Blood clot formation in the leg veins, a condition known as deep vein thrombosis (DVT), is much more likely to develop during winter months than in the summer.

Similar seasonal variation in a related, potentially fatal condition known as pulmonary embolism, in which blood clots travel to the blood vessels of the lung, has already been well documented.

The researchers found a clear seasonal variation in hospitalizations for DVT and for pulmonary embolism. While hospitalizations for DVT were about 18% lower than average during the summer months, they were about 18% higher than average in the winter.

And for pulmonary embolisms, hospitalizations dropped below the average in the summer and climbed as much as 26% above average in the winter, the report indicates.

Although this was not part of the study, the researchers speculated that cold-induced narrowing of blood vessels and reduced physical activity during winter months may produce a reduction in blood flow in the lower limbs.

Winter respiratory tract infections could also influence blood coagulation.

Patients at risk of deep vein thrombosis and/or pulmonary embolism -- and their doctors -- should be informed of this increased risk in winter.

Because people who travel by air in cramped seats for long hours face an increased risk of DVT, the condition has garnered considerable media attention as "economy class syndrome."

British Medical Journal September 15, 2001;323:601-602

People traveling long distances by air are more at risk of a blood clot forming and lodging in their lungs than are people traveling shorter distances.

While airline travel is considered to be a risk factor for developing these blood clots, researchers had not investigated whether the risk increases with longer flights.

According to researchers, being immobile on a plane and having a limited space in which to move increases the risk that blood clots will develop and then travel to and lodge in the lungs, an event called pulmonary embolism.

During long periods of sitting, blood pools in the legs and gets thicker. This in turn increases the risk of pulmonary embolism, which can be fatal. A person may experience no symptoms, or they may feel a pain in the calf or leg, chest pain or shortness of breath. Surface veins may become more visible.

Over the last 30 years, about 100 cases of pulmonary embolism after air travel have been reported.

The authors report that the number of cases was much higher among those traveling more than 3,100 miles than among those traveling a shorter distance. The number was higher still among those who traveled more than 6,200 miles.

According to the report, most of the people with DVTs had traveled in economy class and only three reported that they had left their seats during the flight. About three-quarters said they were completely immobile during the flight.

A relation between the duration of air travel and the risk of pulmonary embolism is strongly suggested by this study.

Several reports have suggested that pulmonary embolism may develop in passengers several weeks after air travel.

To reduce risk, the researchers advise passengers to drink fluids, avoid alcohol, wear loose clothing, avoid crossing their legs, wear elastic support stockings and engage in minor physical activity, such as walking or moving their legs, during flight.

A related editorial calls the findings "the tip of the iceberg" with respect to the number of cases that may really be occurring.

The New England Journal of Medicine September 13, 2001;779-783, 828-829