How Safe is Scuba Diving?

To ask, “how safe is scuba diving?” one could easily answer with another question, “how responsible is the scuba diver?” Scuba diving is a sport with risks, like every other sport. What makes scuba diving seem riskier than other sports is the fear associated with drowning and the nervousness using a scuba system as “life support”. Many people are afraid of water, afraid of sharks, afraid of relying on a strange contraption of hoses and tanks to provide air in an airless environment. In reality, scuba diving is not as dangerous as people seem to believe and in those rare instances of a scuba fatality it is almost always shown diver recklessness was the cause.

most scuba diving fatalities claim the lives of the ignorant, the reckless and the irresponsible

In popular lists of the “most dangerous sports” (including one by Forbes) two kinds of underwater diving are often mentioned. The list does include some sports which strike me as being truly dangerous: base jumping (throwing oneself off a tall building), street luge (sledding down a road at up to 80 mph), bull riding (and anyone who has watched a rodeo must agree). Why scuba diving even makes it on the list is a mystery. I assume the compilers included scuba because they are afraid of it. The statistics just don’t support the assertion.

On the Forbes list free diving, or breath hold diving, is at number two just below base jumping. In free diving a weighted individual plummets to incredible depths while holding their breath. Then after shedding the weights bobs back up to the surface again. It’s worth noting that even though the diver compresses and decompresses very quickly they are not at risk of the bends because their dives are so short they will have little to no nitrogen absorbed. Most fatalities in free diving occur because the diver blacks out from lack of oxygen. This is not scuba.

Scuba diving is not dangerouscompared to some other activities we do without hesitation.

The one kind of scuba diving that makes it onto the “most dangerous” list is cave diving. Granted, there are a lot of things that can go wrong during a cave dive. That’s why we get trained to learn how to do it properly. Injuries and fatalities do occur in cave diving, even sometimes to people who are properly trained and prepared. But, by far, most scuba diving injuries and fatalities become Darwinian forces that tragically claim the lives of the ignorant, the reckless and the irresponsible.

The truth is the number of fatalities decrease every year, even though the number of people diving is increasing dramatically. About half of all reported scuba-related fatalities happen on tourists’ “dive packages”, rather than on organized scuba trips chartered by a dive shop. The most dangerous dive packages are those in which the participants are not trained or certified, but are merely given a short training session before diving accompanied by a scuba chaperone. Responsible associations like PADI and NAUI recommend a complete training and certification before diving in open water, which sometimes does not happen on these organized scuba trips. Novice divers beware.

With proper training a scuba diver is confident. He is prepared, responsible, knows the risks and how to avoid them. Scuba diving, to the properly trained, is not dangerous compared to most other banal activities we do without hesitation.

Scuba fatality estimates range around 5 fatalities per 100,000 divers. About a third of those are the result of heart or circulation problems, most common of which is a heart attack or cardiac arrest (which is tragically almost always fatal if it happens while submerged). Here is a partial list of things that are more dangerous than scuba diving:

  • Riding a bicycle on a city street
  • Being a passenger in an automobile
  • Riding a motorcycle
  • Smoking tobacco
  • Sailing a yacht
  • Fishing from a dock

As for non-fatal injuries it is well known that most injuries happen to scuba divers while they are still on the boat.

The point of this editorial is not to say that scuba diving is safe. Indeed people do get hurt scuba diving. But anyone who puts scuba diving in the same list as riding a bucking bronco or jumping out of an airplane is probably letting their fears take precedence over factual research.

Flying After Diving

When diving on vacation a scuba diver needs to be aware that residual nitrogen poses a health risk upon decompression. This applies just as much to ascending in a jet as ascending to the surface of the water. The difference in air pressure, as a plane ascends to cruising altitude, is roughly equivalent to the last 14-16 feet of ascension in water and the same risks are evident including the bends (decompression sickness) and/or an embolism. If you fly after diving you risk extremely painful gas bubbles forming in your joints and flesh, or tragically in your blood, leading to an embolism and possibly even death.

A good conservative rule is to ensure that you have no residual nitrogen in your body before flying. Following the NAUI dive tables, you should be in letter group A or less (which can take over 9 hours). A safe suggestion is to pass a full 24 hours after your last scuba dive before flying.

Dive computers, which track nitrogen levels, have a “time-to-fly” indicator, which tells a scuba diver how long they need to wait before boarding a plane. Plan your last dive sensibly, so that you aren’t bringing nitrogen along for a painful and possibly fatal return trip.

Scuba Diving Risks

Scuba diving is not a dangerous sport. Scuba diving is riskier than a sport like hockey or baseball, but less dangerous than street luge or mountain climbing. Modern scuba diving equipment is easy to use, very reliable and with the proper training and a responsible attitude scuba diving can be enjoyed safely. In fact, almost all scuba diving injuries and casualties are the result of recklessness or bad judgment.

There are certainly risks involved in scuba diving. Part of certification training is learning about those risks and how to avoid them. The majority of possible health problems are forms of barotraumas, which are all caused in one way or another by changes in pressure. Other possible risks are associated with higher absorption of gases, while other risks are more mechanical and environmental in nature.

Here are some of the risks associated with scuba:

  • Barotrauma (explained by Boyle’s law)
    • alternobaric vertigo 
      Dizziness or disorientation caused by unbalanced pressures in the inner ear. Most commonly experienced by stubborn scuba divers trying to dive with the common cold.
    • altitude sickness 
      Headache caused by a quick ascent, usually associated with airplane travel.
    • barodontalgia 
      Pain caused by tiny bubbles of gasses trapped in the teeth, usually in fillings or caps.
    • decompression sickness, a.k.a. “the bends” 
      Nitrogen coming out of a solution in tissue which is caused by hastened decompression.
    • dysbaric osteonecrosis 
      Rare bone lesions produced by long term exposure to high pressure environments.
    • embolism 
      Nitrogen coming out of a solution in the body. It can be caused by accelerated decompression.
    • arterial gas embolism 
      Gas coming out of a solution in the arteries. It can be potentially fatal.
    • cerebral embolism 
      Gas coming out of a solution in the brain. It can be potentially fatal.
    • lung expansion injury 
      It can be caused by holding breath while ascending.
    • pneumomediastinum 
      Ruptured bronchus or alveoli in the lungs from excessive pressure. May be caused by holding breath while ascending.
    • pressure arrhythmias 
      Abnormal heart rhythms caused by external pressure.
    • tinnitus, Eustachian & inner ear damage, Tympanic membrane rupture and/or hearing loss 
      Inner ear damage can result from diving without equalizing air pressure in the Eustachian tubes. It is complicated or caused by water pressure and blocked sinuses and it can be extremely painful.
  • Non-Barotrauma (explained by Henry’s Laws and Dalton’s Laws)
    • co2 toxicity, a.k.a. hypercapnia. 
      Too much CO2 in the body, usually caused by inadequate exhalation or air consumption during heavy exertion. Symptoms include shortness of breath, headache and/or confusion.
    • nitrogen narcosis, a.k.a. “rapture of the deep” 
      The result of a toxic effect of high pressure nitrogen on nerve conduction. Symptoms are comparable to the effects of alcohol drunkenness.
    • o2 toxicity 
      Toxic effects of absorbing too much oxygen. Symptoms include a burning sensation in the lungs, twitching, dizziness, vomiting and/or seizures.
  • Other physical and health hazards – Scuba Diving
    • dangerous marine life 
      Most common injuries are the result of divers touching poisonous animals such as jellyfish, fire coral, urchins or stingrays. Attacks by large fish are extremely rare.
    • dehydration 
      Dehydration is an inadequate bodily water level. Surprisingly common on boat tours; diving while dehydrated aggravates other health risks including nitrogen narcosis and hypercapnia.
    • hypothermia 
      Hypothermia is a loss of body heat and early symptoms include fatigue and loss of judgment.
    • drowning 
      An obvious risk if for any reason a diver breathes in water instead of air or just simply the loss of air.
    • running out of air 
      Typically caused by irresponsible air management or scuba equipment failure.
    • underwater injury 
      Common injuries include abrasions and cuts (from sharp coral), sprains, bumps and bruises. Studies show more serious injuries occur getting in and out of the boat than actually in the water.

Dive Computers

Scuba computers, or dive computers, are compact digital devices which perform the task of nitrogen management without the need for a watch or dive tables. Combining a timer with a depth gauge, and intelligent software which computes absorption of gases, dive computers are popular with experienced recreational divers. While it is essential that all divers learn about residual nitrogen and decompression using manual dive tables, a computer is a valuable device that enhances your dive experience by automating nitrogen management.stops, if necessary.

A dive computer does not plan a dive for the scuba diver. A scuba diver is still responsible for planning a dive safely in accordance with health guidelines. What the computer does is: keep track of depth and time spent underwater, computes the amount of nitrogen present in the diver’s body, alerts the diver of levels getting too high and guides the diver to make appropriate decompression

On a traditional manual dive table there are only 2 variables used to describe a dive: maximum depth and time. The calculation of absorbed nitrogen assumes that the diver plunged immediately to maximum depth, stayed there for a duration and then bobbed back up to the surface. Then the dive profileis “square” – if it is graphed on an X-Y grid it would look like a rectangular trench. In practice a dive is never like that: a diver will descend slowly, swim around and up and down, change depth to navigate reefs or bottom formations and then slowly ascend to the surface.

The most enticing advantage of a dive computer is longer bottom times. Dive tables with a square profile are necessarily conservative; the exaggerated absorption on the safe side. The accurate curved profile of computerized depth tracking invariably allows the diver to stay down deeper, longer.

Here are some things to consider when choosing a dive computer:

  • Research the dive computer’s brand and model. Look for recognizable manufacturers and consult consumer guides.
  • Is it easy to change the battery without dissembling the unit?
  • Is the dive computer’s interface intuitive, easy to read and backlit?
  • Some dive computers can upload your dive data to a PC for use in scuba planning and logging software. Not all dive computers have this feature. Do you want this feature?
  • Will the dive computer handle gas mixes other than air, such as nitrox?
  • Does the dive computer support decompression management and a “time-to-fly” feature?
  • Does the dive computer use a well-tested algorithm for calculating gas absorption and decompression? For example, the Mares-Wienke Reduced Gradient Bubble Model (RGBM) Algorithm and the Buhlmann algorithm, etc.
  • Are the buttons large, spaced far apart and easy to press while wearing neoprene gloves?

NAUI Dive Tables

Dive tables or dive charts are tables used to determine the amount of nitrogen that is absorbed by your body. The charts provide a guide for the amount of nitrogen which can be safely absorbed with no adverse health effects. The nitrogen content of your body is designated by a Letter Group; from A to L with A being the lowest and L being the highest.

A familiarity with dive tables is required for certification even though there are dive computers which automate the entire process of nitrogen management. The justification is that tables require only your brain. Computers won’t always work. Proper planning and a sensible conservative approach to nitrogen management is always safer than relying on a machine.

Dive tables give you a recommended depths and times for a single dive. The deeper you go the less time you may spend submerged. If you dive shallowly you may spend more time underwater before you reach your nitrogen limit.

Dive tables were first developed in 1915 and the numbers were based on awful inhumane decompression experiments on unsuspecting volunteers. Later, the U.S. Navy developed better tables in the 1930’s using far more humane tests with military volunteers. It is worth pointing out that the U.S. Navy tables are based on experiments on fit young men in the peak of athletic health. Without being derogatory it is simply true that most recreational scuba divers are not buff young athletes with zero body fat. The 1990 NAUI tables are more conservative and are thus appropriate for the “rest of us”.

When you dive underwater the increased air pressure forces more nitrogen to be absorbed into your body. This is called ingassing – forcing gas in to your system. When you return to the surface you will have more nitrogen in your body than before you dove.

As you spend time on the surface your body will expel gases, mostly through normal exhalation. This is called offgassing and is analogous to a tire or balloon with a slow leak; as you spend more time on the surface your nitrogen level will become closer to normal. The period between repetitive dives is called the Surface Interval Time (SIT).

If you decide to dive again before your nitrogen level has returned to normal it is called repetitive diving. If your body already has residual nitrogen then your second dive of the day may not be as long or deep as the first. The dive tables can be used to find your letter group after the first dive, calculate your letter group after the SIT (Surface Interval Time) and start your second dive knowing the depth and/or time limit for the second dive.

A diver goes to a depth of 60 feet in the morning. According to the chart the maximum dive time for 60 feet is 55 minutes, but you stay submerged for only 50 minutes. You would emerge from the water in Letter Group H. If you stayed on the surface for 8 hours, your Letter Group would drop right back to an A. Instead, you go to shore, have some lunch, browse the dive shop and head back out to the water a few hours later. After 3 hours, you have offgassed a lot of nitrogen, but you are still in Letter Group D. As a “D”, your nitrogen level is the same as if your first dive had been 20 minutes; this is known as your Residual Nitrogen Time (RNT). On the second dive, you want to visit the same reef – at 60 feet – but this time because you have 20 minutes-worth of residual nitrogen you can only stay for 25 minutes.

Adjusted Maximum Dive Time = Maximum Dive Time – Residual Nitrogen Time 

NAUI dive tables are copyright protected and may not be reproduced on this website. A diver should carry the tables with them in their gear bag and for this a laminated copy is ideal. Every dive shop will carry a selection of laminated charts and since you may be consulting them before and after every dive it is a good idea to get a good one. Laminated paper copies are good and very inexpensive. A laminated paper print is especially good if it has a sealed key ring hole punched so it can be tied to your bag (so it won’t blow away in windy weather). Some charts are printed on hard plastic – do not buy these. They may seem very sturdy in the store, but after a few weeks of abuse they become brittle and crack. The best and most permanent charts are stamped onto pliable rubber.

Dalton’s Law

Dalton’s Law is named for John Dalton, who stated the law in 1801. The law states that the pressure exerted by a mixture of gases is equal to the sum of the pressures which would be exerted by the gases individually.

P = p(1) + p(2) + p(3) … + p(n)

Where P is the total pressure exerted by the sum of pressures p.

Dalton’s discovery was all gases tend to compress similarly. If you have a mixture of nitrogen and oxygen and you add 100lbs of pressure both the nitrogen and oxygen will compress equally.

In other words, mixed gases will stay in the same proportions under pressure – you will not find some gasses compressing more than others. For instance, imagine squeezing a cloth bag equally full of marbles and popcorn. You know that the popcorn will compress more than the marbles and with sufficient pressure the proportion of marbles and popcorn in the bag (by volume) will change. Dalton’s law assures us that among gases it will not happen.

To a scuba diver this assures us that our bodies absorb gases in the same proportions at any depth. It is this principle which makes it possible to estimate gas levels in our body, plan repetitive dives and avoid the bends.

Henry’s Law

Henry’s Law is named for William Henry, who in 1801 stated that the mass of a gas which dissolves in a volume of liquid is proportional to the pressure of the gas.

e^P = e^{kC}

Taking the natural logarithm of this equation we get the more common version:


Where P = the partial pressure of the gas solute, 
C is the concentration of the gas 
and K = the Henry’s Law constant.

To a scuba diver Henry’s Law tells us that at higher pressure our bodies will absorb more gasses. At great depths, the amount of nitrogen (and other gases) absorbed into our blood and tissue is greater than the amount absorbed at shallow depths. That is why a diver going to 100′ has a greater risk of decompression illness than a diver who dives only 30 feet. Since the shallow diver has absorbed less gas, it is less likely to come out of solution in the body.

Charle’s Law

Charles’ Law states the relationship between temperature and volume of a gas at a constant pressure.

T/V = k

Where T is the temperature of the gas, 
V is the volume 
and k is a constant.

In a nutshell, Charles’ law says that cooling a gas decreases its volume. If the volume remains fixed then the pressure must decrease. A suitable example of this phenomenon is a scuba tank bursting because it is left in the trunk of a hot car in the tropical sun (temperature rises, volume is constant and pressure increases).

Boyle’s Law

Boyle’s Law is named after Robert Boyle (1627-1691). Edme Mariotte (1620-1684), a French physicist, discovered the law independently at roughly the same time, so this law is often known as Mariotte’s or Boyle Mariotte’s law. During scuba diving classes the law is usually called Boyle’s Law. Boyle’s law relates the volume and pressure of a gas held at a constant temperature. Boyle’s law is:

PV = k

Where P is the pressure of gas, 
V is the volume of gas 
and k is a constant. The constant k does not need to be known to understand the relationship between P and V.

Boyle’s law basically says this: 

  • When you increase the pressure the volume decreases. 
  • hen you increase the volume the pressure decreases.

ADD Boyle’s Law Photo

This Law is often applied to demonstrate what happens during ascent and descent and how it compensates the pressure in the BCD, lungs, mask and anywhere else that air is contained. As you descend, pressure increases. As pressure increases volume decreases, so the same amount of air takes up less space. Because of this, as you descend you will notice that your BCD “deflates”. It is not losing air, it is compressing the same air into a smaller volume. You may also notice other effects of airspaces being compressed – wet suits fit less snugly (if you are wearing a thick neoprene suit this is even more apparent) and the airspace behind ears becomes decompressed. When equalizing you need to let more air into your Eustachian tubes to compensate for the reduction in volume.

Likewise, as you ascend pressure decreases and volume increases. A full BCD at depth will become fuller as you ascend – that is why you must release air from your BCD as you ascend. More importantly, it is important to exhale from your lungs as you ascend. If you hold your breath while ascending the air in your lungs expands beyond capacity, which can cause painful internal injuries.

Physics of Scuba Diving

Scuba diving is all about getting air into you while you’re underwater. What complicates scuba is the way air behaves at depth, under pressure, in your body and in your equipment. To dive safely you need to understand all the things intuitively and how they affect the techniques and practice of scuba diving. As part of your scuba certification you will come to understand certain principles of physics, in particular the gas laws which apply to scuba. When you understand the gas laws all the intricacies of nitrogen management, offgassing and even rules of inflating and deflating your BCD will make more sense.

As part of scuba certification, you will learn these four gas laws: