by Donald Daniel, Originated Feb 2004, revised May 2016

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An unknown number of people die each year suddenly and without warning of pulmonary embolisms resulting from blood clots in the veins of the thigh incurred on long flights. The airline industry would rather not warn you of this danger for fear of scaring off customers. I lost a dear lady friend this way. I am not a medical doctor, but have researched the subject as best I can on the internet and in medical books. In my opinion, the following statement should be included in the mandatory safety lecture about seatbelts, oxygen masks, and flotation devices:

For some strange reason sitting still at the 8,000 ft. pressure altitude in the airplane, in very, very dry air, exhaling moisture with every breath, does not make you thirsty. You can easily make an eight hour flight drinking very little and never needing to go to the bathroom. Research has shown that the body responds to the 8000 ft. pressure altitude in the plane by increasing the level of clotting factors in the blood. The resulting very unusually high degree of dehydration combined with high altitude and immobility can cause a clot in a vein in your thigh that can break loose, circulate through your blood stream, and cause a suddenly FATAL pulmonary embolism when you leave the plane. Drink enough water that you have to go to the bathroom every three hours. Stand up at your seat at least every hour. Aspirin, purple grape juice and lemon juice can reduce the tendency of blood to clot. Caffein, alcohol, nicotine, birth control pills and estrogen pills increase the tendency of blood to clot. If you have experienced swelling of the ankles after a long flight you may have had some clotting. If you have ever experienced an unexpected loss of stamina and shortness of breath when climbing stairs after a long flight you may have survived a mild pulmonary embolism. If you have experienced these symptoms after a long flight you should see a doctor to schedule injections of a potent anti-clotting medicine before your next long flight. The web site for the most popular estrogen pill has a "cautions" paragraph that says women on estrogen should see a doctor before a long airplane trip. Two of the most important risk factors, estrogen and birth control pills, apply only to women; the youngest woman on record to have died this way was only 28 years old.

The doctor in whose care my lady friend died said that in view of the swollen ankles and temporary shortness of breath when climbing stairs she had experienced after a long flight two weeks earlier, it was obviously a death from pulmonary embolism, "not uncommon on these long transatlantic flights". The swollen ankles and stair climbing difficulties had completely disappeared 24 hours after the flight. She was on a large dose (1.25 mg.) prescription of estrogen, though she did not take her pill the morning of the flight. Unfortunately she had a large cup of coffee, one risk factor too many, before the fatal flight. Only two tiny cups of water that were mostly ice were served during the eight hour flight. Had she not drank the coffee, and had she brought a large plastic bottle of water, I suspect she would have survived with no symptoms. She normally walked a mile each day, and could not pass a long staircase without climbing it for exercise. My lady friend's death was apparently of natural causes, not foul play, so the county examiner waived an autopsy and declared the probable cause of death "arteriosclerotic cardiovascular disease", in other words, a heart attack, not a pulmonary embolism. Thus she was not added to the statistics on the hazards of air travel. Thus, no one knows just how probable this is. In my own personal experience, I would say probable enough that it should not be ignored in the safety lecture.

If you will make copies of this page and distribute them to your fellow travelers, perhaps the airline industry will eventually be embarrassed into including the problem in their safety lecture.

New legislation could assess the magnitude of this problem. The IRS must be notified of each fatality of a tax paying adult. The airlines have passenger ticket records. If each fatality in the IRS data is compared with airline data, fatalities who took very long plane trips in the year they died can be identified. If the death occurred within two weeks of the flight, it should be added to the statistics of possible blood clot fatalities, even if the official cause of death was stroke, heart attack or an automobile accident. The frequency of these deaths could be compared to actuarial norms to determine the number due to long flights.

This website gets about 360 hits per year from people searching about swollen ankles on flights. A few years ago I only counted 24. Either I made a mistake then in the way I searched my logfiles for the hits, or a higher percentage of people with the problem are searching for information now.

A small fraction of healty people have an inherited condition of thrombophilia, extra propensity to clot, and may be unaware of it. It would seem to put them at extra risk with long distance air travel. The condition can be detected by a blood test.

This condition has been named "economy class syndrome" but occurs just as readily in first class. Another name for the clotting condition is DVT for Deep Vein Thrombosis. Thickening of the blood such as would be caused by dehydration is called "relative polycythemia". Other websites concerning this air travel hazard are and A book about the dangers of estrogen is "The Greatest Experiment Ever Performed on Women", by Barbara Seaman, published by hyperion , 2004. Another site about contributing factors to DVT is

In 2004 the NIH (National Institue of Health) advised postmenopausal women to take estrogen only in low doses and only for short duration.

A food supplement Nattokinase available in health food stores shows promise as an anticlotting agent in a petri dish, but this has not yet been proven in clinical trials in humans. The FAA would be the logical agency to fund the trials.

Some airlines provide a large clear plastic bottle of water in the seat pocket in front of every seat for transatlantic flights. If you do not want to bring your own water bottle, shop around for an airline that provides a water bottle before your trip. An empty grape juice bottle, 2 quarts or 1.89 liters, is about right for a long trip from America to Europe. Now that you are no longer allowed to bring bottles on the flight, you should drink more water than you really want before you get on each flight for a long trip. It might be OK to bring a large EMPTY clear plastic bottle through security, and fill it up with water before getting on the plane.

The Boeing 787 and the Airbus A350 have composite fuselages, not aluminum fuselages like older planes. This will permit higher humidity in the passenger cabin. This should reduce the incidence of blood clots. Low humidity must be maintained in aluminum body planes to prevent condensation of water on the inside of the aluminum skin, which would cause dangerous corrosion.

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