Jogging to stay healthy? If you live in a heavily polluted city like Hong Kong, don't bother: the benefits are probably outweighed by the increased exposure to pollution. We typically inhale six to 10 litres of air per minute, but exercising athletes can increase this by up to 30 times. They also tend to breathe more deeply and through the mouth rather than the nose, bypassing the body's defences such as the mucous membranes of the sinuses and drawing pollution deep into the recesses of the lungs. As the American Lung Association's website notes: 'The interaction between air pollution and exercise is so strong that health scientists typically use exercising volunteers in their research.' For this reason, more organisations such as the American Heart Association and the Lung Association of Canada now recommend avoiding exercising outdoors on smoggy days. For the smog levels at which the US government recommends that all outdoor activity cease, Hong Kong guidelines say no action is necessary. Wong Tze-wai, of the Chinese University of Hong Kong's community medicine department, said studies on local children who exercised a lot in highly polluted areas found that their fitness levels as reflected by Vo2 max (a measure of the body's ability to metabolise oxygen) did not increase with levels of exercise as they would in healthy subjects. WHAT TO DO Avoid running on busy roads. Exercise early or late in the day, when pollution levels are lower. Try to minimise activity on smoggy days. Stick to the gym - exercising indoors does have some benefit. Avoid combinations of high heat, humidity and pollution.