A study set for publication this week in The New England Journal of Medicine confirms that blood clots form more frequently in those traveling longer distances in airplanes than those traveling shorter distances. The study found that long haul airline travel is a major risk factor in the formation of dangerous blood clots. The clots may be fatal if they break free and travel through the body, lodging themselves within the lung, and causing a pulmonary embolism.

Approximately 100 cases of pulmonary embolism have been linked to air travel within the last 30 years. Those suffering a pulmonary embolism may be in grave danger, yet may experience no symptoms. Sometimes calf or leg pain during extended flights and chest pain or trouble breathing indicate pulmonary embolism.

This most recent study found that instances of pulmonary embolism increased dramatically when people traveled more than 3,000 miles and even more so on trips over 6,000 miles. Those traveling shorter distances accounted for far fewer cases of the condition.

Researchers studied all pulmonary embolism cases at France's Charles de Gaulle Airport between November 1993 and December 2000. Of the 125 million passengers passing through the busy airport, 56 received immediate treatment for pulmonary embolism following arrival. Not surprisingly, researchers determined that the majority of the 56 patients traveled in cramped economy class and failed to move about the airplane.

Clearly, there is a link between extended air travel and blood clots. However, scientists admit that they do not completely understand the relationship. Further clouding the subject, some cases of pulmonary embolism do not develop until weeks after air travel.

Researchers recommend that passengers on extended air flights avoid alcohol, drink plenty of water, wear loose, comfortable clothing during flights, and walk or move their legs during the flight to prevent clots from forming.