China's National Stadium, known as the Bird's Nest, and the National Aquatics Center, known as the Water Cube, left, are seen through pollution in Beijing. Authorities last week began strict new measures, including taking half of the city's 3.3 million cars off the road, in an effort to reduce chronic air pollution before the Olympic Games open on Aug. 8.

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 Chinese leadership considers 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.