Health reporter, BBC News
Imagine you are an Olympic athlete who has spent most of your life training towards what could be your one shot at a gold medal.
A finely tuned training programme has enabled you to acclimatise to conditions you are unaccustomed to, such as heat or altitude.
But on the big day there is one aspect you have no control over - the quality of the air you are breathing in.
BBC tests suggest the level of pollution in Beijing is currently well outside international air quality standards.
It has been reported that the International Olympic Committee is being flooded with applications from athletes to use asthma medication during the Games.
But is this an over-reaction or can pollution really affect an athlete's chances?
John Brewer, performance director at the Lucozade Sport Science Academy in Slough, says it absolutely can but the extent of the effect will depend on the event.
The outdoor competitors least at risk of suffering from the effects pollution will be the sprinters who barely take a breath before the race is over and others doing short sharp intensive events such as the shot put and javelin.
Athletes competing for longer lengths of time, such as rowers and medium distance runners may notice the effects more, he says.
And those playing team sports should not be forgotten - hockey players for example have short bursts of activity from which they need to recover quickly.
But competitors who should be most worried are those doing endurance races, such as cycling and most notably the marathon.
Marathon runners, such as Paula Radcliffe may suffer most from pollution
"Marathon runners take about 40 to 50 breaths per minute and there is a real need for oxygen to be transported to the muscles.
"In normal conditions oxygen makes up about 21% of the air, if that's compromised, because the very complex transport process in the lungs is compromised, there will be less oxygen getting to the muscles.
"Add in the heat and the humidity and there could be some major implications," says Brewer.
No one has done a study comparing the effects of say running a marathon in clean air and in a polluted environment.
But what evidence there is suggests there will be an impact.
"If the pollution is still at the level that has been recorded by the BBC on the day of a distance endurance event then it is almost certain that performance times will be affected," Brewer says.
PM10 LEVELS IN CENTRAL BEIJING
WHO maximum: 50 micrograms/cubic metre
"The organisers have to take a responsible attitude to endurance events and be brave enough to change the start time or withdraw athletes if needed - they need to protect the health and wellbeing of the competitors."
The BBC has measured the levels of tiny particles (known as PM10) in the air for 10 minutes at the same time each day.
Research shows the size of the particles enables them to get right down into the part of the lungs where oxygen is transferred into the bloodstream.
Once there they cause irritation and inflammation in healthy people and are liable to exacerbate existing conditions such as asthma.
High levels of PM10 have also been directly linked with higher mortality rates by the World Health Organisation.
Professor Sir Malcolm Green, vice president of the British Lung Foundation said it was important to measure air pollution over 24 hours because it can be affected by rush hour traffic or weather conditions.
"But if the 24-hour average for PM10 is 100 or 150, it is unacceptably high and it would be highly undesirable for people to be doing marathons with that level of pollution.
"And particularly for Olympic athletes who are pushing their heart and lungs to the absolute maximum."
He said closing the roads before the start of the Olympics, as was planned by the Beijing authorities should help.
Such pollution can cause greater problems in those with underlying respiratory conditions.
Paula Radcliffe, who is still hoping to compete despite recovering from a stress fracture, is one athlete who famously suffers from asthma.
"Anything that causes irritation of the lungs is likely to exacerbate an existing condition," Sir Malcolm warned.
For their part the British Olympic Association is increasingly confident that the air quality in Beijing is improving.
They have warned athletes that the heat and humidity are likely to have more of an impact on performance and have taken expert advice on how to mitigate the effects.
Brewer adds: "I was in Seoul in 1998 when they banned the traffic and the problems that had been predicted didn't materialise.
"I hope that's the case this time and don't forget the athletes will have been prepared for it."