Effects of diving on the body
Water pressure squeezes the body's air spaces. Solid and liquid parts do not change much.
- Ears: the eardrum bends inward. A diver must equalise (pinch the nose and blow gently, or swallow) so air enters the middle ear.
- Sinuses and mask: if sinuses are blocked, pain follows. A mask squeezes the face; breathe a little air out through the nose into the mask.
- Lungs: air shrinks going down and expands coming up (Boyle's law). Breathe normally and never hold the breath on ascent.
- Blood: at depth more nitrogen dissolves (Henry's law). Rising slowly lets it leave through the lungs.
- Mind: below about 30 m nitrogen can cause narcosis: slow thinking, a drunk-like feeling. Going up cures it.
- Heat: cold water causes hypothermia. Tiredness and thirst also add risk.
Diving injuries and countermeasures
Many injuries are pressure injuries called barotrauma. Here is each problem with its prevention.
- Ear or sinus squeeze: equalise early and often. Do not dive with a cold.
- Lung overexpansion (air embolism): never hold your breath when rising; keep breathing.
- Decompression sickness ("the bends"): nitrogen bubbles cause joint pain, rash, tiredness, numbness. Rise slowly (about 9 to 10 m per minute), do a safety stop of about 3 minutes at 5 m, and follow dive tables or a dive computer.
- Nitrogen narcosis: stay shallower, or go up if you feel strange.
- Oxygen poisoning: keep within the safe depth for the gas used.
- Hypothermia: wear a suit; end the dive if shivering.
- Running out of air or panic: watch the gauge, dive with a buddy, learn to share air.
First aid
This is awareness, not training. Real first-aid courses are needed.
- Stay safe and bring the diver out of the water with help.
- Check breathing. If there is none, start rescue breaths and chest compressions (CPR) if trained.
- Give 100% oxygen if available. It helps all pressure injuries.
- Lay the diver flat and keep warm and dry. Do not give alcohol.
- Call emergency help and say it is a diving accident. Someone with decompression sickness may need a recompression chamber, where pressure is raised again and lowered slowly.
- Write down the dive depth and time for the doctor.
Try it
In the 3D, press "Fast" and then "Slow" and compare the blood bubbles. At home, shake a bottle of fizzy water and open it fast and slow. Which one foams? That is the idea of decompression.
Key formulas and definitions
- Boyle: P1 × V1 = P2 × V2 (lung air expands on ascent)
- Henry: more pressure, more gas dissolves in blood
- Safe ascent rate about 9–10 m per minute
- Air use at depth = surface use × absolute pressure
- Rules: equalise down, breathe out up, rise slow
Worked examples
1. A diver at 30 m has 4 atm around. A breath holds 4 L of air at that depth. If the diver rose to the surface with the breath held, what volume would it want to fill?
P1V1 = P2V2. 4 × 4 = 1 × V2, so V2 = 16 L. A lung cannot hold that, so it could burst. Breathe out while rising.
2. How long should an ascent from 36 m take at 9 m per minute?
36 / 9 = 4 minutes (plus a safety stop of about 3 minutes at 5 m).
3. A diver breathes 20 L of air per minute at the surface. How much per minute at 30 m? How long does a 1600 L supply last at 30 m?
Absolute pressure = 4 atm, so use is 20 × 4 = 80 L/min. Time = 1600 / 80 = 20 minutes.
Common mistakes
- Equalising only when the ear starts to hurt. Do it early and often.
- Holding the breath on ascent because it feels safe. It is the most dangerous mistake.
- Diving with a cold because the sinuses seem fine.
- Giving alcohol or hot drinks to an injured diver.