Dr. Jeff Hersh: Why do divers get ‘the bends’?

Dr. Jeff Hersh More Content Now

Q: Why do divers get sick if they ascend too fast?

A: Almost 10 million people in the United States scuba dive; there are about 1,000 injuries and 100 fatalities from this activity every year. To better understand diving injuries it is necessary to understand some basic physical principles and how they affect divers.

* Pressure on a specific gas increases how much of that gas dissolves in a liquid. This is the principle used to make soda; high pressure is used to force more carbon dioxide to dissolve in water. When the soda is opened at normal atmospheric pressure (the pressure at sea level), the carbon dioxide effervesces.

* Divers, and the liquid in their body (about two-thirds of the human body is water), are placed under increased pressure as they dive; every 33 feet (10 meters) of depth increases the pressure by 1 atmosphere.

* For a gas at a given temperature, the volume decreases with increased pressure (and vice versa). You can think of this as the higher pressure “squishing” the gas.

* Certain air-filled body cavities cannot sufficiently change volume in response to changing pressures during descent or ascent, possibly leading to complications (barotrauma).

Barotrauma is the most common type of diving injury and lung (pulmonary) barotrauma is the second most common cause of death in diving accidents (second to drowning). Minor barotrauma to the eardrums or sinuses is common. Pulmonary barotrauma occurs from the diver putting their lungs under higher pressure during descent, and then during ascent their alveoli (the small sacs in the periphery of the lungs) can “pop,” especially if they hold their breath (confining the expanding air) as they ascend.

The compressed air a diver breathes must equal the pressure they are subjected to, or else the air would not enter their lungs. This pressurized air (70 percent of air is nitrogen) will dissolve in the blood more than it would at atmospheric pressure. If they ascend too fast the air will form bubbles (like the bubbles in soda). These bubbles can act like emboli (objects that can travel in the blood stream, possibly clogging small blood vessels), causing insufficient blood supply to body tissue. This is called decompression sickness; it occurs in about 3 per 10,000 dives.

Type I decompression illness, the most common type, is when the musculoskeletal system and/or the skin are involved; joint pain (most commonly elbow or shoulder) is the most common symptom. The affected patient may walk bent over to try to alleviate the pain, which is often worsened by “stretching” the connective tissue; this is how the condition became known as “the bends.” The skin may also be compromised, for example with an itchy rash, most commonly on the upper torso.

Type II decompression sickness is often more severe, with the nervous and/or breathing system being affected. Symptoms may be mild, such as a headache, or more severe, like weakness or paresthesia (from spinal cord involvement), or more severe such as paralysis. When the brain is involved, the patient may lose their balance (called “the staggers”), develop confusion, vision changes, speech changes, memory impairment or other symptoms. Lung involvement (only 5 percent of decompression patients) can cause breathing compromise (“the chokes”), and may be fatal.

Decompression illness is treated with hydration and supplemental oxygen (to increase blood flow and minimize ischemia) and when severe, by placing the patient in a hyperbaric oxygen chamber. The high pressure can re-dissolve the bubbles, and then the patient can be slowly “decompressed.”

Nitrogen narcosis (“rapture of the depths”), another possible diving condition, is where the higher level of nitrogen dissolved in the blood (generally at depths over 100 feet) may cause symptoms similar to being drunk, or more severe symptoms such as hallucinations and loss of consciousness (typically at depths over 300 feet).

Diving injuries are diagnosed from the symptoms and the history of a recent dive(s); symptoms from diving injuries typically occur within minutes to hours of the dive, and essentially always within 24 to 48 hours.

Prevention of diving injuries includes ensuring the diver ascends slowly (to allow gradual depressurization), and stays within recommended duration and number of dives (depending on the depth of the dive(s), and for which there is specific guidance); it also includes factoring in whether the diver will subsequently go to conditions of low pressure such as taking a flight or ascending to a high altitude. Other measures that can help minimize diving injuries include avoiding dehydration, avoiding alcohol and following safe diving recommendations (including diving with a buddy and ensuring the diver has the skills for the dive they are attempting).


Jeff Hersh, Ph.D., M.D., can be reached at