It is considered a real possibility in certain weather conditions, as levels in the capital of several pollutants are so high that they are in breach of EU limits. London has the highest levels of the toxic gas nitrogen dioxide (NO2) in the EU, and has received a series of legal warnings for failing to comply with European laws governing PM10s, tiny specks of particulate matter 10 microns across (a micron is a millionth of a metre). The capital's air quality is also affected by the gas ozone, created by pollutants from vehicle exhausts reacting with sunlight.
Experts say that the risk for the Games is that in certain summer weather conditions – in particular, a "temperature inversion" in which on still, hazy days, a layer of warm air traps pollutants close to the ground – the pollution levels could go so high as to affect athletes' health and performance.
Temperature inversions are common, and affect people more in the summer, according to the Met Office. "It's not a rare thing. It can happen all the time," a Met Office spokesman said yesterday.
"If we have a high-pressure temperature inversion period, there may well be high levels of ozone and nitrogen dioxide and these could induce coughs, breathlessness and other problems," said Professor Sir Malcolm Green, spokesman for the British Lung Association.
Frank Kelly, professor of environmental health at King's College London, said: "...Athletes, such as marathon runners and cyclists, need to breathe very hard. If it's a high-pollution day, they will be taking in large amounts of pollution. Their chests may tighten up, they may feel pain and shortness of breath, and for certain conditions such as asthma they may need medication..."
At the last Olympics in Beijing, when the Chinese government issued a blanket ban on more than half the city's cars and shut down polluting industries, at a cost of £6bn. There is no sign of London taking such action .