Dirty old town: exercise early or very late to avoid pollution, say experts / AFP/Getty

Britain's cities are among the worst polluted in Europe and it's not just Olympic athletes who'll suffer. Jamie Merrill reports on the toxic threat we face – and how urban exercisers can avoid it

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 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.

The majority 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 particulates, 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 next week. "It could have an effect on competitors during the Games, 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. 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 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.


Pollution-free fitness: The essential guide to working up a sweat in the smog

Steer clear of busy streets

Exercising out of town is, of course, much better for you, but there are still things you can do in town to limit the extent air pollution can damage your health. "It is amazing how much you can reduce your exposure to particulate emissions by simply moving away from the source of the pollution," says Professor Frank Kelly. "So avoid busy roads and use side streets and parks instead." If you commute to work along a main road you should consider altering your route to reduce long-term exposure.

Get out early

Ozone pollution, what we often see as a hazy smog, is produced when pollutants, such as NO2, react with sunlight. "The main way to avoid ozone pollution is to get your exercise in early or late in the day," says Professor Kelly. "This avoids the poor air quality we can see when the temperature rises in the afternoon and NO2 becomes fixed in the atmosphere".

Stay informed

With air pollution, forewarned is definitely forearmed. There are probably only 15 major periods of concern for most healthy people each year – many more in certain local areas. To stay one step ahead of the smog you can check online for poor air-quality areas and in particular which sort of dangerous emission is high. For example. If NO2 levels are high, get your exercise in early but if PM10 levels are high you'll want to avoid busy roads. You can check Defra's Daily Air Quality Index and the London Air Quality Network in the capital for more information (defra.gov.uk; londonair.org.uk). Professor Kelly recommends downloading the free app from the London Air Quality Network to your smartphone.

Use protection

This one's tricky; should your wear a respiratory mask or not? The jury is still out. The British Lung Foundation advises that people can wear a mask when levels of pollution are very high. However, they also note that “there is not yet conclusive evidence that they are effective at UK air pollution levels”, adding that "in some cases they can even make breathing and exercise more difficult". Professor Kelly however isn't convinced and argues they will not "protect you against tiny PM10 particles that will just go straight through the filter". There is some evidence though, Professor Kelly admits, that a mask fitted with an activated charcoal filter will help to protect you from NO2 and ozone pollution. Similarly, science is split as to what benefit may be gained from taking antioxidant supplements. They could help to protect against oxidative damage but little research has been carried out. That said, it won't harm to eat antioxidant-rich fruit and vegetables, such as broccoli, red peppers and melons.

When to call it a day

For some people with pre-existing respiratory conditions, exercising in heavy pollution is a bad idea. The Defra website has a scale of pollution alerts depending on your condition. "For most people, though, exercise is an important part of staying healthy, and exercise should only be avoided if levels of pollution are very high," says Dr Keith Prowse. "And if you prefer to avoid pollution by exercising in an air-conditioned gym, remember to remain in the air-conditioned environment until your breathing has returned to its normal rate."