Heat and humidity levels will be of more concern to competing athletes at the Beijing Olympics than pollution levels, according to Spanish athletics chief Jose Maria Odriozola.

"I was there two years ago at almost exactly the same time as the Olympics will be held for the world junior athletics championships," Odriozola told a news conference on Tuesday after announcing the Spanish athletics team for the Games.

"My eyes didn't water and the athletes did not complain about the pollution, only about the heat and humidity.

"I'm convinced there will be no more of a pollution problem than in other big cities. I believe the Chinese when they say there will be no pollution."

Marathon runner Jose Maria "Chema" Martinez said the conditions were going to be extremely difficult to cope with in his event.

"The marathon is going to be conditioned by the heat and humidity, and if you add the problem of pollution then you could say it is better to stay at home," he told reporters.

"It is going to be very hard and the athletes that cope best will win the medals. I trust the Chinese government will live up to its pledge (about pollution levels), because if not it will be very embarrassing for them."

Beijing's chronic pollution has been one of the biggest worries for Olympic organizers, who promised a green Games and have enacted special measures to reduce the problem.

They have raised the prospect of more pollution controls, in addition to keeping nearly half of Beijing's 3.3 million cars off roads and shutting many factories near the capital, but have refused to give details, insisting air quality is improving.