Health warnings were issued to asthmatics and the elderly yesterday as it was revealed that air pollution in London has risen to its highest level for a decade during the heatwave.
Air-quality monitors in Enfield, north London, recorded pollution levels of 131 parts per billion (ppb) - almost three times the safe limit set by the World Health Organisation - as temperatures reached a record 35.4C (95.7F) in the capital on Wednesday.
Data collected for the Government showed scores of sites in the South-east - the worst-affected region - with ozone levels of 80-100ppb. This is about twice the norm for August.
Experts on environmental pollution said the ozone level was the highest recorded for about a decade. Ozone acts as a protective shield in the upper atmosphere but is a health hazard at ground level.
The highest level of ozone ever recorded in Britain is 250ppb at Harwell, Oxfordshire, in 1976, before strict EU laws restricting vehicle emissions were introduced. Although such peaks have been eradicated by innovations such as the catalytic converter, scientists from the National and Environmental and Technical Centre, which advises the Government, remain concerned about the gradual rise in aggregate ozone levels. They believe this is linked to climate change and global warming.
Yesterday afternoon slightly cooler temperatures and a light breeze caused the ozone levels in the capital to return to the August average of about 50ppb, but air-quality experts warned of a continuing risk with temperatures forecast to rise again tomorrow.
The current wave of pollution has been caused by hot and still weather combined with pollution that has blown in from the Continent.
The Environmental Research Group at King's College London said the first half of the year had produced alarmingly high levels of pollutant dioxides, which are converted into ozone in sunny conditions.
Frank Kelly, the director of the group, said: "This year is unique in terms of air pollution due to a combination of factors, and these latest highs in ozone levels are a serious concern for public health. Ozone can remain in the atmosphere for many days and is often transported over long distances.
"It is for this reason that a reduction in ozone levels can only be achieved through European-wide action. Studies have shown that European ozone levels have increased rapidly since 1940."
The ozone layer refers to naturally occurring ozone in the upper atmosphere, which protects the earth. However, ground-level ozone is linked to a number of respiratory illnesses.
The Department for Environment, Food and Rural Affairs issued a warning of the health hazards posed by ozone levels. It urged people to avoid exercise outdoors in the afternoon and warned that those suffering from lung diseases, including asthma, should be aware that their symptoms might worsen. It said the elderly were particularly at risk.
According to a government report, air pollution causes 1,600 people who suffer from breathing problems to die prematurely every year. A further 1,500, mostly suffering from asthma, are admitted to hospital because their symptoms become worse.
As the heatwave continued yesterday, speed restrictions remained in place on the railways because of heat causing rails to buckle. Network Rail said there had been 21 instances of rails buckling from Monday to Wednesday. A rail at Tonbridge, Kent, reached a temperature on Wednesday of 56C (132.8F).
The bookmaker William Hill has lengthened the odds against the temperature in England, Scotland or Wales hitting 100F from 5/4 to 6/4.
Graham Sharpe, a spokesman for the company, said: "The previous record temperature, set in 1990, happened at the end of a sustained heatwave and the current one shows few signs of ending yet, leading punters to continue to place hefty bets. One client from Nottingham staked £400 at odds of 6/4."
* Regularly updated information on air pollution is available on www.airquality.co.uk or Defra's freephone helpline on 0800 556677.Reuse content