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ALTITUDE-INDUCED 

DECOMPRESSION 

SICKNESS

Tiny Bubbles, BIG Troubles

Decompression sickness (DCS) describes 

a condition characterized by a variety of 

symptoms resulting from exposure to low barometric 

pressures that cause inert gases (mainly nitrogen), 

normally dissolved in body fluids and tissues, to 

come out of physical solution and form bubbles. 

DCS can occur during exposure to altitude (altitude 

DCS) or during ascent from depth (mining or diving). 

The first documented cases of DCS (Caisson 

Disease) were reported in 1841 by a mining 

engineer who observed the occurrence of pain and 

muscle cramps among coal miners exposed to airpressurized mine shafts designed to keep water out. 

The first description of a case resulting from diving 

activities while wearing a pressurized hard hat was 

reported in 1869. 

ALTITUDE-INDUCED 

DECOMPRESSION SICKNESS 

Altitude DCS became a commonly observed 

problem associated with high-altitude balloon and 

aircraft flights in the 1930s. In present-day aviation, 

technology allows civilian aircraft (commercial and 

private) to fly higher and faster than ever before. 

Though modern aircraft are safer and more reliable, 

occupants are still subject to the stresses of high 

altitude flight—and the unique problems that go with 

these lofty heights. A century and one-half after the 

first DCS case was described, our understanding 

of DCS has improved, and a body of knowledge 

has accumulated; however, this problem is far from 

being solved. Altitude DCS still represents a risk to 

the occupants of modern aircraft. 

Tiny Bubbles 

According to Henry’s Law, when the pressure of 

a gas over a liquid is decreased, the amount of 

gas dissolved in that liquid will also 

decrease. One of the best practical 

demonstrations of this law is offered 

by opening a soft drink. When the 

cap is removed from the bottle, gas 

is heard escaping, and bubbles can 

be seen forming in the soda. This 

is carbon dioxide gas coming out 

of solution as a result of sudden 

exposure to lower barometric 

pressure. Similarly, nitrogen is an inert gas normally 

stored throughout the human body (tissues and 

fluids) in physical solution. When the body is 

exposed to decreased barometric pressures (as in 

flying an unpressurized aircraft to altitude, or during 

a rapid decompression), the nitrogen dissolved in 

the body comes out of solution. If the nitrogen is 

forced to leave the solution too rapidly, bubbles form 

in different areas of the body, causing a variety of 

signs and symptoms. The most common symptom is 

joint pain, which is known as “the bends.” 

Trouble Sites 

Although bubbles can form anywhere in the body, 

the most frequently targeted anatomic locations are 

the shoulders, elbows, knees, and ankles. 

Table 1 lists the different DCS types 

with their corresponding bubble 

formation sites and their most 

common symptoms. “The bends’’ 

(joint pain) account for about 60 to 

70% of all altitude DCS cases, with 

the shoulder being the most common 

site. Neurologic manifestations 

are present in about 10 to 15% of all DCS cases,

with headache and visual disturbances being the 

most common symptoms. “The chokes” are very 

infrequent and occur in less than 2% of all DCS 

cases. Skin manifestations are present in about 10 

to 15% of all DCS cases. 

Medical Treatment 

Mild cases of “the bends” and skin bends (excluding 

mottled or marbled skin appearance) may disappear 
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during descent from high altitude, but still require 

medical evaluation. If the signs and symptoms 

persist during descent or reappear at ground 

level, it is necessary to provide hyperbaric oxygen 

treatment immediately (100% oxygen delivered in 

a high-pressure chamber). Neurological DCS, “the 

chokes,” and skin bends with mottled or marbled 

skin lesions (see Table 1) should always be treated 

with hyperbaric oxygenation. These conditions are 

very serious and potentially fatal if untreated. 

Facts About Breathing 100% Oxygen 

One of the most significant breakthroughs in altitude 

DCS research was the discovery that breathing 

100% oxygen before exposure to a low barometric 

pressure (oxygen prebreathing), decreases the risk 

of developing altitude DCS. Oxygen prebreathing 

promotes the elimination (washout) of nitrogen from 

body tissues. Prebreathing 100% oxygen for 30 

minutes prior to initiating ascent to altitude reduces 

the risk of altitude DCS for short exposures (10-30 

min. only) to altitudes between 18,000 and 43,000 ft. 

However, oxygen prebreathing has to be continued, 

without interruption, with inflight 100% oxygen 

breathing to provide effective protection against 

altitude DCS. Furthermore, it is very important to 

understand that breathing 100% oxygen only during 

flight (ascent, enroute, descent) does not decrease 

the risk of altitude DCS and should not be used in 

lieu of oxygen prebreathing. 

Although 100% oxygen prebreathing is an effective 

method to provide individual protection against 

altitude DCS, it is not a logistically simple or an 

inexpensive approach for the protection of civil 

aviation flyers (commercial or private). Therefore,

at the present time, it is only used by military flight 

crews and astronauts for their protection during high 

altitude and space operations. 

Table 1. Signs and symptoms of Altitude Decompression Sickness.

DCS Type Bubble Location Signs & Symptoms (Clinical Manifestations)

BENDS Mostly large joints 

of the body (elbows, 

shoulders, hip, wrists, 

knees, ankles)

• Localized deep pain, ranging from mild (a “niggle”) to excruciating. Sometimes a 

dull ache, but rarely a sharp pain. 

• Active and passive motion of the joint aggravates the pain. 

• Pain can occur at altitude, during the descent, or many hours later.

NEUROLOGIC 

Manifestations

Brain • Confusion or memory loss 

• Headache 

• Spots in visual field (scotoma), tunnel vision, double vision (diplopia), or blurry 

vision 

• Unexplained extreme fatigue or behavior changes 

• Seizures, dizziness, vertigo, nausea, vomiting and unconsciousness may occur

Spinal Cord • Abnormal sensations such as burning, stinging, and tingling around the lower chest 

and back

• Symptoms may spread from the feet up and may be accompanied by ascending 

weakness or paralysis 

• Girdling abdominal or chest pain 

Peripheral Nerves • Urinary and rectal incontinence

• Abnormal sensations, such as numbness, burning, stinging and tingling 

(paresthesia) 

• Muscle weakness or twitching 

CHOKES Lungs • Burning deep chest pain (under the sternum)

• Pain is aggravated by breathing

• Shortness of breath (dyspnea)

• Dry constant cough 

SKIN BENDS Skin • Itching usually around the ears, face, neck arms, and upper torso

• Sensation of tiny insects crawling over the skin 

• Mottled or marbled skin usually around the shoulders, upper chest and abdomen, 

accompanied by itching

• Swelling of the skin, accompanied by tiny scar-like skin depressions (pitting edema) 
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PREDISPOSING FACTORS 

Altitude 

There is no specific altitude that can be considered 

an absolute altitude exposure threshold, below 

which it can be assured that no one will develop 

altitude DCS. However, there is very little evidence 

of altitude DCS occurring among healthy individuals 

at altitudes below 18,000 ft. who have not been 

SCUBA (Self Contained Underwater Breathing 

Apparatus) diving. Individual exposures to altitudes 

between 18,000 ft. and 25,000 ft. have shown a low 

occurrence of altitude DCS. Most cases of altitude 

DCS occur among individuals exposed to altitudes 

of 25,000 ft. or higher. A US Air Force study of 

altitude DCS cases reported that only 13% occurred 

below 25,000 ft. The higher the altitude of exposure, 

the greater the risk of developing altitude DCS. It 

is important to clarify that although exposures to 

incremental altitudes above 18,000 ft. show an 

incremental risk of altitude DCS, they do not show 

a direct relationship with the severity of the various 

types of DCS (see Table 1). 

Repetitive Exposures 

Repetitive exposures to altitudes above 18,000 

ft. within a short period of time (a few hrs.) also 

increase the risk of developing altitude DCS.

Rate of Ascent 

The faster the rate of ascent to altitude, the greater 

the risk of developing altitude DCS. An individual 

exposed to a rapid decompression (high rate of 

ascent) above 18,000 ft. has a greater risk of 

altitude DCS than being exposed to the same 

altitude but at a lower rate of ascent. 

Time at Altitude 

The longer the duration of the exposure to altitudes 

of 18,000 ft. and above, the greater the risk of 

altitude DCS. 

Age 

There are some reports indicating a higher risk of 

altitude DCS with increasing age. 

Previous Injury 

There is some indication that recent joint or limb 

injuries may predispose individuals to developing 

“the bends.” 

Ambient Temperature 

There is some evidence suggesting that individual 

exposure to very cold ambient temperatures may 

increase the risk of altitude DCS. 

Body Type 

Typically, a person who has a high body fat content 

is at greater risk of altitude DCS. Due to poor blood 

supply, nitrogen is stored in greater amounts in fat 

tissues. Although fat represents only 15% of an adult 

normal body, it stores over half of the total amount 

of nitrogen (about 1 liter) normally dissolved in the 

body. 

Exercise 

When a person is physically active while flying at 

altitudes above 18,000 ft., there is greater risk of 

altitude DCS. 

Alcohol Consumption 

The after-effects of alcohol consumption increase 

the susceptibility to DCS. 

Scuba Diving Before Flying 

SCUBA diving requires 

breathing air under 

high pressure. Under 

these conditions, 

there is a significant 

increase in the amount 

of nitrogen dissolved in 

the body (body nitrogen 

saturation). The deeper the SCUBA dive, the greater 

the rate of body nitrogen saturation. Furthermore, 

SCUBA diving in high elevations (mountain lakes), 

at any given depth, results in greater body nitrogen 

saturation when compared to SCUBA diving at sea 

level at the same depth. Following SCUBA diving, if 

not enough time is allowed to eliminate the excess 

nitrogen stored in the body, altitude DCS can occur 

during exposure to altitudes as low as 5,000 ft. or 

less. 

WHAT TO DO WHEN ALTITUDE 

DCS OCCURS 

• Put on your oxygen mask immediately and switch 

the regulator to 100% oxygen. 

• Begin an emergency descent and land as soon as 

possible. Even if the symptoms disappear during 
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descent, you should still land and seek medical 

evaluation while continuing to breathe oxygen. 

• If one of your symptoms is joint pain, keep the 

affected area still; do not try to work pain out by 

moving the joint around. 

• Upon landing seek medical assistance from an 

FAA medical officer, aviation medical examiner 

(AME) military flight surgeon, or a hyperbaric 

medicine specialist. Be aware that a physician not 

specialized in aviation or hypobaric medicine may 

not be familiar with this type of medical problem. 

Therefore, be your own advocate. 

• Definitive medical treatment may involve the use 

of a hyperbaric chamber operated by specially 

trained personnel. 

• Delayed signs and symptoms of altitude DCS can 

occur after return to ground level whether or not 

they were present during flight. 

THINGS TO REMEMBER 

• Altitude DCS is a risk every time you fly in an 

unpressurized aircraft above 18,000 feet (or at 

lower altitude if you SCUBA dive prior to the flight). 

• Be familiar with the signs and symptoms of 

altitude DCS (see Table 1) and monitor all aircraft 

occupants, including yourself, any time you fly an 

unpressurized aircraft above 18,000 ft. 

• Avoid unnecessary strenuous physical activity 

prior to flying an unpressurized aircraft above 

18,000 ft. and for 24 hrs. after the flight. 

• Even if you are flying a pressurized aircraft, 

altitude DCS can occur as a result of sudden loss 

of cabin pressure (inflight rapid decompression). 

• Following exposure to an inflight rapid 

decompressions do not fly for at least 24 hrs. In 

the meantime, remain vigilant for the possible 

onset of delayed symptoms or signs of altitude 

DCS. If you present delayed symptoms or signs of 

altitude DCS, seek medical attention immediately. 

• Keep in mind that breathing 100% oxygen during 

flight (ascent, enroute, descent) without oxygen 

prebreathing prior to take off does not prevent the 

occurrence of altitude DCS. 

• Do not ignore any symptoms or signs that go away 

during the descent. In fact, this could confirm that 

you are actually suffering altitude DCS. You should 

be medically evaluated as soon as possible. 

• If there is any indication that you may have 

experienced altitude DCS, do not fly again until 

you are cleared to do so by an FAA medical officer, 

an aviation medical examiner, a military flight 

surgeon, or a hyperbaric medicine specialist. 

• Allow at least 24 hrs. to elapse between SCUBA 

diving and flying. 

• Be prepared for a future emergency by 

familiarizing yourself with the availability of 

hyperbaric chambers in your area of operations. 

However, keep in mind that not all of the available 

hyperbaric treatment facilities have personnel 

qualified to handle altitude DCS emergencies. To 

obtain information on the locations of hyperbaric 

treatment facilities capable of handling altitude 

DCS emergencies, call the Diver’s Alert Network at 

(919) 684-8111. 

For More Information 

If you are interested in learning more about altitude 

DCS, as well as the other stressors that may affect 

your performance and/or your health during flight, 

we encourage you to enroll in the Physiological 

Training Course offered by the Aeromedical 

Education Division (Airman Education Programs) 

at the FAA Civil Aerospace Medical Institute in 

Oklahoma City. A similar course is also available at 

US military physiological training facilities around the 

country through an FAA/DOD Training Agreement. 

For more information about any of these courses, 

call us at (405) 954-4837. 

Medical Facts for Pilots

Publication AM-400-95/2

Written by: J. R. Brown & Melchor J. Antuñano, MD

Prepared by: Federal Aviation Administration

 Civil Aerospace Medical Institute

Aeromedical Education Division

To order copies of this brochure and 

others listed below, contact

FAA Civil Aerospace Medical Institute

Shipping Clerk, AAM-400

P.O. Box 25082

Oklahoma City, OK 73125

(405) 954-4831
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Other Pilot Safety Brochures Available

Number Title

AM-400-94/2 Alcohol and Flying: A Deadly 

Combination

OK05-0270 Carbon Monoxide: A Deadly Threat

AM-400-03/2 Deep Vein Thrombosis and Travel

AM-400-91/1 Hypoxia: The Higher You Fly, the Less Air...

AM-400-98/3 Hearing and Noise in Aviation

AM-400-97/1 Introduction to Human Factors in 

Aviation

AM-400-92/1 Over the Counter Medications and Flying

AM-400-98/2 Pilot Vision

AM-400-95/1 Smoke!

AM-400-00/1 Spatial Disorientation: Visual Illusions

AM-400-03/1 Spatial Disorientation: Why You Shouldn’t 

Fly By the Seat of Your Pants 

AM-400-01/1 Physiological Training Courses for Civil 

Aviation Pilots

AM-400-05/1 Sunglasses for Pilots: Beyond the Image

To view these pilot and passenger safety brochures, 

visit the Federal Aviation Administration’s Web Site

www.faa.gov/pilots/safety

Physiological Training Classes for 

Pilots

If you are interested in taking a one-day aviation 

physiological training course with altitude chamber 

and vertigo demonstrations or a one-day survival 

course, learn about these courses by visiting this 

FAA Web site:

www.faa.gov/pilots/training/airmen_education