BEIJING — The Chinese capital was shrouded in thick gray smog on Sunday, just 12 days before the opening ceremony of the Beijing Olympic Games. One expert warned that drastic measures enacted to cut vehicle and factory emissions in the city were no guarantee skies would be clear during competitions.

The pollution was among the worst seen in Beijing in the past month, despite traffic restrictions enacted a week ago that removed half of the city's vehicles from roadways.

Visibility was a half mile in some places. During the opening ceremony of the Athletes' Village on Sunday, the housing complex was invisible from the nearby main Olympic Green.

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

"We try to be hopeful. Hopefully we are lucky during the games as we were with Atlanta, Athens and Barcelona," she added.

The city's notoriously polluted air is one of the biggest questions hanging over the games, which begin on Aug. 8. On Sunday, temperatures of about 90 degrees, with 70 percent humidity and low winds, created a soupy mix of harmful chemicals, particulate matter and water vapor.

Athletes have been trickling into Beijing and were expected to begin arriving in large numbers this week — though some were headed to South Korea, Japan and other places to avoid Beijing's air for as long as possible. Some Olympic delegations, including the U.S. Olympic Committee, are making protective masks available to their athletes.

The Beijing Municipal Environmental Protection Bureau said Sunday's air was "unhealthy for sensitive groups."

The Chinese leadership consider the Beijing Olympics a matter of national prestige, and efforts to clean up the environment were part of its meticulous preparations for an event it hopes will dazzle the world. Choking air pollution and visitors shocked at the environmental conditions would be an embarrassment for a government that wants to show itself is a modern nation.

"Hosting a successful Olympics and a Paralympics are now top priority of the country," Chinese President Hu Jintao said Saturday during a meeting with top Communist Party officials, the official Xinhua News Agency reported.

Du Shaozhong, deputy director of the Beijing Municipal Environmental Protection Bureau, blamed the thick haze on a combination of fog and light winds that were unable to blow away the pollution.

"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.

"The air quality in August will be good," he said.

Du noted that compared to days with the same weather conditions a year ago, pollution levels had decreased by 20 percent. He did not give specifics.

Du was supported by Dr. Patrick Schamasch, an orthopedic surgeon who is the IOC's medical and scientific director. Schamasch said the IOC was monitoring Beijing's air. He said particulate matter on Sunday "was a little bit higher than what's expected but nothing dramatic."

He said readings for ozone, carbon monoxide, sulfur dioxide and nitrogen dioxide fell within 2005 guidelines set by the World Health Organization.

He did not disclose the exact levels, saying it was not the IOC's practice.

"Today there is nothing critical preventing an athlete from running, except the visibility," he said. "I can tell you it's mist more than smog."

Schamasch said conditions were "not worse" than in other cities that hosted the games, mentioning Los Angeles, Atlanta and Athens.

Beijing's drastic pollution controls include pulling half the city's 3.3 million vehicles off the roads, closing factories in the capital and a half-dozen surrounding provinces, and halting most construction. Some 300,000 heavily polluting vehicles, such as aging industrial trucks, have been banned since July 1.

Veerabhadran Ramanathan, an atmospheric scientist who is leading a study of the impact of Beijing's pollution controls, said the direction and strength of the wind will be a main factor in whether the air will be clean during the Olympics.

"There's only so much you can do with local emission reduction," he said.

Wind can blow pollution in from thousands of miles away. Conversely, a lack of wind can create stagnant conditions in the city, allowing pollution to accumulate.

"I applaud the Chinese government for doing this locally, but the thing is, as scientists we all knew it may not make a major impact," said Ramanathan, of the Scripps Institution of Oceanography at the University of California San Diego. "You're basically at the mercy of the winds."

Jacques Rogge, president of the IOC, has warned that outdoor endurance events will be postponed if the air quality is poor. The world's greatest distance runner, Haile Gebrselassie of Ethiopia, has decided not to run the marathon because the city's pollution irritates his breathing.

AP Sports Writer Stephen Wade contributed to this report.