Dive Physics and Physiology Foundations

Why Physics Matters Underwater

Water is about 800 times denser than air. As a diver descends, the weight of the water column above adds pressure rapidly. At the surface, a person experiences 1 atmosphere of pressure (1 atm, also written 14.7 psi or 1 bar). Every 33 feet of seawater (fsw) adds another atmosphere. So at 33 fsw the total (absolute) pressure is 2 atm; at 66 fsw it is 3 atm; at 99 fsw it is 4 atm. Understanding what that pressure does to gases inside and around the body is the foundation of diver safety.

Boyle's Law: Pressure and Volume

Boyle's Law states that, at constant temperature, the volume of a fixed amount of gas varies inversely with the absolute pressure applied to it:

P₁ × V₁ = P₂ × V₂

In plain language: double the pressure, halve the volume. Halve the pressure, double the volume.

For divers, the most important gas space is the lungs. A diver who takes a full breath at the surface (roughly 6 liters) and then descends to 33 fsw (2 atm) without exhaling would have only 3 liters of lung volume—the air is compressed. That compression is harmless as long as the diver keeps breathing normally from the regulator, which continuously delivers air at ambient pressure.

The danger runs the other way on ascent. If a diver holds a full breath and rises, that trapped air expands. Expanding gas in the lungs can over-pressurize and rupture lung tissue—a serious injury called pulmonary barotrauma. This is why the first rule of scuba is: never hold your breath; always breathe continuously.

Boyle's Law also explains squeeze injuries in ears, sinuses, and mask during descent when a diver fails to equalize, and it explains why a buoyancy compensator (BCD) must be vented during ascent.

Dalton's Law: Partial Pressures

Dalton's Law states that the total pressure of a gas mixture equals the sum of the partial pressures of each component gas:

P_total = P_gas1 + P_gas2 + P_gas3 …

Dry air is approximately 21% oxygen (O₂) and 79% nitrogen (N₂). At the surface (1 atm), the partial pressure of oxygen (PO₂) is 0.21 atm and the partial pressure of nitrogen (PN₂) is 0.79 atm. At 66 fsw (3 atm total), PO₂ rises to 0.63 atm and PN₂ rises to 2.37 atm. The body responds to partial pressures, not percentages.

This matters for two major physiological effects:

• Nitrogen narcosis becomes noticeable for many divers at PN₂ values typically encountered below about 100 fsw on air. Narcosis causes impaired judgment similar to alcohol intoxication. Recreational diving limits (no deeper than 130 fsw on air) reduce but do not eliminate this risk.
• Oxygen toxicity is a concern when PO₂ exceeds 1.6 atm; recreational air diving keeps PO₂ well below this level at the depth limits taught by PADI.

Decompression Sickness (DCS)

As ambient pressure rises, nitrogen dissolves into blood and tissues according to Henry's Law (the amount of gas that dissolves in a liquid is proportional to its partial pressure). Tissues absorb nitrogen steadily during a dive. If a diver ascends too fast, dissolved nitrogen can come out of solution as bubbles—much like carbonation fizzing when a soda bottle is opened quickly. These bubbles cause decompression sickness (DCS).

Type I DCS primarily affects the skin (rash, mottling) and musculoskeletal system (joint and limb pain, sometimes called "the bends").

Type II DCS is more serious and involves the spinal cord, brain, inner ear, or lungs. Symptoms include neurological deficits, paralysis, extreme vertigo, or difficulty breathing.

Common DCS signs and symptoms to recognize include: joint pain, skin mottling, unusual fatigue, numbness or tingling, weakness, dizziness, and shortness of breath. Symptoms typically appear within one hour of surfacing, though they can be delayed.

Prevention: follow dive tables or a dive computer, ascend no faster than 30 feet per minute, perform a 3-minute safety stop at 15 fsw on every dive, stay within no-decompression limits, stay well hydrated, and avoid flying for the recommended interval after diving.

Barotrauma Overview

Any gas-filled space in or around the body can be injured by unequal pressure:

• Ear squeeze / reverse squeeze – pain in middle ear; equalize early and often on descent
• Sinus squeeze – facial pain around sinuses
• Mask squeeze – blood pooling under the mask if it is not equalized by exhaling through the nose
• Pulmonary barotrauma – the most life-threatening form, caused by breath-holding on ascent; can lead to arterial gas embolism (AGE), requiring immediate recompression

Emergency Response: DCS Recognition and Oxygen

As a divemaster candidate, you are expected to recognize and initiate response to a diving emergency. A structured five-step approach is required:

1. Scene safety – ensure the area is safe for rescuers and the injured diver

2. Recognition – use a systematic symptom checklist to identify possible DCS or AGE

3. Oxygen delivery – administer 100% oxygen at a high flow rate via non-rebreather mask as soon as possible; breathing oxygen accelerates nitrogen off-gassing and reduces bubble size

4. EMS activation – call emergency services and contact DAN (Divers Alert Network) for guidance

5. Diver positioning – lay the diver flat (supine); do not place them in the head-down position, which is no longer recommended by DAN or PADI

Oxygen first aid is the single most important pre-hospital intervention for DCS. Divemasters must be familiar with the portable oxygen unit, including how to assemble it, confirm flow, and apply the mask correctly. Refer to DAN Emergency Oxygen Provider guidelines for hands-on practice requirements.

Safety Summary

• Never hold your breath while breathing compressed gas underwater
• Ascend slowly; perform safety stops
• Never skip equalization when you feel pressure
• Know your emergency action plan before every dive
• Oxygen first aid is not optional—it is the standard of care