Avoid the bends when you scuba off distant shores.

From Bonaire to the Great Barrier Reef, distant seas lure thousands of sport divers to explore their crystalline waters and exotic sights every year. And after traveling all that way, these enthusiasts naturally want to enjoy their exotic explorations of the watery deep until the last possible minute -- often just hours before they board a flight to return home.

But soaring above the clouds so soon after emerging from the depths of the ocean can be dangerous. According to ongoing research conducted by the Duke Center for Hyperbaric Medicine and Environmental Physiology at Duke University Medical Center, the combination of diving and flying can intensify one of the most serious dangers of diving: decompression sickness, popularly known as "the bends."

Decompression sickness occurs when the body isn't given enough time to adjust to the pressure changes associated with diving. We have one atmosphere of weight (air pressure) on us at sea level. Every 33 feet of sea water adds another atmosphere to the pressure, causing nitrogen to go into solution in the blood.

If a diver ascends from a dive too quickly, the decrease in pressure allows the nitrogen to come out of solution and form bubbles that percolate out of the blood and tissues -- something like opening a bottle of carbonated beverage.

These nitrogen bubbles expand as pressure continues to decrease, potentially leading to a variety of symptoms. High levels of the bubbles can cause complex reactions in the body, usually in the spinal cord or brain. In its mildest state, the bends present as a dull, achy pain, often felt in a joint such as the elbow or shoulder. Severe cases can cause paralysis, unconsciousness, or even death.

All divers are exposed to some degree of risk for decompression sickness; sometimes, though very rarely, it occurs even when decompression guidelines have been followed. An individual's actual risk of getting the bends, however, is very small: there are only about two cases in every 10,000 recreational dives. That's because standard diving instruction stresses the importance of taking precautions against the condition by ascending slowly after a dive, breathing properly, and limiting the time spent underwater.

What many divers may not know, however, is the need for continued precautions once they are out of the water and preparing to head home.

The reduced cabin pressure experienced during flight increases the decompression stress. The risks of flying after diving were first reported more than 40 years ago when a pilot and copilot aboard an intercontinental commercial aircraft were incapacitated by decompression sickness less than four hours after diving.

In 1992, in an effort to establish guidelines for safer flying after diving, Divers Alert Network (DAN) researchers began conducting studies in the hyper/hypobaric chambers of the Center for Hyperbaric Medicine and Environment Physiology at Duke University Medical Center.

"Exposure to reduced barometric pressure after diving can increase a diver's risk of decompression sickness," says Richard Vann, PhD, a DAN scientist and assistant research professor in Duke's hyperbaric medicine service. "When divers board an airplane, where the pressure in the cabin is only two-thirds to three-quarters of an atmosphere, they open themselves to that risk unless they remain at sea level long enough to allow the elimination of excess nitrogen from tissue."

So the question is: How long is long enough?

To answer this query, Vann and his colleagues, Edward Thalmann, MD, (who passed away in 2004) and Neal Pollock, PhD, began a series of studies at the Duke Hyperbaric Center. Study participants begin by sitting in a hyperbaric chamber during a simulated dive. After a prescribed period of time resting at the surface, participants later return to the chamber for four hours, where the reduced pressure of a commercial airline flight is simulated.

The intervals at sea level pressure between participants' "dives" and "flights" vary from five minutes to 22 hours, depending on the dive. When several participants experience mild symptoms of the bends for a given surface interval, the researchers know that the surface interval doesn't need to be any shorter.

To find out how exercise and immersion might affect the time needed to wait on the surface after diving before flying is safe, DAN is conducting further studies in which divers exercise in a warm swimming pool inside the hyperbaric chamber before participating in a flight simulation.

Avoiding Decompression Sickness

The following guidelines were the consensus of a DAN workshop on flying after diving. They apply to air dives followed by flights at cabin altitudes of 2,000 to 8,000 feet for divers who do not have symptoms of decompression sickness. The recommended preflight surface intervals (amount of time between the dive and flight) do not guarantee avoidance of decompression sickness. Longer intervals will further reduce the risk of decompression sickness risk.

• For a single no-decompression dive, a minimum preflight surface interval of 12 hours is suggested.
• For multiple dives per day or multiple days of diving, a minimum preflight surface interval of 18 hours is suggested.

According to Vann, "Vacationers can enjoy recreational diving with little risk of paying for it on the flight home if they follow these simple guidelines."

If you're a certified diver, at least 18 years old, and interested in participating in flying after diving research you may be qualified to participate in the Flying After Diving Research Study. For more information, call (919) 684-2948, ext. 260 or visit hyperbaric.mc.duke.edu.

Decompression Sickness: Signs & Symptoms

Symptoms

• Unusual fatigue
• Skin itch
• Pain in joints or muscles of the arms, legs or torso
• Dizziness, vertigo, ringing in the ears
• Numbness, tingling, and paralysis
• Shortness of breath

Signs

• Skin may show a blotchy rash
• Paralysis, muscle weakness
• Difficulty urinating
• Confusion, personality changes, bizarre behavior
• Amnesia, tremors
• Staggering
• Coughing up bloody, frothy sputum
• Collapse or unconsciousness

Note: Symptoms and signs usually appear 15 minutes to 12 hours after surfacing -- in severe cases, symptoms may appear immediately or even before the dive is completed. Delayed occurrence of symptoms is rare but can occur, especially if air travel follows diving. The quicker treatment begins, the better the chances of a full recovery.

More information about decompression sickness and other diving health issues can be found at dukedivemedicine.org.