Beijing was swathed in smog on Friday just two weeks ahead of the Olympics as its notorious pollution defied aggressive steps aimed at clearing the air for next month's Games.
However, Chinese officials brushed off concerns over the city's stubborn smog, which has triggered a warning by IOC chief Jacques Rogge that some events could be postponed if air quality is poor.
"Sometimes it looks like it's a foggy day, but the air quality is actually good," Sun Weide, a spokesman for the Beijing Olympic Organising Committee, told AFP.
"Our confidence is based on our 10 years of effort (to clean up the air). We are now implementing a continued plan to ensure clear air during the Olympics."
Chinese officials routinely refer to the city's smog as "fog".
Friday's smog came despite a broad last-ditch campaign kicked off last weekend to reduce air pollution by closing factories and banning more than a million cars.
Beginning on Sunday, cars with odd and even number plates are allowed on streets only on alternate days. Beijing had earlier taken 300,000 heavily-polluting vehicles off the road.
The city experienced a few days of air quality this week that was better than usual, raising hopes that the measures were working.
But by Wednesday, the familiar grey cloak had re-emerged and on Friday the air pollution index in the city averaged 130, or "light pollution."
By comparison, it stood at around 65, or "good" last Sunday, the first day of the driving restrictions.
Du Shaozhong, deputy head of the Beijing Environmental Protection Department, told reporters on Friday that major air pollutants emitted by cars, such as carbon dioxide, had dropped by 20 percent from July 1 to Friday.
He said the city had seen 22 "blue sky" days during that period, two more than the same same stretch last year.
Pollution is a major threat to the August 8-24 Games and Rogge warned last year that poor air quality during the Games could result in the suspension of some events, particularly endurance races such as the marathon.
Haile Gebrselassie, the Ethiopian marathon record-holder, has already pulled out of the Beijing race over fears the pollution could affect his asthma.