British Olympic chiefs are advising athletes not to use anti-pollution masks in Beijing because they are not convinced the device actually works.
Beijing's smog remains a worry but the British Olympic Association (BOA) is worried the mask may hinder, not help.
"I'm not sure there is any scientific support for using it," said BOA chief medical officer Dr Ian McCurdie.
"It could be detrimental - if you're not used to a mask to suddenly put one on could be counter-productive."
McCurdie, who was appointed by the BOA earlier this month, said the final decision on using a mask in training would be left to individual athletes.
"We're not encouraging people to use it but if our athletes feel it helps them they're perfectly welcome to use it," he told BBC Sport.
But McCurdie, a rehabilitation expert who shares his time between the BOA, Chelsea Football Club and the Royal Ballet, said there were also doubts as to whether the device complied with Olympic rules.
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"But I don't think it's something we'll see on the day because I'm not sure where it sits within the regulations about the use of equipment and potential enhancement of performance."
McCurdie's comments would appear to draw a line under an issue that has seen the BOA make a succession of U-turns.
It initially rejected pressure to drop the mask, then said no British team members would be allowed to wear them, before changing its mind once more to say athletes could wear them if they wanted while claiming they would not help performance.
Participants in the marathon, road cycling, triathlon, open water swimming and road walking have most to worry about from the pollution in Beijing and the International Olympic Committee (IOC) has already acknowledged that these events may have to be postponed if conditions are too dangerous.
British marathon runner Mara Yamauchi took part in a recent test in Beijing. She finished fifth but was 20 minutes down on her personal best and afterwards complained of a dry throat and irritated eyes.
Yamauchi said she would consider wearing a mask in Beijing this summer, adding "anything that can help me perform my best on the day is worth trying".
The IOC has always publically said the legality of anti-pollution devices was a matter for each sport's governing body to decide.
Privately, however, it has felt there was no need to make a ruling on masks as it believed they do little to filter out the worst pollutants and actually make breathing harder during exercise.
One expert, who advised the International Association of Athletics Federations on the issue, said "there was no point banning something that gives you a disadvantage".
The mask, which was developed by UK Sport and Brunel University at a cost of "between £20,000 and £30,000", has been a controversial topic ever since it was unveiled in January at a British training camp in South Africa.
"There is concern about (the pollution) but it will face all the athletes and there is a limit as to what you can do about it"
It said the device could also be used in new, dusty buildings - such as velodromes - and other cities with air quality issues, including London.
But from the moment Haile Gebrselassie, the men's world record-holder for the marathon, confirmed he would not be running the distance at the Games due to fears that Beijing's pollution would exacerbate his asthma, the mask's fate has become entwined with widespread concern about the city's air quality and China's sensitivities to that concern.
The IOC has been forced to strongly defend Beijing's efforts to tackle its pollution problems and has also published its own data that suggests the situation, while not "ideal", will be far from dangerous.
The Americans, who like the British considered using masks, have recently admitted they will not let their athletes use them in competition to avoid embarrassing their hosts.
And women's marathon world record-holder Paula Radcliffe told BBC Sport that she was more concerned by Beijing's heat and humidity than the pollution, which she suggested had been exaggerated as a factor in the race and would be the "same for everybody".
Even Gebrselassie, who initially said he had "no intention of committing suicide in Beijing", has since claimed his comments were taken out of context and he is not running the marathon at the Games because he wants to attempt a new record in Berlin in September.
For his part, McCurdie believes there are no long-term health risks for any athlete in Beijing this year and says from a competitive point of view the conditions will not be a factor.
"There is still concern about (the pollution) but it will face all the athletes and there is a limit as to what you can do about it," he said.
"Most athletes have a 'control the controllable' philosophy - they worry about things they can do something about and don't worry about things they can't do anything about.
"Their performances may suffer on the day - so we may not see any world records - but that is the same for every athlete.
"So it remains a level playing field. I don't think people have serious long-term concerns about spending a few days in Beijing and doing some exercise."
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