Women who spend most of their time sitting down when they get home from work may be more likely to get a potentially fatal blood clot on the lungs than those who are more active, according to new research.

The big study, carried out on nurses in the USA, is the first to show that a sedentary lifestyle can lead to pulmonary embolism‚ where a blood clot travels up from the deep veins in the leg and eventually into the lung. The symptoms include chest pain, difficulty breathing and coughing.

It is already known that people who play sport and are physically more active are less likely to suffer pulmonary embolism, but the study published on the website of the British Medical Journal is the first to show that sitting about raises the risk.

The study, by Dr Christopher Kabrhel from Massachussetts general hospital, investigated the leisure habits of nearly 70,000 nurses in the USA‚ many of whom would be on their feet for most of the working day. However, over an 18 year period, the researchers found that those who sat for longer than six hours a day when they were not working had twice the risk of a pulmonary embolism of those who sat for less than two hours a day. The results held good even after taking into account age, overweight and smoking habits.

The increased risk women run by sitting down for hours is not a big one ‚ an editorial published with the study says it is only slightly higher than that run by women who are on the pill or who take long-haul flights. But, it says, "if the findings are valid they may have major public health ramifications". The study also found that inactivity correlated with heart disease and high blood pressure. "Prolonged periods of physical inactivity could be one of the hidden mechanisms that link arterial disease and venous disease," said James Douketis, director of vascular medicine at McMaster University, Hamilton, Ontario in Canada.