A study, published in the journal Occupational Environmental Medicine, found that the poisonous gas carbon monoxide, found in the exhaust fumes of cars, severely reduced exercise performance.

It is known that carbon monoxide can produce abnormal heart rhythms in patients with existing coronary heart disease.

But the research, carried out in Haifa, Israel, suggests that the gas can work its way into the skeletal muscles of healthy people and reduce their capacity for exercise.

And these effects occurred after even a short exposure to low concentrations of the gas.

The invisible, odourless, killer

Carbon monoxide is especially dangerous because it is invisible, and cannot be detected by smell.

It works by binding with a substance called haemoglobin in red blood cells.

Haemoglobin is the substance which the body uses to transport oxygen around the body - but if the molecules are taken out of action by being linked with carbon monoxide, the body can be starved of oxygen.

The effects of the gas are culmulative - it takes weeks for the affected haemoglobin to be removed from the body.

Although high levels of carbon monoxide, such as those produced by a faulty gas appliance, can kill, even low levels produced by car exhaust fumes can impair the body.

The researchers gave enough gas to their healthy, non-smoking volunteers to increase blood concentrations of carbon monoxide - but not enough to produce any symptoms of poisoning.

They found that, after breathing carbon monoxide, 13 out of 15 men could not keep exercising for as long.

In once case, the subject found he could work out for only 15 minutes with carbon monoxide, as opposed to 20 minutes without.