Climbers tackling Everest receive a radiation dose five times more than the average annual exposure of a UK nuclear power worker, new research suggests.
Scottish mountaineer Bob Kerr gathered cosmic radiation readings on an expedition to the mountain last year.
His findings have been published by the Society for Radiological Protection.
Mr Kerr, who works as a radiation protection adviser, said no-one had died on Everest because of cosmic radiation.
However, he added that the level of exposure carried a "small one in 10,000 risk" of developing a fatal cancer some time in later life.
Mr Kerr, 36 and a member of Assynt Mountain Rescue Team, took a Geiger counter to Everest in May last year.
He reached a height of 26,000ft (7,924m) before temporarily losing his vision and was forced to descend.
A Nepalese guide, Dorje Khatri, took the equipment to the summit. Earlier this month, he was among 16 Sherpa guides who were killed in an avalanche on the mountain.
Mr Kerr, of Portskerra, said the reading gathered by the veteran guide suggested climbers were exposed to a radiation dose of 1milliSievert.
He said: "If someone received this level of dose at ground level, and if it was not due to cosmic radiation, then under the UK's Ionising Radiations Regulations 1999 this would be classed as a 'significant dose' and would be at the annual public dose limit.
"It is interesting to note that climbing Everest gives you five times more radiation dose than the average UK nuclear plant worker gets in a year."
He said most guides and climbers were likely to be unaware of the exposure.
Mr Kerr said: "They don't realise that as they ascend into the heavens their exposure to natural background radiation from outer space - cosmic rays - increases."