There's nothing sweeter than the smug feeling that you're getting the better of the rat race. While your fellow commuters struggle in on the bus, you sail by on your bike or jog along overtaking them at every traffic jam on your way to work. But what if instead of doing your body a favour, you're really exposing it to dangerous air pollution?
Exercising or playing sport in Britain's cities can be a dirty affair. We have some of the worst air quality figures in Europe, London is one of the EU's the worst offenders for poor air quality and as many as 30,000 people across the country die prematurely each year because of poor air quality – more than die from alcohol abuse, obesity or in car accidents.
Most of this pollution comes from emissions of toxic gases and particulates from car exhausts and dirty diesel engines. These include nitrogen dioxide (NO2) gas and PM10 and PM2.5 particulates2w, tiny specks of oily and partially combusted matter.
There is no known "safe" exposure level to PM10s and air quality is so poor in some places, and particularly in the capital, that the UK is regularly in breach of EU limits and is threatened with a total of £300m in fines.
"The last time there was this much concern about air pollution in London was during the 'Great Smog of 1952'," says Simon Birkett, director of air pollution watchdog Clean Air in London.
"Levels of nitrogen dioxide are more than twice World Health Organisation guidelines and legal limits near London's busiest streets."
According to Birkett, we face a "perfect storm" of air quality concerns in the run up to the Olympic Games this week. "It could have an effect on competitors, particularly in endurance events. Athletes who may be prone to conditions such as asthma may get a cough or increased breathlessness," adds Dr Keith Prowse, a medical adviser to the British Lung Foundation.
Professional athletes aren't the only people who need to be concerned though. "Risks from exercising in areas of poor-air pollution include the aggravation of pre-existing lung conditions such as asthma and chronic obstructive pulmonary disease. Even in those with no history of chest disease there may be irritation to the whole respiratory tract, from the nose down to the small tubes in the lungs as a result of fumes and particulates," says Dr Prowse.
A 2005 study from Edinburgh University showed exposure to diesel pollution while exercising causing blood vessels to become less flexible and produce less of a protein that breaks down blood clots in the heart.
This is because exercise increases how deeply you breath and more particles bypass the nasal filters (your nose hairs) that trap dangerous particles. Other studies have shown links between air pollution and various forms of cancer. And it's not just athletes and fitness fans in London that need to worry. Twelve UK cities including Bristol, Brighton, Birkenhead, Liverpool, Newcastle Preston, Tyneside and Sheffield face legal action from the action from the European Commission over air quality. And 28 zones across the country are expected to fail to meet legally binding EU limits by 2015.
The weather plays a big part, too. "Air pollution is worse in very calm conditions, high pressure and when there is no wind to disperse it," says Dr Prowse. Air pollution leves are already rising according to Defra and if it stays hot during Games we may see a "smog episode".
This is when on still, hazy days, a layer of warm air traps pollutants like NO2 and PM10s close to the ground. Smog episodes are not uncommon and there have been five already this year.
So does that mean you should avoid exercise then? "No," says Professor Frank Kelly, of the environmental health department at King's College London. "Not exercising at all is far worse than exposing yourself to air pollution."
The NHS recommends 150 minutes of aerobic exercise a week and missing out on this can lead to long-term health problems – figures published last week in medical journal The Lancet show that lack of physical activity is responsible for as many deaths as smoking.
"You just need to take the right precautions, stay informed and understand what sort of pollution you are dealing with," says Professor Kelly.