Aug. 9 (Bloomberg) -- Angel Chen and her 10-year-old daughter, Brenda, had to leave the Bird's Nest stadium to cool off during the opening ceremony for the Beijing Olympics, an option that won't be available to athletes.

``It's so hot and gives me such a headache that I have to come out for a break,'' Chen, who lives in Los Angeles, said. ``Why not put some air conditioners inside? It would be a great show if it's not this hot.''

Competitors including tennis player Rafael Nadal sweltered and spectators struggled to cool themselves with handheld fans during the hottest and most humid Olympics opening ceremony since at least 1996. The weather during the four-hour show highlighted the challenges for athletes who'll run and cycle their way through Beijing's smog over the next two weeks.

The audience of 91,000 at the stadium and 10,000 team members endured temperatures of 32 Celsius (90 Fahrenheit) and 88 percent humidity by drinking water late into the night. Beijing officials said the $17 billion spent to improve air quality at the Olympics didn't guarantee that the weather would cooperate. The average high temperature on Aug. 8 in Beijing is 29 Celsius, according to the Weather Underground Inc. Web site.

``The weather conditions aren't conducive to the dispersal of pollutants,'' Guo Hu, who heads the Beijing Meteorological Observatory, said yesterday. ``Temperatures are high, humidity is high, one must take precautions against heat strokes.''

Asthmatic athletes also may be at an increased risk of being affected, or even withdrawing from events.

Organizers fired more than 1,000 rain dispersal rockets yesterday afternoon and evening to ``intercept a stretch of rain moving toward the stadium,'' Guo Hu, head of the city's weather bureau, told the state news agency Xinhua.

Paramedics

Paramedics last night had tables every six seating sections to treat fans who suffered from heat stroke and dehydration. Thousands of drinking fountains lined the concourse. One woman filled a rubber glove with cold water and held it on her head as she watched the ceremony in the partially covered stadium.

Wu Wenhan, running one of the paramedic units, estimated that 20 to 40 spectators had been sent to the hospital, half with heatstroke. Two patients she tended to almost fainted, he said.

``We've prepared medicines and ice packs to treat heatstroke,'' he said. ``It's too humid and stuffy.''

Thousands waved fans they brought with them to beat the heat, which was worse in the upper levels of the building.

``It's uncomfortably hot and I'm not used to this,'' said 44-year-old Virginia Rottschaefer from Minneapolis, who is in Beijing for business.

Past Olympics

Dirk Nowitzki, who carried the German flag into the stadium, stopped several times to wipe sweat from his brow and face as the National Basketball Association All-Star celebrated with his teammates after their entry parade.

Nearby on the field, perspiration dripped off Spain's most recognizable Olympian, Nadal. His chief rival on tennis courts around the world, Roger Federer of Switzerland, also glistened after serving as his country's standard bearer.

The International Olympic Committee said this week it doesn't expect heat and air quality to be a concern at the Beijing Olympics and pointed to similar worries before Games in Athens, Los Angeles and Seoul.

Records on Weather Underground -- a provider of worldwide weather conditions and forecasts -- show temperatures and humidity were lower at the opening ceremonies in Athens four years ago and at Sydney in 2000. Atlanta's humidity reached 84 percent on July 20, 1996, while temperatures fell to 77 Fahrenheit at night.

High-Pollution Days

For Beijing, the IOC said it will reschedule events if smog endangers athletes. Pollution levels were almost twice the maximum recommended by the World Health Organization yesterday, less than 24 hours before cyclists struggle for six hours over the 152-mile road race.

``It'll be a long, hot, sweaty day,'' said Austrian rider Thomas Rohregger. ``It's beautiful countryside, but it would be great to see the sun.''

The Beijing government on July 20 restricted cars to driving on alternate days, closed construction sites and ordered some factories to cut production or shut down. They toughened the measures after a run of high-pollution days.

Randy Wilber, senior sports physiologist for the U.S. Olympic Committee, said 27 percent of America's Olympic athletes historically have had asthma, and that the number probably would increase in Beijing. Asthma causes airways to swell and produce mucus, reducing oxygen supplies to straining muscles.

Beijing had its first run of four straight days within the WHO recommendations in a year last week, then levels of dust and dirt particles built up again over the past four days.

``Amazing, but I need a bit of air,'' said 41-year-old Gilles van der Werff from Luxembourg as he tried to lean into a slight breeze on the concourse at the Bird's Nest. ``Just silly hot up here.''

To contact the reporter on this story: Erik Matuszewski in Beijing at matuszewski@bloomberg.netLi Yanping in Beijing at yli16@bloomberg.net

Last Updated: August 8, 2008 21:56 EDT