Sitting through long movies can trigger the formation of dangerous blood clots, according to the New Zealand Medical Journal.

Researchers say it's not just airline passengers who can suffer from DVT, or deep-vein thrombosis. DVT has been blamed in a number of cases in which travellers have died from a clot.

DVT occurs when a person is immobile for a long period of time causing a clot to form in the leg. The clot is fatal if it travels to the lungs or brain.

Researchers examined the case of Phyllis Caton of Raumati, north of Wellington. Caton developed sudden severe pain in her right leg within a half hour of leaving a cramped movie theatre. She had just watched the three-hour The Lord of the Rings: The Two Towers

Caton's leg began to swell the next day and she was treated in hospital for a clot.

Richard Beasley of the Medical Research Institute of New Zealand says Caton's habit of sitting at a computer for two to three hours at a time probably increased her risk of getting a clot.

"There are many situations where you can put yourself at risk, and this is often not recognized by the medical profession," said Beasley.

Beasley advised people who are sitting for a long time to do foot exercises while seated, not to cross their legs and if possible, to stand up and walk around regularly.