Safe Snorkeling

by Carl R. Gwinn, PhD, and Katrina A. Gwinn, MD

Snorkelers exit the water at the Casino Point Dive Park on Catalina Island, California

During a beach vacation, snorkeling is an easy way to enjoy the underwater world without a lot of trouble or expense. It is often presented as “easy”. However, snorkeling is dangerous. Drowning while snorkeling is the leading cause of death among all water-based activities in Hawaii. And, most of those deaths may not technically be drowning at all.

During a 9-year period, 204 people died while snorkeling in Hawaii, as compared with 25 while scuba diving and 50 while freediving. Of those 204 deaths, all but 20 were tourists. The state of Hawaii has put some effort into learning why. Their study concluded that some, and possibly most snorkeling-related drownings were actually due to SIROPE (Snorkeling-Induced Rapid Onset Pulmonary Edema). In pulmonary edema, the lungs fill with fluid, as they do during drowning. But, the fluid isn't from outside the body. It leaks from the body's vascular system into the lungs. The victim doesn't struggle as during drowning, but quickly and quietly loses consciousness. In SIROPE, the process usually takes less than 5 minutes. After death, autopsies can't distinguish drowning from pulmonary edema.

Lungs have a difficult job: absorbing oxygen into the bloodstream and expelling carbon dioxide from it. That task is accomplished through an enormous, specialized, microscopically-folded membrane, with blood on one side and air on the other. With each breath, the rib cage and abdomen expand, momentarily decreasing gas pressure inside the lungs and drawing in fresh air. To exhale, they contract to increase pressure and expel the breath. The balance of pressure created during this process is essential to normal lung function and is carefully maintained physiologically. The lungs of a healthy young male can develop a pressure difference from the outside air equivalent to a depth of about 16 cm of water, or 6 inches. People who are older, who suffer from cardiovascular disease, or who are breathing rapidly, might develop half that: equivalent to 3 inches of water. Usually, that is plenty to provide the lungs with the fresh air they need. But when the lungs and the muscles that drive their expansion and contraction are pushed to their limits or beyond, fluid leaks through the membrane of the lungs and starts to fill the tiny air spaces, because the pressure difference between fluid and gas is compromised across that specialized, highly-maintained membrane. The victim loses consciousness quickly as the supply of oxygen is cut off.

Breathing during snorkeling is harder than breathing on the beach, even without chasing beautiful fish. A snorkeler breathes through a narrow tube, possibly with a valve at the top to block splashes. That constricts airflow, requiring more chest muscle effort. A snorkeler's body is mostly underwater, and the rib cage and abdomen have to expand against surrounding water. It's usually just a few inches deep--but that's close to the limit of most people's lungs. Snorkelers also have to deal with waves and current. Those waves may look small from the beach, but how far does that snorkel poke above the peaks, when the snorkeler swims face-down? Some water is going to come down the snorkel, even if you don't surface dive. Expelling water from the snorkel, or breathing with water inside it, makes the lungs' work even harder.

How can you stay safe while snorkeling? Start out easy: wait a couple of days after air travel, and start out snorkeling in water shallow enough to stand up in. If you feel out of breath, or need to pant rapidly, stop snorkeling and stand up. If the water is too deep, float or tread water without exertion. Lift your head above water and take off your mask and snorkel so that you can breathe through your mouth and nose. (Placing your mask on your forehead is a sign of distress--which you may want to send.) Relax and get your breath. Wait until you breathe normally. Call for help if you need it.

Always snorkel with a buddy. Wear fins--they reduce swimming effort a lot. Wear a mask, but not a full-face mask: a disproportionately high percentage of SIROPE victims were wearing full-face masks. At least at first, or if conditions look dicey, bring along a flotation device such as a boogie board, and make sure you are within easy distance of shore. If you are caught in a current, swim perpendicular to the current, or float with it until it has dissipated.

Why do we recommend that you wait a day or two after your flight before snorkeling? Hawaii's study found some evidence that SIROPE was more common just after air travel. Jet cabins are pressurized to 8,000 ft altitude, or 75% of pressure at sea level. That's about 7.5 feet of water. It's difficult to avoid getting dehydrated. It can take a few days for your body to settle down.

Familiarize yourself with your gear and with the layout and conditions at your site. Hawaii's study found that more than half of SIROPE survivors had trouble with their equipment, were caught in current or waves, or panicked. More than 80% were in water over their heads if standing up. More than half were strong or intermediate swimmers, and most had snorkeled before, often recently. Experience and strength aren't a substitute for caution.

If your heart or lung health is not good, consider giving snorkeling a miss. Hawaii's study found significant incidence of SIROPE among snorkelers with cardiovascular disease, although healthy and young individuals also succumbed.

Before a surface dive, take only one or two deep breaths. If you take more, or hyperventilate, you can lose consciousness on the way back up. (This is called shallow-water blackout). Shallow-water blackout occurs because a certain fraction of oxygen in your lungs, times the pressure, is needed to keep you conscious. You use up oxygen while you hold your breath. Higher pressure at depth keeps you conscious despite a lower fraction of oxygen in your lungs; but that fraction may not suffice when the pressure drops as you approach the surface. Meanwhile, the urge to breathe comes only when carbon dioxide in your lungs reaches a certain level. Hyperventilation expels carbon dioxide so that you don't have that urge. You can black out without ever feeling out of breath.

Why does scuba diving have fewer fatalities than snorkeling? We believe that scuba diving is safer (as long as you are properly trained), although this may seem counterintuitive. Scuba regulators keep the air pressure in the diver's lungs the same as that in the surrounding water, to within about an inch of water depth, and can provide air just about as fast as the diver can inhale. At depth, currents are weaker and waves have less effect. Scuba certification requires classes and in-water training. In fact, most of our certification classes were about what to do if something goes wrong. Scuba divers usually dive with a buddy or a group, so minor crises usually don't become major.

Many activities are dangerous: crossing the street, taking a shower, driving a car. They are much less dangerous, and can even be fun, as long as you understand the hazards and take a little care. Snorkeling is in the same category, but much more fun. You can visit another world, using only a few items that you can carry with one hand. Is it worth it? As long as you know and manage the possible risks, well, why did you travel to the beach to begin with?

A snorkeler enjoys a brief surface interval off Kosrae, in Micronesia.