People who watch television for prolonged periods are at an increased risk of developing a fatal blood clot.

According to the findings from Japan, those who watch TV for five hours or more per day significantly increase their risk of suffering a pulmonary embolism (PE). This is the name given to a blood clot that lodges in one of the blood vessels that supplies the lungs with blood.

It is a very serious condition, one that can cause death within hours if not medically treated.

A PE usually results from deep vein thrombosis (DVT) - a blood clot in the veins, often in the legs, which breaks away and moves to the lungs.

"The association between prolonged sitting and PE was first reported among air raid shelter users in London during World War II. Nowadays, a long haul flight in an economy class seat is a well-known cause of PE that is called ‘economy class syndrome'," noted public health researcher, Toru Shirakawa, of Osaka University.

He and his team followed the progress of more than 86,000 people over an 18-year period. All were aged between 40 and 79.

They found that those who watched TV for at least five hours each day were twice as likely to suffer a fatal PE compared to those who watched TV for less than two-and-a-half hours per day.

For the study, TV watching was divided into three groups - less than 2.5 hours, 2.5 to 4.9 hours and five hours or more per day. The risk of death from PE was then calculated, taking into account other risk factors, such as smoking, diabetes and body mass index.

Aside from the overall increased risk among those who watched TV for more than five hours per day, the researchers found an even higher risk among those over the age of 60 who watched TV for this long. They were six times more likely to suffer a fatal PE compared to those who watched TV for less than 2.5 hours.

Those over 60 who watched TV for between 2.5 and 4.9 hours per day saw their risk of a fatal PE tripled.

"We showed that prolonged television viewing may be a risky behaviour for death from PE. Leg immobility during television viewing may in part explain the finding. To prevent the occurrence of PE, we recommend the same preventive behaviour used against economy class syndrome. That is, take a break, stand up, and walk around during the television viewing. Drinking water for preventing dehydration is also important," explained Mr Shirakawa.

He called for more public awareness of the risk of PE that can arise as a result of ‘lengthy leg immobility'.

The results of these findings were presented at the recent ESC (European Society of Cardiology) Congress 2015 in London.