Thousands of homeowners could be exposed to higher levels of the deadly gas radon than they realise because of a bizarre interaction with the moon.
Scientists have discovered that natural levels of the radioactive gas, which is thought to kill up to 2,000 people in the UK each year, are affected by changes in the lunar cycle, because it alters the gravitational pull on the Earth.
The study raises doubts about the reliability of home-testing kits for radon, which are often used to show house buyers their new property is safe.
Robin Crockett, a radon expert at the University of Northampton, said: "Somebody could measure radon for a week and show a level about 40% below the true value." Monitoring is needed for at least 14-15 days, he said, to cover the transition between new moon and full moon.
Companies that make the one-week tests market them as a convenient alternative to three-month tests, which the government recommends.
Radon can cause lung cancer. The gas occurs naturally inside rocks, and seeps from the ground into buildings. Levels vary across the country, but are higher in Cornwall and parts of south-east England.
Dr Crockett and colleagues at the University of Bradford and Northampton general hospital discovered the effects of the moon while carrying out routine radon measurements for the government. Writing in the journal Geophysical Research Letters, they say the lunar cycle changes the amount of stress in the Earth's crust and drives the water table up and down. Combined with ocean tides, which squeeze and relax the ground, this can change radon emissions during a full moon and a new moon by up to 46%.
The scientists say: "One-week measurements could erroneously indicate that a building with 'safe' levels is unsafe, if measured around a tidal-cyclic radon maximum, or that a building with 'unsafe' levels is safe."
Brian Ahern, chairman of the Radon Council, a trade body that promotes home testing, insists the tests are reliable: "I've tested radon levels for a year at a time and haven't detected any changes with the moon."
But he adds that radon readings close to the safe level should be checked using a longer-term test.