With only 11 days to go until the start of the Beijing Olympics, this was the smoggy scene in the Chinese capital yesterday.

Visibility was down to half a mile in some parts, including the National Stadium, while the Athletes' Village complex could not be seen from the nearby Olympic Green.

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This was the smoggy scene in Beijing yesterday

The city's notoriously polluted air has cast a cloud over the Games, with organisers threatening to postpone events if it is bad.

City officials confidently - and possibly unwisely - predict that air quality will be good for the Games. Their efforts to curb pollution include taking half of Beijing's 3.3million vehicles off the roads and closing factories.

Today state media reported drastic measures - pulling more cars from the roads and shutting down additional factories - could be taken if the air quality does not improve.

Haze: Pollution today in Beijing was at its worst for a month, but officials say air quality will be good by the time the Olympics start on August 8

The grayish haze was one of the worst seen in Beijing in the past month despite tough traffic restrictions imposed a week ago to help reduce pollution.

The city's notoriously polluted air remains one of the biggest question marks hanging over the games, which begin on August 8.

'The air quality in August will be good,' Du Shaozhong, deputy director of the Beijing Municipal Environmental Protection Bureau, told reporters on Sunday. He did not explain the reasons for his optimism.

Du blamed the thick haze on a combination of fog and light wind unable to blow away the pollution, but he said pollution levels now are 20 per cent lower than one year ago in similar weather conditions. He did not provide any details.

'Our job is to decrease the pollution as much as possible, but sometimes it is very common to have fog in Beijing at this time,' Du said.

Pollution: Tough traffic restrictions have been in force since July 1 to improve the air quality in the Chinese capital, but athletes will be at the mercy of the winds

Olympic athletes have been trickling into Beijing, but are expected to begin arriving in larger numbers this week.

Some, though, were headed to training sites in South Korea, Japan and other places to avoid the Beijing air until the last possible minute.

'No, it doesn't really look so good. Yesterday was better but the day I arrived, Tuesday, was awful,' said Gunilla Lindberg, an International Olympic Committee vice president from Sweden who is staying in the Athletes' Village.

Jacques Rogge, president of the IOC, has warned that outdoor endurance events will be postponed if the air quality is poor.

Drastic efforts to curb pollution include pulling half of Beijing's 3.3 million vehicles off the roads, closing factories in the city and in a half dozen surrounding provinces, and halting most construction in the capital.

Some 300,000 heavily polluting vehicles, such as aging industrial trucks, have been banned since July 1.

Experts have said that while the measures are sure to reduce pollution, they are not a guarantee for blue skies during the games.

Wind can blow pollution to Beijing from thousands of miles away, while a lack of wind can cause chemicals and particulate matter to build up in the city.

'There's only so much you can do with local emission reduction,' said Veerabhadran Ramanathan, an atmospheric scientist at the Scripps Institution of Oceanography at the University of California San Diego.

He is leading a team that is studying the impact of Beijing's pollution reduction measures.

'You're basically at the mercy of the winds,' he said.