The United Nations' environment boss has rebuked global media for a "frenzied focus" on Beijing's pollution and "amnesia" over past hosts' similar problems.

Achim Steiner, who heads the UN Environment Programme (UNEP) and was in Beijing to see the Olympics opening, said many of the 30,000 journalists covering the Games were over-fixated on China's smog and had displayed short-term memories.

"After all, air pollution was a major concern in Los Angeles 24 years ago," he said. "Though few now seem to recall the dramatic scene at the end of the women's marathon when the Swiss competitor was seen staggering and stumbling from exhaustion, the heat and, perhaps, the effects of air pollution."

Gabriele Andersen-Scheiss' weaving finish and collapse over the finishing line horrified spectators and became one of the abiding images of Los Angeles 1984. TV shots of Beijing's cloying smog have already become a fixed image of this Games.

But "air quality was also an issue for subsequent Olympic Games in Barcelona, Atlanta, Seoul and Athens", Steiner added in a comment piece published by Chinese media.

"Without doubt Beijing is facing a huge challenge. There are real and understandable concerns for the health of competitors, especially those in endurance and long-distance events ... But the current frenzied focus is marked by considerable amnesia.

Athletes are competing under a blanket of haze as the Olympics gets going, rather than the blue sky Beijing hoped for.

Steiner, U.N. under-secretary as well as executive director of Nairobi-based UNEP, said athletes were right to be worried about the smog but China deserved praise for reducing pollution over the past decade even as its economy has ballooned.

He said Beijing's new Olympic forest and other climate friendly initiatives would improve air quality in the future.

"For us the Olympic Games are not the be-all and end-all. They are the means to a larger end," he later told reporters.

"It's not just trees that have been planted, it's an ecological experiment," he said of Beijing's Olympic park. "It's a whole new green lung for the people of Beijing."

Beijing has closed, cleaned up or relocated some 200 factories in preparation for the Games and added sanitation plants to treat more than 90 percent of the city's waste water, Steiner said. Some 50 percent of the city is now forested and 60 percent of energy generation comes from natural gas.

New vehicle emission standards are higher than the United States, Steiner said, while 4,000 new buses were powered by natural gas in the largest such fleet in the world.

Olympic venues have also been designed to be green. The athletes village uses water reclaimed from a sewage treatment plant to heat and cool systems. "Beijing is striving to be part of the Green Team," Mr Steiner said.

With the changes, China reckons it can offset within a few years the more than one million tonnes of carbon dioxide likely generated by the 2008 Games. The UNEP will sit down with Beijing officials in six months to evaluate their progress.