The cloud hanging over the Olympics

How bad is the air in Beijing?
It's pretty awful. Beijing's thick, nasty combination of carbon monoxide, sulphur dioxide, nitrogen dioxide, and assorted particulate matter routinely fails China's own relatively lenient air quality legislation; it fails the guidelines of the World Health Organisation (WHO) several times over. Often a dense, yellowish haze blankets the entire metropolis - or 'Greyjing', as it has been nicknamed. Researchers estimate that more than 400,000 Chinese people die prematurely from air pollution annually, with as many as 25,000 deaths in Beijing alone.

Why is there so much smog?
The problem dates from well before China's recent, ferocious economic development and is by no means confined to Beijing. In his brutal mid-century remodelling of the city, Mao Zedong tore down temples and palaces, declaring that he wanted to "look out from Tiananmen Gate and see smokestacks". In the past decade those factories have been running overtime. Since China was awarded the Olympics in 2001, the nation's industrial output has increased 80 per cent. Most of this growth has been fuelled by primitive, coal-fired furnaces and boilers, which spit out tons of choking hydrocarbons. Car ownership has also tripled in the last decade, with 3.3m cars now on Beijing's roads. Natural factors are at work, too. Dust storms from the Gobi Desert fill the air with soil, and since mountains surround Beijing on three sides, pollutants tend to stay trapped there, like soup in a bowl.

What does all this mean for athletes?
Considerable unease. The run-up to the Games has been marked by dire warnings and fears that the pollution could hamper athletes and harm spectators alike. Last year a WHO air quality expert told the BBC that people with heart and lung problems should not visit Beijing at all. Haile Gebrselassie of Ethiopia, the world's fastest long-distance runner, announced in March that he would not be running in the marathon, although he will still take part in the 10,000m. "It's going to be the hardest marathon in history," he said. "I'd love to go for it, but health is my first priority." The air is so dirty you can taste it, says US triathlete Jarrod Shoemaker. "You can feel the grit in your teeth." But there have been some unexpected benefits: