Beijing ? Heat, humidity and haze cloaked the start of China’s long-awaited Olympic Games in a sultry blanket Friday.

Temperatures in the National Stadium, known as the Bird’s Nest, averaged about 30 degrees Celsius (86 degree Fahrenheit) in advance of opening ceremonies, and moderate humidity made the air feel heavier.

The noxious air has been a major headache for Olympic organizers, with athletes voicing concerns over the potential impact to their health and performance. Beijing’s air pollution regularly reaches levels two or three times above what the World Health Organization considers safe.

On Friday, the official air pollution index for Beijing was at 94, similar to levels of moderate pollution recorded earlier in the week. The WHO recommends levels below 50 for healthy air, while China considers anything above 100 to be harmful to sensitive groups including children and the elderly.

For U.S. swimmer Garrett Weber-Gale, the first view of the skies over Beijing as he was flying in earlier this week was a shock.

“I really noticed the first day that it was pretty smoggy. We landed on the plane, and I think we were all like, ‘Wow. Is this weather? Is this smog? Is this fog? What is it?’ It was kind of surprising to see that,” he said.

Beijing officials on Friday continued to insist that the murky haze enveloping the city is not the result of pollution, but instead is fog created by moisture vapor in the air. Visibility, they say, is not necessarily an indicator of air quality.

“The air quality, I think it’s good. It looks a little bit misty. You cannot judge the air quality by its appearance. You have to trust the scientists,” said Wang Wei, executive vice president and secretary general for Beijing’s Olympic Organizing Committee.

China has invested billions in “greening” the capital, and implemented drastic measures to ensure air quality for the Olympics. Last month, some 2 million cars were taken off roads, constructions sites were shut down and dozens of factories were closed across the capital. Similar measures were taken in surrounding provinces.