunshine. The principal source of both nitrogen oxides and hydrocarbons is road traffic.

However, contrary to popular expectation, higher levels of ozone are seen outside of cities because the reaction takes time to occur and so the worst pollution is often seen downwind of the source of the road traffic emission of the chemicals.

Not all of the ozone found in the UK is the result of emissions in the UK. High ozone levels in southern and eastern areas can be as a result of emissions in continental Europe, if the wind is in the right direction.

The 25 sites that repeatedly broke the limit were:

Five sites recorded pollution levels more than double the health standard. These were:

• Bournemouth (Sat 9th).
• Harwell (Sat 9th).
• Portsmouth (Sat 9th).
• Wicken Fen (Weds 6th).
• Wigan Leigh (Sat 9th).

Ozone causes coughing and choking, headaches, eye, nose and throat irritation and chest discomfort on deep breathing. It aggravates asthma symptoms and recent studies in California have suggested that ozone can cause asthma. Government health experts have estimated that ozone pollution causes up to 12,500 premature deaths and up to 9,900 additional hospital admissions for respiratory problems every year.

Friends of the Earth's Pollution Campaigner Tony Bosworth said: "Our record-breaking sunshine has brought a lot of smog, breaching health standards nation-wide.”

“And because of the way ozone pollution travels, people who tried to escape the heat of our cities for a breath of fresh air will have found they got just the opposite.”