Researchers at the Leiden University Medical Centre in the Netherlands have found that sitting by the window on a plane can double a person's risk of a potentially deadly blood clot, the 'British Journal of Haematology' reported.

In fact, the dangers are greater for obese passengers, who are six times more likely to develop a blockage than those who prefer to sit next to the aisle, the study has found.

According to them, one theory could be that passengers find it easier to fall asleep by a window – and, sleeping for several hours at a time, particularly during longhaul flights, is known to raise the chances of a blood clot.

What's worrisome is that a DVT clot can cause death if it travels through a passenger's bloodstream to the lungs and causes a pulmonary embolism.

"Passengers in a window seat had a two-fold increase in risk compared to those in an aisle seat, while there was no increase in risk in those in middle seats. One explanation may be that passengers are in a more cramped position in a window seat," the 'Daily Mail' quoted the researchers as saying.

Moreover, the study found that flying business class reduced the risk by about 30 per cent. But, preventative methods such as drinking water, made no difference, while "the use of elastic stockings was associated with an increased risk of DVT, which is difficult to explain", according to them.