One of the US athletes said the masks were a precaution
A group of US cyclists who arrived in Beijing wearing respiratory masks have apologised to China's Olympic Games organisers, a US sports official said.
US Olympic Committee chief executive Jim Scherr said the four athletes had written an apology to the Beijing Olympic committee.
"They were overly cautious," he said, adding they regretted the action.
There have been concerns about the haze over the city but Olympic officials say air quality is good.
Scherr said about one-third of the US team had been issued with face masks as a precaution against air pollution in the Chinese capital.
In an attempt to clear the city's notoriously smoggy skies, city officials have restricted traffic and shut down many factories.
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The IOC says said the air did not pose athletes or visitors a problem unless they had a respiratory problem such as asthma.
But levels of particulate matter measured by the BBC in the city on Wednesday gave readings well above the World Health Organisation's recommended target level.
In fact, at 186 micrograms per cubic metre, one reading was almost four times the WHO target of 50.
Meteorologists said there was a good chance the hazy skies over the city would lift in time for Friday's opening ceremony.
"It probably wasn't the most opportune time for these athletes to wear these masks," admitted Scherr.