Standing on the beach, every inch of your body is subject to about 14.7 pounds of pressure from the atmosphere around you. But plunge 33 feet underwater – a typical depth for a first-time diver – and that pressure doubles, thanks to the weight and density of the water. At 66 feet below, the pressure is three times greater. And at 99 feet, it is four times greater.

Consider just how intense the effects of all that pressure might be on your body the next time your ears throb as you touch bottom in a 20-foot swimming pool.

"The challenge with dive medicine is that your doctor may not be a diver, and he doesn't know these basic things about pressure," said Dr. Eric Johnson, a Staff Physician at Teton Valley Hospital, in Driggs, Idaho; past president of the Wilderness Medical Society; a NAUI diving instructor; and a Mount Everest base camp physician. "The physics of what happens underwater is what makes dive medicine different than regular medicine."

SAFE SPORT

Luckily, few boaters end up seeing a doctor after diving because scuba is a pretty safe sport. Some 500,000 to 750,000 new divers are certified every year, Johnson says. Only about 1,000 divers, novice and expert alike, suffer decompression sickness during that same period (more on that in a minute), and fewer than 100 divers die annually worldwide. Statistically speaking, you are more likely to die in a car accident on the way to your dive site than after you clip on your weight belt and take to the water in search of eagle rays and reef sharks.

In fact, the primary medical problems most divers face are as much boating-related as they are scuba-related: sunburn, heat illness, bumps and bruises and seasickness. For these, doctors know what to do.

What can confuse doctors are the effects of drugs that divers take, because underwater pressure can intensify side effects in unpredictable ways. "The only drug that I know of that has been thoroughly studied underwater is Sudafed," says Johnson, who spoke at the week-long "Medicine for Mariners and Safety at Sea" conference at The Bitter End Yacht Club on Virgin Gorda in January. "That's because the U.S. Navy uses it. We really don't know the effects of other drugs underwater. That includes everything from seasickness patches to Lipitor."

What really confuses doctors who aren't divers are scuba-related problems that look like other illnesses. Decompression sickness is a classic case.

Decompression sickness is a function of all that underwater pressure. As a diver descends, the hydrostatic pressure of the seawater around him dissolves gases from the tanks, especially nitrogen, into his body at a faster rate than he would experience on land. Divers are trained to ascend slowly, with great care, so that the body has time to release the dissolved gases at a safe rate.

If the diver resurfaces too quickly, nitrogen bubbles form in the body, and the diver gets decompression sickness, or "the bends." The nickname comes from the intense pain that can be felt in elbow and knee joints when suffering decompression sickness.

THE BENDS

The bends are one of the most discussed scuba-related medical problems, but are not common in general. As just one example, a recent study in British Columbia (a far more challenging diving environment than, say, the Bahamas) showed that fewer than 10 divers in 100,000 suffer from decompression sickness. Even still, divers fear the bends because those fast-exiting bubbles can go anywhere – including blood vessels, heart, lungs and brain.

And, unfortunately, it's not always possible to predict who will get the bends, or why.

"The vast majority of people who get bent nowadays did nothing wrong," Johnson says. "Eighty-five percent followed their dive tables and training exactly. You can minimize your chances of decompression sickness, but it's a matter of chance. If you're not willing to get bent, then you shouldn't dive."

Also, unfortunately for divers, the onset symptoms of decompression sickness look a heck of a lot like the flu. Some 60 percent of divers with the bends say they feel aches and pains. Others complain of various combinations of dizziness, weakness, headache, nausea and fatigue. In general, these symptoms begin an hour after the diver surfaces, but sometimes, they can take days to appear and strengthen, just like a nasty winter's cold.

A lot of doctors who are not trained in dive medicine may not even consider the actual dive a factor in the illness.

TREATING DECOMPRESSION SICKNESS

The first thing you need if you get the bends is a way to contact the nearest recompression chamber. Many divers subscribe to DAN – Divers Alert Network – which offers insurance starting at just $25 per year to cover what might turn out to be a $40,000 emergency helicopter ride to the nearest hyperbaric chamber, , Most dive operators have information about the nearest chambers, but if you're diving from your own boat, you can get that same information through diversalertnetwork.org. And until the rescue chopper arrives, the best thing you can do is inhale 100 percent oxygen (versus mixtures like Nitrox, which some divers use), lie on your side with your head low and drink fluids if you are able.

Other emergencies might require specialized care that can be incredibly difficult to anticipate. "We occasionally have divers with bad dentists," Johnson says. "The air gets in during a diver's descent, say beneath a filling, and then when the diver surfaces, the air expands and can't get out without popping the filling." In these cases, too, you would likely need to call for immediate medical assistance.

The biggest thing to remember, Johnson says, is that even if you have only passed a beginner-level scuba course, you may know far more about dive medicine than the doctor who reaches you first.

"He may try to send you up in the pressurized cabin of an airplane to get you to a trauma center," Johnson says with a smirk of disbelief. "You may be in pain, and you may be frightened, but you need to insist on the proper treatment. Underwater pressure changes everything."