Beijing has vowed the Olympic Games will take place under blue skies, not the murky haze that has shrouded the city recently.
Officials have spent billions of dollars on cleaning up the capital with factories dozens of miles away closed down, construction halted, and over half the city's 3.3 million cars cleared from the roads.
Still, even when Beijing says the air is clear, athletes and their coaches may have cause for concern because Chinese air pollution standards are outside World Health Organisation guidelines.
Experts say the pollution index China uses to tell ordinary citizens whether the air is safe - a "blue sky" day - is seriously flawed.
