Scuba Diving

General Principles


  • Diving includes multiple activities performed in an aquatic environment.

  • Technology allows people to enjoy the underwater experience.

  • An estimated 2.8–3 million US divers participate each year.

    • People dive for various reasons, including new experience, unique environment and surroundings, new challenges, jobs, military duties, sport, and environmental awareness.

Types of Diving

  • Breath-hold diving (apnea, free, or skin diving)

    • Snorkel is the main piece of equipment used; may use fins for propulsion

    • Often performed in conjunction with spear fishing, shellfish gathering, snorkeling tours, and competitions

  • Recreational scuba (self-contained underwater breathing apparatus)

    • Enjoyment of unique environments

    • Performed in several locations worldwide

  • Commercial, technical, and advanced recreational diving

    • A wide range of gases used for respiration, depending on the characteristics of the event

    • Specialized training for wreck, cave, night, and exploratory diving

    • Various gases are used; decompression techniques may be required

Environments for Diving

  • Multiple different underwater environments, each with different risks and techniques involved

  • Ocean water is the most common location for recreational diving

  • Freshwater lakes and rivers

  • Ice diving

  • Cave and wreck diving

  • Pools for competition and training

Physiology of Diving

  • Pressure at sea level on the surface is 1 atmosphere absolute (ATA).

    • Pressure in Denver, Colorado (elevation of 5280 feet) is 0.8 ATA, but at 6 feet under sea level, it will be 1.2 ATA.

    • Every 33 feet of seawater (FSW) below the surface increases the pressure by 1 ATA.

  • Saltwater is approximately 775 times denser than air.

    • Increased density provides buoyancy to the diver.

    • Creates a sense of weightlessness, which can lead to disorientation or allow a freedom of movement for trained divers with neuromuscular disabilities.

  • Increased viscosity of water increases resistance to movement.

    • Contributes to the need for increased physical fitness of the diver and importance of fitness for diving evaluation

  • Increased heat capacity of water

    • Conductive heat loss in water is 25 times faster than that in air, which leads to an increased risk of cold illness and need for thermal protection while diving.

  • Human body’s response to the aquatic environment

    • Air spaces subjected to increased pressure and rapid pressure changes with depth changes.

    • Thermoregulation can be a challenge.

    • Shivering may hasten cooling in the water.

    • Diving reflex can occur as a response to facial immersion and leads to bradycardia, peripheral vasoconstriction, lower temperature, and shunting of blood to the core.

Physics of Diving

Boyle Law

  • For a fixed amount of gas at a uniform temperature, volume (V) and pressure (P) are inversely related.

V 1 / P 1 = V 2 / P 2
  • As diver descends and surrounding environmental pressure increases, the volume of air-filled flexible wall structures (e.g., lung or gastrointestinal tract) will decrease; the volume of these structures will subsequently increase on ascent.

  • Liquid and liquid/solid organs (e.g., blood, bone, muscle, and organs) will equally transmit pressures in all directions according to the Pascal principle, which states that pressure is distributed equally over surfaces and transmitted equally through compartments containing gas or liquids.

  • Air-filled rigid-walled structures (sinuses, middle ear, and face mask air space) will remain at surface pressure at a depth because of their inability to change volume.

  • The pressure gradient is responsible for barotrauma injuries.

  • The gradient can be resolved by equalizing spaces during depth changes.

  • Gradient change is the greatest near the surface.

Dalton Law

  • The total pressure exerted by a mixture of gases is equal to the sum of the pressures that would be exerted by each of the gases if it alone was present and occupied the total volume.

  • Air pressure at sea level (1 ATA) is composed of nitrogen (0.79 ATA) and oxygen (0.21 ATA), with trace amounts of carbon dioxide (CO 2 ), water vapor, and other gases.

  • At greater depths, the air will be breathed in at the ambient pressure.

    • 33 FSW has a pressure of 2 ATA, which will be composed of nitrogen (1.58 ATA) and oxygen (0.42 ATA).

  • Increases the partial pressures of the inspired gases, and the body is sensitive to changes.

Oxygen (O 2 )

  • Our bodies tolerate a wide range of O 2 pressure (0.158–2 ATA) and extract enough to meet metabolic requirements without developing toxicity.

    • Below 0.158 ATA, the body will experience hypoxia, and the diver will develop air hunger, fatigue, confusion, loss of consciousness, and death of tissues.

  • Brain cells are most sensitive and can die if deprived of O 2 for only 4 minutes.

  • Oxygen toxicity can occur if the partial pressure is too high for too long.

    • Effects can include nausea, disorientation, visual changes, and seizures; life threatening underwater

    • Enriched air mixtures have varying concentrations of O 2 and different depth limits because of the increased partial pressures of O 2 .

Henry Law

  • The amount of gas that will dissolve in a liquid is directly related to the pressure (P) of the gas. Thus, G (PD) ::P, where G (PD) is the gas physically dissolved in a liquid phase and :: stands for proportional.

  • With descent to greater depths and pressures, the number of molecules of each gas dissolved in body tissues increases.

  • Factors that affect speed at which dissolution occurs include temperature, solubility coefficient, and cellular metabolism of the gas.

  • Different tissues absorb and release gases at different rates.

  • Tissue saturation eventually occurs if the diver stays at depth long enough.

  • Upon ascent, partial pressure decreases and gas will leave the solution phase. If ascent is too rapid and/or decompression stops are not used, bubbles may form in the local tissue or the bloodstream.


  • Usually considered an inert gas because it does not unite chemically with other substances in the body.

  • No effects at normal sea-level atmospheric pressures (0.79 ATA).

  • Nitrogen can interact with cells at increased pressures and lead to nitrogen narcosis.

  • Effects of nitrogen at depth are comparable to taking one alcoholic drink for every 50 FSW descended, and at 750 FSW, it has anesthesia-like effects. This effect is also called “rapture of the deep” and can cause confusion and strange behaviors.

Portions of the Dive

  • Surface: Surface swimming or wading before going underwater; uses energy and increases exposure to cold temperatures

  • Descent: Involves changes in pressures of 1 ATA per 33 FSW; need to have equilibration of pressure in closed air spaces

  • Bottom time: Traditionally, the amount of time spent at lowest depth; however, with the advent of dive computers, it has evolved to the amount of time spent underwater. Activity at depth can change respiratory needs and risks of complications. Good buoyancy management can decrease the O 2 consumption and conserve the air supply.

  • Ascent: Risk of injuries because of changes in pressure and release of absorbed gas back into local tissues and bloodstream.

  • Surface interval: Allows the body to resume its usual physiologic status and normalize tissue gas concentration before the next dive; complications or injuries encountered during the dive may not present until the diver is at the surface.

Incidence of Dive-Related Injuries

  • Difficult to determine because exact number of divers and dives performed each year are unknown

  • Decompression sickness (DCS)

    • Approximately 1.52 DCS events for males and 1.27 for females per 1000 dives

  • The mortality rate for recreational divers is estimated to be 1 death per 200,000 dives

  • Increased age, decreased physical conditioning, and history of chronic disease were risk factors

  • Dehydration, exercise level during the dive, hypothermia, and hyperthermia may also play crucial roles

Safe Dive Profiles

  • Follow standard recommendations on dive tables or dive computer algorithms for duration of dives at various depths, with decompression stops as needed.

    • US Navy Air Dive tables

      • Based on rectangular or square profiles that assume that the diver directly descends to the deepest depth and stays at that depth until returning to the surface.

      • Designed for safety to minimize risks of DCS, O 2 toxicity, and nitrogen narcosis.

  • Dive computers are now very common and easy to use.

    • Allow flexibility in dive profiles

    • Potential for fewer calculation errors compared with tables

    • Provides planning information: dive profiles, safety stops, ascent rate alarms, decompression times, and dive intervals

  • Recommended that most sport scuba divers perform only no-decompression dives to minimize risks

  • No dive is 100% safe because of individual variation in conditions, health, physiology, and equipment.

    • Diving within skill limits, equipment, and certification makes diving safer.

    • Keep ascent rate below 30 feet/minute.

  • Always dive with a buddy.

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