DEAR DOCTOR K:

I'm planning a diving vacation. A friend told me it's not safe to fly within 24 hours of diving. Why not?

DEAR READER:

I'm not a diver myself, so I had to do some research to answer your question. The concern with flying soon after diving (or rising too quickly to the surface of the water after a deep dive) is something called decompression sickness. It's also known as "the bends."

When you scuba dive, you breathe compressed air from a tank. That air contains high amounts of two gases, oxygen and nitrogen -- higher than are normally in the air around us.

The deeper you dive, the more pressure you're under. That causes the nitrogen to stop being a gas; it dissolves into your blood, becoming a liquid, where it remains during your dive. Your body slowly eliminates the nitrogen, primarily in your urine.

As you swim back toward the surface after a deep dive, the water pressure around you decreases. If this transition occurs too quickly, the liquid nitrogen forms bubbles in your tissues or blood that can damage the walls of blood vessels. These bubbles also can block normal blood flow in the same way that little blood clots can.

Decompression sickness can cause a variety of symptoms: joint pain, dizziness, headache, difficulty thinking clearly, extreme fatigue, weakness in the arms or legs, or rash. The symptoms you get depend on the organs in which blood flow is blocked.

It can take a while for the symptoms of decompression sickness to appear. You can get back up to the surface of the water without symptoms, but then experience them for up to 24 hours thereafter.

If you experience any of the symptoms of decompression sickness after scuba diving, get to a doctor as soon as you can. The key to treatment is the use of a hyperbaric oxygen chamber. This is a high-pressure chamber that provides 100 percent oxygen. The treatment drives nitrogen back into its liquid form so that it can be cleared from the body gradually, over a period of hours. Hyperbaric treatment is most successful if given within several hours after symptoms start.

To minimize your risk of decompression sickness while diving:

-- Dive and rise slowly in the water, and don't stay at your deepest depth longer than recommended. Scuba divers typically use dive tables that show how long they can remain at a given depth.

-- Don't drink alcohol before diving. Alcohol makes the brain more vulnerable to injury by the nitrogen bubbles.

-- Avoid hot tubs, saunas or hot baths after diving.

You asked about not flying. As a plane climbs higher, the surrounding air pressure gets progressively lower. Although the plane keeps cabin pressure higher than the real air pressure outside the plane, the cabin pressure still is lower than the air pressure on the ground. That encourages any remaining liquid nitrogen in your blood to form bubbles and cause decompression sickness. For that reason, you should wait 24 to 48 hours after diving before flying.

(Dr. Komaroff is a physician and professor at Harvard Medical School. To send questions, go to AskDoctorK.com, or write: Ask Doctor K, 10 Shattuck St., Second Floor, Boston, MA 02115.)

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