As top U.S. athletes make China their home for the 2008 Beijing Olympics, concerns are arising over the poor quality of the air blanketing Beijing in a dark haze.
"I have bad allergies, so I started to cough because of the bad air out there," Erinn Smart, a U.S. fencer, told The New York Times on Monday.
• Click here to view photos of athletes preparing for the 2008 Beijing Olympics.
Despite Olympic organizers' promise that there would be blue skies for the Games, thick smog was rampant Monday, and the United States issued special masks to its athletes ahead of their trip to China, The Times reported.
Organizers say the smog is from humidity, not pollution, and the conditions could subside when the region is hit with storms this week.
The International Olympic Committee has said it would cancel some endurance events if pollution levels are too high on the day of competition, The Times reported.
Meanwhile, police tightened security in China's far western Xinjiang region on Tuesday in the wake of a deadly attack on police that heightened security jitters ahead of the Beijing Olympics.
A full security alert was issued for government offices, schools and hospitals in Kashgar, and police numbers were boosted along roads leading into the city, the official Xinhua News Agency reported.
Officers boarded vehicles at checkpoints to search passengers' bags and during routine street patrols they used handheld devices to check people's identity cards, Xinhua said.
Sixteen officers were killed and another 16 injured Monday in an attack in which two assailants rammed a dump truck and hurled explosives at a group of policemen in Kashgar, near the Afghanistan-Pakistan border on the far side of China from Beijing.
China's Olympic organizers immediately said security precautions were in place for Beijing and other Olympic venues ahead of the Games' Friday start.
China has made safety a major priority for the Olympics, mobilizing hundreds of thousands of police and military forces and local residents as part of a huge security net over the capital. But with so much security focused on Beijing, the Xinjiang attack underscored the vulnerability of areas far from the capital.
The Associated Press contributed to this report.