BEIJING — Smog? What smog?

Pollution concerns evaporated Friday under a picture-perfect canopy of blue skies and white clouds on the first day of the Olympic Games' signature track-and-field events.

Heavy showers that drenched the city on Thursday cleared away much of the pollution, giving Beijing its first genuinely sunny day since the opening ceremony kicked off a week ago. Temperatures hovered around a comfortable 28 degrees Celsius (82 Fahrenheit) with little humidity and a light breeze.

The heavy haze, along with the heat and humidity, that had shrouded the city for days disappeared, giving spectators and athletes alike a clear view from the 91,000-seat Bird's Nest stadium.

Air quality levels reflected the crystalline skies. Beijing's official air pollution index was at 17 — the lowest recorded since the city undertook drastic measures starting in July to curb emissions of major pollution.

"First day I can breathe freely, it's great," said Australia's Tamsin Lewis, the world indoor 800-meters champion, who qualified for the semifinals during the morning. "One of my competitors walking out said it reminded her of Melbourne for the Commonwealth Games, so there you go."

The city's air pollution has been the focus of major concern for months leading up to the Games. Beijing typically has air that is two to three times dirtier than most Western countries. In a major bid to clear up the skies, city officials shut down scores of factories, stopped all construction and removed 2 million vehicles from the road for a two-month period.

The efforts have seen mixed results. Since the measures began in July, Beijing had only a handful of days where the level of particulate matter — tiny dust particles that are the worst pollutant — fell within the range of what the WHO considers healthy.

The Associated Press has been taking independent air samples from the Olympic Green, the primary sports thoroughfare, since mid-July. Readings taken between 3 and 4 p.m. on Friday showed 42 micrograms of particulate matter per cubic meter, down from 97 on Thursday. It was well below the high during the games of 604, set on Sunday.

Health experts say that breathing polluted air can affect respiratory functions and particularly worsen problems for those with asthma or allergies. The biggest concern has been whether the performances of outdoor endurance athletes, who would be exposed to dirty air for more than one hour, would be adversely affected.

Athletes, including world-renowned distance runner Haile Gebrselassie, an asthmatic who holds the world record in the marathon, opted out of some competitions over concerns about Beijing's smog and heat. Instead of running the marathon, he will compete in the 10,000 meters.

Athletes from the U.S. and several other countries chose to train far outside Beijing, in some cases in neighboring Korea or Japan to avoid being exposed to the air pollution.

"I would suggest that in the long endurance outdoor sports, you can see an impact. These athletes are talking about seconds apart. Long-term exposure in these conditions, it can certainly take seconds off," said Peter Dingle, associate professor in health and environment, at Murdoch University in Perth, Australia.

But on Friday, none of those scenarios materialized for the athletes competing in front of an appreciative crowd.

"I was expecting the worst and we put measures in place if it was necessary. But it hasn't been an issue, not since I've been here," said Kylie Wheeler of Australia, who is competing in the two-day women's heptathlon. Half of the events are held before noon while the reminder will take place after sundown to avoid the midday heat.

Wheeler said the haze in recent days has not been a problem during actual training: "It's visible. You can see it, but it doesn't bother me," she said.

British runner Tyrone Edgar, who qualified in the men's 100 meters with the fastest time, said he didn't worry about pollution because it would affect everyone equally. "We were all going to be in the same situation so it doesn't matter at all," he said.

Friday's weather left everyone feeling upbeat.

Nataliia Dobrynska, of Ukraine, who won her qualifying heat in the 100-meter hurdles, proclaimed: "It's the Olympics. It's a happy day. Smog? No problem."