The haze was back on Monday after several more or less clear days, bringing with it new concern about pollution affecting the Beijing Olympics, reported dpa.
"We are looking into what is happening. Yesterday was a wonderful day. Today, is it hazy? Is it pollution? I don't know," said International Olympic Committee marketing chief Gerhard Heiberg.
Local organizing committee (BOCOG) spokesman Sun Weide said the Air Pollution Index (API) was below 100 and therefore suitable for competition.
According to the forecast, today's air quality is grade two. The API is under 100," said Weide. "We thing this is suitable for outdoor activities, including sports events.
The IOC has announced contingency plans for endurance events such as cycling road races, marathons and triathlons of the air quality is too bad during the Games which open on Friday and run until August 24.
"We are most concerned for the athletes," said Heiberg.
China has moved and closed down factories and is imposing drastic traffic restrictions in Beijing to reduce pollution. Further restrictions could apply during the Games if necessary.