According to data from Beijing Environmental Protection Bureau, air quality on Monday was still considered a "blue sky day".
But the BBC’s own test found one major pollutant, particulate matter, was almost six times higher than the recommended level.
The World Health Organization’s target is 50 micrograms per cubic metre. But we recorded levels in Beijing at 292 micrograms on Monday.
The test was done at a time of day when many Olympic events will take place.
From the International Herald Tribune:
The body's reaction to pollution exposure is immediate, said George Thurston, a professor of environmental medicine at NYU School of Medicine.
"Your body says, 'This air is bad; breathe less of it,' and that's a defensive mechanism," Thurston said. "For athletes, that means they will go into oxygen debt sooner and will start cramping up. At an event like the Olympics, that could be disastrous."
And if the smog isn't enough, how about the heat? From Australian doubles specialist, Sam Stosur:
"It's very hot, very humid and the sun's not even out yet so it can only get worse from here... It's unpredictable and probably the hottest conditions we are going to play in all year."