The World Health Organization (WHO) recently released the findings of a study that suggests that travelers who travel for four hours or more double their chances of developing deep vein thrombosis. According to the WHO, almost one in 6,000 long-haul travelers is at risk for thrombosis, which can have some unpleasant and potentially serious consequences.

The WHO points out that very short travelers whose feet do not touch the ground and very tall people who are cramped in small seats are at most risk. Frequent travelers, women taking birth control pills, the obese, and those who have disorders related to blood clotting are also at risk of developing dangerous blood clots due to immobility during travel.

The assistant director-general for non-communicable disease and mental health for WHO, Catherine Le Gales-Camus, points out that whether travelers are traveling by car, bus, train, or plane, venous thromboembolism can occur when someone is not moving much for fours or more during travel.

The problem occurs because travelers who are immobile do not experience regular muscle contractions that would occur with movement. As a result, blood starts to pool in the legs, creating good conditions for blood clots, or thrombus, in deep veins. Some sufferers of thrombosis experience swelling, soreness, or cramps in the areas affected, while other victims have no symptoms. Whether a victim has symptoms or not, however, a blood clot can be very dangerous if it travels to the lung. There, it can block blood flow and cause pulmonary embolism. Symptoms such as difficulty breathing and chest pain may develop; the injury may prove fatal if the victim does not get medical help.

Health experts estimate that about 2 billion people travel by airplane annually; many more take trips by land. In both cases, these travelers sit still for long periods of time. Currently, studies suggest that one person suffers from thrombosis for every 20 long-haul flights with 300 passengers. Although aviation accidents are far more likely to cause personal injury to travelers, thrombosis can develop into a potentially life-threatening injury, and it is very easy to prevent.

There are a few things that people can do to prevent thrombosis even on long-haul journeys. Travelers should move their calf muscles in their seats using up and down movements that move the ankle joints and feet, according to the WHO. Also, travelers might want to leave their seats and move around for a few minutes where possible. Travelers who are traveling by car may wish to pull over periodically to stretch their legs and walk around.

Passengers who are traveling by air or by overland transportation should avoid alcohol and sleeping pills, according to the medical experts at the WHO. Alcohol and sleeping pills do not encourage passengers to move around and may contribute to the ideal conditions for thrombosis. Passengers should also avoid wearing tight clothing, say the WHO experts. Tight clothes can constrict circulation and contribute to problems.