The US athletes who arrived at the Beijing airport wearing face masks were identified on Wednesday as four track cyclists.
A USA Cycling spokesman confirmed the identity of the four as Jennie Reed, Sarah Hammer, Bobby Lea and Michael Friedman -- all cyclists who compete indoors.
Reed is the world champion in women's keirin while Hammer is another of the US team's best track medal hopes in the women's individual pursuit.
The four arrived at the airport wearing close-fitting face masks covering their noses and mouths as they made their way through the arrivals area of the airport.
Beijing's chronic pollution, a source of respiratory illness, has been one of the biggest worries for Games organisers who have had to deflect international criticism over air quality. The city was covered in haze on Wednesday, two days before the Games begin.
US Olympic officials said the decision by the cyclists to wear masks appeared unnecessary.
"It is an athlete's choice. However, it did not seem necessary today," US Olympic Committee spokesman Darryl Seibel said on Tuesday evening. "I am certain that it was in no way intended to be disrespectful."
A USA Cycling spokeswoman said she did not believe the cyclists were trying to make any statement by wearing the masks.
The US Olympic Committee has not given any indication that it would sanction the athletes in any way. US officials are expected to make a statement on the subject later in the day.