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Questions and Answers on
Physiology and Medical Aspects of Scuba Diving


Lawrence Martin, M.D. Copyright 1997


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Scuba quiz
Myths & Misconceptions
Disclaimer & Invitation

Brief History of Diving
Recreational  Diving
The Respiratory System
Explanation of Pressure

Water & Physical Laws
Unequal Air Pressures
Decompression Sickness
Oxygen Therapy
Gas Pressure at Depth

Dive Tables & Computers
Stress & Diving
Non-air Gas Mixtures
Women & Diving
Medical Fitness for Diving
Asthma & Diving
The Great Debate

All About DAN
Scuba Training Agencies
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Diving Odds N' Ends

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Recreational Scuba Diving: An Overview


Scuba is an acronym for 'self-contained underwater breathing apparatus'. There are several ways one can go under water (breath-hold, helmet diving, submarine, etc.). "Diving with scuba" signifies using apparatus that is completely carried by the diver and not connected to the surface, hence self-contained.


An estimated 2.5-3.5 million Americans participate in recreational scuba diving. Another 500 thousand become certified each year in the United States. In the early days of scuba diving (roughly 1950-1970), participants were predominately young men. Today the sport is enjoyed by people in all age groups and both sexes. And why not? Just about any teenager or adult who enjoys the water and can swim, and who does not have heart or lung impairment, can learn to scuba dive. It does not take great physical strength or unusual exercise tolerance. All it takes is the desire, plus some basic classroom and in-water training.

Scuba originally began with military and commercial applications, where it is still employed. However, by far its widest use, in terms of number of participants, is recreational. Recreational scuba today is like other any other sport that requires specialized equipment and training, such as snow skiing, sailing, mountain climbing and horseback riding. This section will provide an overview of recreational scuba diving, and explain how it differs from other forms of diving with scuba equipment.


Scuba apparatus for the recreational diver consists of (Figure 1):

Figure 1. Front and side view of scuba diver equipped ready to jump into the water. She is carrying a compressed air tank, and is wearing a buoyancy compensator (BC), mask with snorkel, and fins. Her mouthpiece is attached to the second stage regulator, and it in turn is attached by hose to the first stage regulator on top of the tank. The alternate second stage regulator is shown over her left arm. Note: this diver is not wearing a weight belt; weights are carried inside her BC.

  • a tank of compressed air carried by the diver on his or her back.
  • a first stage regulator attached to the tank that serves to lower air pressure delivered to the diver.
  • a second stage demand regulator and mouthpiece, that delivers air on inhalation and closes on exhalation.
  • a face mask that covers the diver's eyes and nose, to allow for both underwater vision and equalization of air pressure within the mask.
  • an extra second stage regulator and mouthpiece, carried by the diver in case of emergency (when attached to a long hose, this extra second stage is called an octopus).
  • two submersible gauges, one to display an accurate depth and the other to show how much air remains in the tank (or, alternatively, a single gauge that combines both functions, e.g., as part of a dive computer).
  • an inflatable vest (buoyancy compensator, BC) worn to provide a means of establishing positive buoyancy when needed.
  • a weight belt and weights (or some other type of weight system), worn to compensate for the positive buoyancy of the diver and scuba apparatus.
  • fins to facilitate self-propulsion in the water.
  • a wet suit or other type of body protection to prevent hypothermia (and secondarily to prevent cuts and abrasions under water); in cold water gloves and a hood may be worn for hypothermia protection also.

Optional equipment for scuba diving numbers many items, such as a snorkel, dive knife, small tank of extra air, dive computer, compass, dive lights, writing slate, whistle, and inflatable signaling device. Depending on the circumstances (e.g., lights for night diving, a snorkel for surface swimming) some of these items may at times be essential, but they are not part of basic scuba equipment.

With the basic scuba equipment a qualified diver can safely remain underwater for anywhere from a few minutes to over two hours; the time limit for any given dive will depend on the depth of that dive, the rate of air consumption, and the profile of any dives made within the previous 6-12 hours.


Potential hazards that every scuba diver must be aware of include de-compression sickness, air embolism, hypothermia, physical exhaustion, injuries from marine life, boating accidents, sunburn, and out-of-air catastrophes. Despite the list of potential problems, recreational diving is actually a safe sport if the diver is healthy and trained properly, each dive is carefully planned and dive limits are followed. The most serious problems, and how to avoid them, are covered in the entry level scuba course.

The entry level course, also known as 'basic open water' or 'basic certification,' takes about 30 hours. About 10 hours is spent in classroom instruction and the remainder in supervised pool and open water diving. 'Open water' means a natural body of water open to the sky. Upon completion, a certification or 'C' card is issued. The C card contains the new diver's picture, date of course completion and signature of the instructor. The basic open water course is fairly standard and is given under the auspices of one of several national training agencies (see Appendix B).

Many people are first exposed to scuba through the "resort course." This is an introductory lesson offered at a resort or on a cruise, and is always taught by an instructor (i.e., one who is also qualified to teach the standard open water course). The resort course is highly variable; it may include only a brief lecture on a crowded bus on the way to the dive site, or a leisurely morning pool session before the afternoon open water dive (I have seen both methods). Because the resort course contains no textbook and very little in the way of theory or skills testing, it does not certify for independent diving. It is also good only for that resort and for the time you are a guest there. If you return six months or a year later to the same resort you would have to start all over with another resort course. (One exception is the Club Med resort course, which offers 'Club Med Certification'. The student completing a Club Med resort course can dive at any other Club Med around the world, but always with a Club Med instructor and after an initial checkout dive.)

Resort courses are fine for introduction to scuba, but anyone who has enjoyed the experience is encouraged to take a standard certification course and learn the necessary skills and theory. Standard certification is also more practical. The C card allows one to rent or buy scuba equipment anywhere in the world, and to engage in recreational diving without supervision by an instructor.

The certification process distinguishes recreational scuba diving from most other sports that use specialized equipment (e.g., snow skiing, mountain climbing). Without a C card (i.e., without certification) one should not go scuba diving (unless accompanied by a professional scuba instructor).


In college the lowest academic rank is the "instructor," usually someone in graduate school who is assigned to teach and work under a "professor." In the world of scuba the instructor is the highest level. A scuba instructor is the only person trained and qualified to teach scuba diving. He or she works under the auspices of one of the national scuba training agencies (see Appendix B). The scuba instructor can be assisted by other scuba professionals who are in training to become a scuba instructor; they may have the title of dive master (the entry level position for a scuba professional) or assistant instructor. However, only someone with instructor status can teach the basic open water course.


Diving was revolutionized by the development of a workable demand regulator, co-invented in 1943 by Jacques Cousteau and Emil Gagnan. The "sport" of scuba diving did not catch on for another 10 years. Today the greatest use of scuba equipment is for recreational diving. Recreational scuba diving, as taught by national certifying agencies, is defined as diving that:

  • Uses only compressed air as the breathing mixture.
  • Is never done solo.
  • Does not exceed a depth of 130 feet.
  • Has a depth-time profile not requiring a decompression stop; if necessary one can ascend to the surface without stopping.
  • Does not require specialized training beyond the basic open water course.

Scuba equipment is also widely employed by two other types of divers, loosely categorized as "professional" and "technical." By definition, these two groups are not constrained by the RSD criteria listed above.

Professional diving is done for military, governmental, commercial, or scientific purposes. Professional divers are paid to dive. They have a specific mission for each dive, e.g., lay a mine or recover a bomb (military), look for a body or a weapon (law enforcement agency), explore for oil (commercial), map an ancient wreck or examine a new species of fish (scientific).

Technical diving is the term for all diving that exceeds recreational limits but is not engaged in for profit. Although many, if not most, technical divers consider themselves involved in a recreational activity, the nature of their diving and type of equipment used exceed the boundaries of RSD. To be sure, there is not universal agreement on what constitutes technical diving, and any attempt to define it tends to degenerate into heated discussion and semantics. For the novice or basic open water diver, the following activities can be considered to be "technical diving."


  • cave diving
  • ice diving
  • very deep diving (to greater than 130 feet)
  • mixed gas diving (using gas mixtures other than air, such as nitrox or trimix)
  • deep-penetration wreck diving
  • diving with specialized life-support equipment (e.g., O2 re-breathers)

Much of technical diving is taught by agencies other than the national open water certification agencies. However, recently PADI and other national agencies became involved in nitrox certification, although the activity is still not considered part of basic open 'recreational diving.' Still, the trend is to expand the envelope of technology to encompass more and more divers, and it is conceivable that some of today's advanced technology (e.g., rebreathers)will one day be routinely used by open water recreational divers. Clearly, the distinction between 'technical' and 'recreational diving' is becoming less distinct over time. (No matter how diving activities are classified, no one should engage in any diving activity unless certified in that activity or else supervised by a qualified instructor.)


RSD requires that each diver be accompanied under water by a buddy who can share air or provide other assistance. Scuba apparatus is designed to carry an extra demand regulator. This is required in case one diver runs out of air and has to share a single tank with his or her buddy. Ideally, the buddies should have similar training and skill levels; it does no good to have a buddy who dives deeper or stays down longer than you do. Buddies should stay close together and always be aware of each other's location.


The 130 foot limit is an arbitrary depth originally adopted by the U.S. Navy because it gave navy divers about 10 minutes of bottom time on compressed air; going any deeper on air made no sense to the Navy because the time available to do useful work was simply too short. As with many diving issues in the early days of scuba (e.g., the 'no decompression' limits), the Navy standard was also adopted by the recreational training agencies.

Some experienced recreational divers do go deeper than 130 feet, and yet still stay within no decompression limits. However, since the risks of the bends, running out of air, and nitrogen narcosis increase as you go deeper, the training agencies feel that some arbitrary limit must be set and have stayed with 130 feet. Thus, although one can dive deeper and stay within no decompression limits, diving deeper than 130 feet a) is not taught by the recreational training agencies, and b) must be undertaken with great care and an understanding of the increased risks.


"Decompression" always occurs when we go from a higher to a lower ambient pressure. Thus all compressed gas diving is decompression diving. When recreational diving is referred to as "no decompression diving" it really means "no mandatory decompression stop diving." A decompression stop should never be necessary within the guidelines of RSD.

A decompression stop is often necessary in dives deeper or longer than allowed in RSD; it provides time for some of the excess nitrogen that entered the tissues to "gas off" (diffuse into the blood stream and then be ventilated out by the lungs), thereby minimizing risk of decompression sickness (DCS). Based on experiments, plus much trial and error experience, the amount of excess nitrogen remaining in the tissues after a planned, mandatory decompression stop should not cause DCS.

Since professional and technical divers often spend longer periods under water and/or dive deeper than recreational divers, they must know when to stop on ascent and how long to wait before surfacing. By contrast, each recreational dive is planned so that the diver can ascend continuously to the surface without encountering decompression sickness; the diver is still decompressing on the way up, but doesn't have to stop to allow further decompression. The basic assumption is that diving is inherently made safer by avoiding dives that require decompression stops. This is both the philosophy and practice of recreational diving today.

The "no decompression stop" limits in RSD are based on maximum depth, time under water, and extent of any preceding dives, all factors which directly affect tissue nitrogen uptake. The limits are set in dive tables known to every certified diver, and are incorporated into all dive computers.

Excess nitrogen, which enters tissues due to the increased ambient pressure under water, determines the "no-decompression stop" limits. A non-repetitive dive (no previous dive within a specified time period, typically 6-12 hours) to 130 feet has an allowable "actual bottom time" (measured from the time dive commences to start of ascent) of only about 5-10 minutes for a dive to 130 feet. Beyond this brief time span the diver risks developing decompression sickness from a continuous ascent. In contrast, at 35 feet, on a non-repetitive dive, the 'no decompression' bottom time is about 205 minutes.

Although RSD is always planned for no decompression stop required, it is routine practice to make a 3 to 5 minute "safety stop" at 15 feet before surfacing from any dive deeper than about 40 feet. This is a decompression stop but it is not mandatory, just added for extra safety to the dive; hence the term safety stop. In theory, when adhering to the recreational diving tables one should not experience DCS if a safety stop is not made. Nonetheless, all dives deeper than about 40 feet should incorporate a safety stop, and it is universally practiced. If any dive requires a decompression stop for any reason, it has exceeded the limits of recreational diving.

1. The reason RSD is limited to 130 ft. is:
a. compressed air won't support a diver below that level
b. there is not much to see below 130 feet.
c. nitrogen narcosis begins to appear at that depth.
d. decompression stop becomes mandatory.
e. none of the above
2. Which of the following items is(are) never used while scuba diving?
a. eye goggles
b. ear plugs
c. snorkel
d. gloves
3. If a diver inadvertently exceeds the recreational dive limits he should:
a. prolong the safety stop, air permitting.
b. surface and ask the divemaster/instructor what to do.
c. begin exercising underwater to increase nitrogen off loading.
d. not worry about it, and make the usual safety stop.
4. A diver goes to 50 feet depth for 30 minutes, then begins her ascent, stopping at 15 feet for three minutes. True or false: this is a decompression stop to lessen the risk of developing the bends.
5. True or false: a divemaster is qualified to teach the basic scuba certification course.



Within the universe of recreational diving there are many levels, based on extent of training, experience, or both. I have already commented on the resort course, which is not formal scuba training but just an introductory scuba experience. The following paragraphs pertain to the certified diver.

Training. One can take several courses beyond basic open water, and they can be divided into two "tracks." The first track is for people who want more training but have no intention of ever teaching scuba. With such courses as advanced diving, stress and rescue, underwater photography, ice diving, and deeper diving (60-130 feet), one can gain additional knowledge and skills, all under the purview of recreational diving.

The second track is for people who plan to turn professional (within the recreational purview) and teach scuba. They will take courses leading up to the instructor level (advanced diving, stress and rescue, divemaster, assistant instructor, instructor). Once an individual achieves instructor level he or she is a full-fledged professional within recreational scuba and can teach the basic certification course.

Unless they have taken special training, scuba instructors are not trained to exceed the recreational limits. Thus they are not technical divers and would need further training to do mixed gas diving, for example. Of course many scuba instructors do obtain training in these and other technical diving skills.

Experience. Most recreational divers never take more than the basic open water course. However, after making many dives over the years they become highly experienced and adept in all the important skills. In recent years scuba agencies have developed a way to recognize the more experienced divers, by creating "dive cards" showing the number of dives obtained, in round numbers. Thus, with proper documentation you can get a card that shows you have done anywhere from 50 to 5000 lifetime dives. The utility of this type of card is that it can quickly show your level of experience, such as when you go to rent equipment or sign on for a boat dive. Experience counts for much in scuba diving (as with any activity), and the cards are a way of acknowledging this.


As its name indicates, sport surface-supplied compressed air diving (sometimes shortened to 'sport surface-supplied air,' SSSA) separates the compressed air source from the diver. For this reason it is not "scuba," which is self-contained (the scuba diver always carries his/her own air source). SSSA uses a small gasoline- or electric-powered air compressor that can sit on a boat, dock or a floating inner tube, so it can literally go to any dive site and remain over the heads of the divers (Figure 2). The compressor can provide air to more than one diver at the same time.

Each SSSA diver is tethered to the compressed air source by a long hose. The entire apparatus is sometimes called a hookah, after the Egyptian term for a pipe that cools water, but "surface-supplied compressed air" is the more correct term. At the end of each hose is a second stage regulator and mouth-piece, just like in scuba diving; exhaled air is expelled into the water. For recreational divers this technique has been pioneered by Brownie's Third Lung, Ft. Lauderdale, FL (1-800-327-0412).

SSSA diving is limited by the strength of the air compressor, the length of the hose and, as in scuba, the level of diver training. As sold by Brownies, each unit serves at least two divers; some can serve three or four divers at 30 feet. Other units allow deeper diving, up to 90 feet for two divers. In principle SSSA diving is the same as professional helmet diving, except that the depths are generally less, no helmet is worn by the diver, only compressed air is used (professionals may used mixed gases), and the amount of surface support (i.e., other people on the surface) may be minimal to none. The box below lists advantages and disadvantages of SSSA compared to scuba.

Figure 2. Air compressor inside rubber tube, as used for surface-supplied compressed air diving for two people. (Courtesy Brownies' Third Lung.)

SSSA vs. Scuba

Principal advantages of SSSA diving

  • Can provide unlimited supply of compressed air.
  • Allows diving to full limit of dive tables without running out of air; only small amount of gasoline required for dives that would otherwise require several tank refills.
  • Eliminates need to carry a heavy tank into the water.
  • Eliminates need for first stage regulator.
  • Each system designed to be used by at least two divers, fostering buddy system; buddy always close by and connected.
  • Since there is no tank and therefore no changing tank pressure, diver's buoyancy tends to stay constant throughout dive.

Principal disadvantages of SSSA diving

Requires purchase and maintenance of air compressor; requires use of hydrocarbon fuels (or access to source of electricity).

  • Limited mobility under water.
  • Limited depths (depending on compressor size).
  • Possible compressor failure; SSSA diver often encouraged to carry a small tank of compressed air for emergency surfacing.
  • Cannot be used for wall or cave diving, or penetration wreck diving.
  • Equipment is not resort based (as is scuba), so one usually has to travel with the compressor or make special arrangements to have it available.
  • Unlimited air supply requires special diligence not to exceed dive table limits.

SSSA diving is very popular in some areas. It is particularly useful for long, shallow, underwater tasks, such as lobster hunting, fish harvesting, cleaning a boat bottom, or examining a shallow wreck that doesn't require penetration.

Although SSSA divers may stay shallow, and hence minimize the risk of DCS, they are still under the same risk of barotrauma inherent in all compressed air diving. For this reason people engaging in SSSA diving should be properly trained and certified. Unlike scuba, however, some proprietors allow purchase of equipment without evidence of proper certification (Brownies' Third Lung requires certification). It is a mistake to think that SSSA diving is somehow "safer" than scuba, or that it does not require training and learning about compressed air hazards. Anyone diving with surface-supplied air needs to fully understand the risks inherent in compressed air diving.


The future is now. The technology that exists today, mostly in the realm of technical diving, may become part of recreational diving tomorrow. There is unending debate within professional diving circles about most of this technology, and whether it should enter the realm of recreational diving sooner rather than later (see section on Nitrox diving, Section L). The future of RSD will likely see advances in two main areas.

Dive safety

  • Spread of hyperbaric chambers to all popular dive sites; development of affordable, portable hyperbaric chambers that can be carried on a dive boat.
  • Universal use of dive computers air integrated to the scuba tank, eliminating the high pressure hose connection.
  • Use of full face masks that remain on the diver's face if there is any panic or loss of consciousness (absence of a mouthpiece reduces the risk of drowning in these situations).
  • Implementation of underwater voice communication technology.
  • Development of "heads up" displays for dive data; instead of data displayed on a wrist device or on a dangling console, all data will appear inside the diver's mask, available for reading with only a turn of the eye. Critical information may flash, assuring the diver's attention.
  • Implementation of diver-specific algorithms (adjusted for the diver's age, weight, and perhaps other characteristics) for dive computers. Such algorithms may become available from studies that correlate dive accident experience with a wide range of dive profiles in a diverse group of divers.

Extension of bottom time or distance traveled under water

  • Use of nitrox, perhaps with a depth limiter to prevent diving too deep and incurring risk of oxygen toxicity.
  • Use of stronger tanks (carbon fiber) that can hold higher pressures and thus more cubic feet of gas.
  • Underwater scooters that allow the diver to range further on a given dive with the same amount of air.
  • Use of re-breathing "closed circuit" scuba technology.

Much of this technology and equipment are available now, if not for the recreational diver than in technical circles. Just as other once-new technology gradually became accepted (e.g., dive computers), it seems likely that much of today's "cutting-edge" technology will be adopted by recreational divers in the coming years. Cost and complexity will certainly be a limiting factor, however. Many recreational divers may not want to be burdened with more equipment or ever more complicated and expensive technology. Only time and the marketplace will tell.

1. e.
2. a,b,c
3. a.
4. True
5. False


For the sport diver, the training manuals of NAUI, PADI and SSI provide much useful information on physics and physiology of diving, as well as on all the diving skills. In addition, the following six books are highly recommended reference works for those who wish to read further on dive physics and physiology; the last three are textbooks marketed mainly to physicians and other medical professionals.

Hornsby A, Brylske A, Shreeves K, Averill H, Seaborn C. Encyclopedia of Recreational Diving. International PADI, Inc., Santa Ana, CA.; 1989.

NOAA Diving Manual, 3rd Edition. U.S. Department of Commerce, National Oceanic and Atmospheric Administration, Washington, D.C.; 1991.

U.S. Navy Diving Manual, Vol. 1 (Air Diving) and Vol 2 (Mixed-Gas Diving), Best Publishing Co., Box 30100, Flagstaff, AZ; 1993.

Bennett P, Elliott D, editors. The Physiology and Medicine of Diving. 4th edition. W.B. Saunders Co., Philadelphia; 1993.

Bove AA, Davis JC, editors. Diving Medicine. 2nd Ed., W.B .Saunders., Philadelphia; 1990.

Edmonds C, Lowry L, Pennefather J. Diving and Subaquatic Medicine. Butterworth Heinemann, Oxford; 1992.

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