BEIJING — As Olympic and city officials continued to defend the air quality here, athletes were faced with the decision Friday of whether to attend the opening ceremony in the thick smog and, if they do go, whether to wear a mask to combat the pollution. 

Matt Reed, a triathlete for the United States, said he would wear a mask during part of the opening ceremony, but was cognizant of the attention it might receive.

Earlier this week, four cyclists on the U.S. team wore masks when they arrived at the airport, then were scolded by the United States Olympic Committee for embarrassing their Chinese hosts. The cyclists subsequently issued a written apology through the U.S.O.C.

“I won’t wear it walking out,” Reed said after a workout at Beijing Normal University on Friday morning. “But when we’re standing there not doing anything, on the field, I’ll wear it.”

On the long-awaited day that these Olympics officially opened, the sky was smoggy and the local air quality rating was 94, well above the limits in the United States. Still, anything below a 101 qualifies as a “Blue Sky Day” here, according to the standards set to monitor air quality for the Olympics.

Local environmental officials said at a hastily organized news conference that they were happy with the air quality here, after so much effort to clear the air for these Games.

“We have gone through a very extraordinary process to improve its air quality and we have achieved outstanding results,” said Du Shaozhong, deputy director and spokesman of the Beijing municipal environmental bureau.

Du said Beijing had met the World Health Organization’s international standards of air quality, which was what the Beijing Olympics organizers had promised the International Olympic Committee in 2001, when the city was awarded the Games. Later, Du said that the city had met the standards set in 1999, not more updated standards that include measures of tiny dust particles that cause most of the pollution here.

On Thursday, Jacques Rogge, the I.O.C.’s president praised Olympic organizers for their efforts, which have improved the air in this city by implementing measures like shutting down coal-burning factories or reducing cars.

John Coates, the president of the Australian Olympic Committee, on Thursday was among the first Olympic officials to acknowledge the smog at the Olympic Village was caused by pollution — and was not simply fog from the high humidity.

“I don’t know how you reverse some of these things,” Coates said. “Let’s hope that’s one of the legacies of these Games, that the realization of the damage that’s been done and will continue to be done unless they are more careful.”

For the time being, athletes are left to deal with the air, which was gray on Friday when five of the six American triathletes arrived from their training camp in Cheju, South Korea, to take part in the ceremony. They will leave Beijing again on Saturday morning, returning on Aug. 15. The women’s triathlon is on Aug. 18, and the men race on Aug. 19.

None of the American triathletes plan to wear the masks during the actual races, but will wear them as much as possible while they are outside. They wore them while walking around the athletes’ village and while riding buses to and from the American training center, where they stayed indoors to train.

“I’m definitely conscious,” Reed said of the attention placed on what has become a delicate matter for American and Chinese officials. “I don’t want to offend anyone, but I’m out for my own health, really. And I’m sure that they know that the air quality is not good, so I don’t see it’s that bad wearing a mask.”

Last September, pollution levels were similarly high during a World Cup triathlon. American triathlete Jarrod Shoemaker wore a mask up until race time, and then was the top American finisher (11th overall), clinching an Olympic berth.

“It’s different than what I remember from last year, in that the pollution looked more yellow and dark,” Shoemaker said. “It’s whiter this year. I think it’s a combination of a different time of year and little bit more mist and humidity in the air. So it doesn’t look as bad, but you can still kind of taste it.”

Still, the chance to take part in the opening ceremony was too big a lure to keep most of the American triathletes in South Korea. Only Sarah Haskins decided to continue her training there and not make the 90-minute flight for two nights in Beijing. Triathlon officials encouraged the athletes to come to the opening ceremony.

“We’re still far out enough from our competition that a little exposure isn’t going to really affect them that much,” the USA Triathlon team leader Scott Schnitzspahn said. “But we still don’t want them running around sucking on a tailpipe.”

In the months leading up to the Olympics and even this week, Rogge said that the I.O.C. could postpone events because of poor air quality up to 72 hours before the events. The sports most affected would be ones like road cycling, triathlon and the marathon in which athletes would be most affected by noxious air because of their prolonged exposure to the pollution.