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.