TEAM GB stars are being told not to breathe in through their mouths because Rio’s air could give them cancer.
Experts have warned athletes to wear face masks when not competing and stay indoors as much as possible.
Local pollution could damage airways, spark heart attacks and cause cancer in the long term, says air purifier firm Blueair.
The advice came as tennis ace Andy Murray practised his Olympic flag-bearer duties – and nearly poked a shocked Princess Anne’s eye out.
Team GB’s patriotic opening ceremony outfits were also on display.
The Games headlines to date have been dominated by fears over the Zika virus, but now athletes are being warned to beware of the thick, polluted air.
Blueair founder Bengt Rittri also told spectators to “mask-up” if they want to protect their health.
He said: “Our advice to all athletes in Rio is to wear pollution face masks when ever possible. It is a city suffering from severe air pollution caused by unfettered traffic and industrial activities.”