Athletes should avoid exercising by the road side, even though levels of the more noxious air pollutants have been controlled in the UK: that is the key message of a major review of exercise and outdoor ambient air pollution published in the British Journal of Sports Medicine.

Scientists from Brunel and Surrey universities surveyed the literature to establish the levels at which air pollutants are considered damaging to humans in general and athletes in particular, the current ambient levels of these pollutants in the UK and whether athletes are particularly at risk. The six major urban air pollutants examined were: carbon monoxide, nitrogen oxides, ozone, particulate matter, sulphur dioxide and volatile organic compounds.

'Air pollution continues to be a matter for concern despite falling levels of some of the major pollutants,' they explain. 'Many of the effects of air pollution on human health have long been established, but no clear consensus has been reached on the effects of ambient air pollution on the exercising athlete and sport performance.'

Athletes may be at particular risk of inhaling pollutants for three reasons:

l. There is a proportionate increase in the quantity of pollutants inhaled with increases in ventilation during exercise;
2. A larger fraction of air is inhaled through the mouth during exercise, effectively bypassing the normal nasal filtration mechanisms;
3. The increased air flow velocity carries pollutants deeper into the respiratory tract.

The researchers' advice to athletes can be summarised as follows:

Stay away from traffic while exercising. There is an exponential decline in concentrations of many air pollutants with increasing distance from busy roads, and it is advisable to exercise whenever possible in open rural or park land. If you can't avoid the roads completely then try to avoid the rush hour, when nitrous oxides, carbon monoxide and volatile organic compounds are likely to accumulate;

Check the pollution forecasts and bulletins and heed any warnings. If it is cold and smoggy, exercise indoors;

Take particular care when travelling. Some countries have less stringent pollution control regulations than the UK and air quality could be considerably different as a result. Climatic and geographic conditions can result in much higher accumulations of pollutants in, for example, Los Angeles, so even in 'developed' countries dangerously high pollution levels may occur;

Beware of indoor risks. Indoor environments pose more risk of carbon monoxide poisoning than the open road, so athletes should keep away from smoky environments and avoid car journeys in congested traffic before competition and training;

Avoid exposure to elevated levels of ozone (which can cause respiratory problems) on hot bright days in the UK by exercising in the early morning or late evening;

Asthmatic athletes beware. Sulphur dioxide is unlikely to be a problem for athletes with normal lungs at current ambient levels but may cause problems for asthmatic athletes, who should take their medication before exercise and carry an inhaler while exercising;

Gain protection from your diet. Athletes should protect themselves by eating a diet rich in antioxidant fruit and vegetables and/or taking antioxidant supplements as there is some evidence that these can reduce the detrimental effects of some pollutants.

Finally, athletes should bear in mind the duration of their exposure to pollutants. 'The length of time spent exercising is another very important factor,' conclude the researchers. 'Ultra-marathon runners and others participating in long endurance events - for example walking and cycling - are likely to be most at risk from the negative and harmful effects of pollution, exacerbated by exercise.'

Br J Sports Med 2001 Aug 35(4), pp214-22

Isabel Walker