Travellers who have had recent health scans may trigger security alerts at airports because they are carrying the radioactive after-effects of the procedures, doctors warned yesterday.

Patients should routinely be given cards explaining their treatment to help prevent false alarms, a team at the Royal Brompton hospital, London, said. Writing in the Lancet medical journal, the team revealed how a 55-year-old commercial pilot who underwent a heart check involving radioactive thallium last year triggered detector alarms at Moscow airport two days later. He was released after extensive questioning but caused another alert at the airport four days later. This time, the officials releasing him gave him a card explaining the problem.

Patients undergoing thyroid scans and treatments as well as checks on bones and blood flow to the heart muscle may set off alerts up to a month afterwards, the writers said.

The problem has been reported before - two patients who had had thallium tests were seized by White House security staff in Washington DC and another case involved detectors in a bank vault.

But, with millions of such procedures being conducted worldwide each year, Professor Richard Underwood and his colleagues said patients should be better prepared for possible problems, given heightened global tensions.

"It should be standard practice to issue patients with an information card after diagnostic or therapeutic procedures using radioisotopes."