Effects of Pressure on Divers: Laws, Risks, Prevention

Types of pressure you feel underwater

When you enter the water you immediately begin to feel forces that are invisible at the surface. Understanding these pressures is the first step to preventing discomfort and injury — and it helps explain many of the physiological responses you notice while freediving or scuba diving.

• Atmospheric pressure: the weight of the air column above you at the surface. At sea level this is approximately 1 bar. It is the baseline pressure that exists even before you enter the water.
• Hydrostatic pressure: the weight of the water column above you. In seawater, hydrostatic pressure increases by roughly 1 bar for every 10 metres of depth. A convenient rule of thumb: hydrostatic pressure (bar) ≈ depth (m) ÷ 10.
• Absolute pressure: the total pressure acting on your body below the surface. This is the sum of atmospheric and hydrostatic pressure and can be written as P (bar) = (depth ÷ 10) + 1.

Note that the largest fractional change in pressure happens in the first 10 metres: pressure doubles between the surface and 10 m. That rapid relative change is why equalization and careful technique matter most in shallow-to-moderate depths.

Boyle’s Law: how gas volume changes with depth

Boyle’s Law captures how a gas volume responds to a change in pressure (assuming temperature is roughly constant): pressure and volume are inversely related, so P × V = constant. For divers this becomes a practical calculation for air spaces in the body and in equipment.

A simple working formula for a freediver is:

Vdepth = Vsurface ÷ ((depth ÷ 10) + 1)

So, if you have 6 litres of air at the surface:

• At 10 m (2 bar absolute): 6 L ÷ 2 = 3 L
• At 20 m (3 bar absolute): 6 L ÷ 3 = 2 L

Practical implication: all enclosed gas spaces shrink on descent and expand on ascent. That includes your lungs, mask, sinuses and middle ear. A small change in depth near the surface produces a large relative volume change — that is the reason equalization should start early and be repeated frequently.

Common pressure-related injuries (squeezes and barotrauma)

The umbrella term for many of these problems is barotrauma: tissue injury caused by pressure differences between an internal air space and the surrounding environment. In freediving, “squeeze” is commonly used to describe the sensation and early stages of those problems.

Ear squeeze

If the middle ear cannot be ventilated via the eustachian tube, the middle ear pressure falls behind ambient pressure and the eardrum is pulled inward. Symptoms include ear fullness, pain, muffled hearing, and in severe cases bleeding or eardrum rupture. Equalize early and often; stop descending if you cannot equalize.

Sinus squeeze

Blocked sinus passages trap air that then compresses and causes localized pain (forehead, cheeks, teeth). Congestion from illness or allergies is a common contributor. Avoid diving when congested and allow time for recovery before returning to depth.

Mask and hood squeeze

Air trapped in your mask or under a tight hood will compress and can bruise facial tissues or increase difficulty equalizing. Prevent mask squeeze by starting the dive with the mask gently equalized and periodically exhaling through the nose into the mask during descent; for hoods, break the seal before descent or allow water in so trapped air can escape.

Reverse block

On ascent the pressure inside an air space must be able to vent outward. If passages are blocked (for example after a prior squeeze or with congestion), expanding gas can become trapped and produce pain or longer-lasting symptoms. Ascend slowly and do not force maneuvers if you feel a block developing.

Prevention summary: equalize early and often, avoid diving while congested, ascend or stop if equalization fails, and adjust equipment seals (mask/hood) so air can move as pressure changes.

Pulmonary effects and risks at depth

The lungs are flexible and will compress as ambient pressure increases. For most recreational freedives this compression is manageable, but there are physiological limits and risks if you push depth repeatedly or attempt extreme depths without appropriate training.

A protective response at very deep breath-holds — sometimes called a blood shift — redistributes blood toward the chest cavity to maintain organ volume and protect delicate structures. This is a normal reflex in well-trained freedivers, but it has limits and does not make deep breath-holds risk-free.

Two pulmonary problems to be aware of:

• Lung squeeze / pulmonary edema: fluid accumulation or small-scale bleeding in the lungs can occur when the mechanical stress of compression is high or when repeated deep exposures stretch vascular structures. Symptoms may include chest discomfort, coughing, or blood-tinged sputum.
• Pulmonary barotrauma on ascent: for breathing divers the most significant pulmonary risk comes when trapped gas expands during a rapid ascent. Freedivers generally breathe a single breath at the surface before descending, but ascent discipline and respecting personal limits remain essential.

If after a dive you experience chest pain, persistent cough, coughing blood, or shortness of breath, seek medical evaluation promptly. These signs require professional assessment.

Gas absorption and decompression considerations

As ambient pressure rises, more inert gas (chiefly nitrogen when breathing air) dissolves into body tissues — a consequence of Henry’s Law. When pressure decreases on ascent, that gas must be eliminated safely. If ascent is too fast or tissue inert-gas load is high, dissolved gas can form bubbles and cause decompression illness (DCS).

Key points for freedivers and breath-hold diving:

• Single shallow freedives pose a much lower DCS risk than long, breathing dives, but repetitive deep breath-hold dives with short surface intervals can increase DCS likelihood.
• Signs of decompression illness include joint or muscle pain (the “bends”), numbness or tingling, weakness, dizziness, breathing difficulty, or cognitive changes. These symptoms require immediate medical attention.
• Prevention relies on sensible limits: plan fewer deep repetitions, allow adequate surface intervals, avoid rapid repeated deep drops, and follow the conservative ascent practices taught in your training.

If DCS is suspected, stop diving, administer oxygen if available, keep the person comfortable and immobile, and seek emergency care with facilities capable of recompression therapy.

Practical safety habits and quick FAQs

A few practical habits make most pressure-related problems easy to avoid:

1. Equalize early and often. Start before you feel pain; small, frequent equalizations are easier and safer than forced, late attempts.
2. Pre-equalize at the surface — gentle jaw and swallowing movements can loosen the eustachian tubes and make descent equalization easier.
3. Hydrate and avoid diving when congested. Dehydration thickens mucus; congestion blocks passages and greatly increases squeeze risk.
4. Check mask and hood fit. Make sure seals allow venting of trapped air and that you can break the seal if needed.
5. Respect limits and recovery. Allow adequate surface intervals between deep repetitions and follow progressive training plans.

Quick FAQs

Q: How does pressure affect divers?
A: Pressure compresses enclosed gas spaces, increases the amount of gas dissolved in tissues, and can lead to tissue injury or gas-bubble formation without proper equalization and disciplined ascent practices.

Q: What are the first signs of decompression sickness?
A: Joint or muscle pain, numbness or tingling, unexpected fatigue, confusion, difficulty breathing, or unusual neurological symptoms. Stop diving and seek medical help immediately if these appear.

Q: What is the '120 rule' in diving?
A: Different groups may reference various operational rules; the safer approach is to follow the surface-interval and ascent protocols you learned with your instructor or training facility rather than rely on an informal rule of thumb.

Q: Why depressurize slowly?
A: Slower ascent and adequate surface intervals give dissolved gases time to leave tissues as dissolved molecules rather than forming harmful bubbles, reducing the risk of decompression illness.

For hands-on equalization drills, guided practice of these pressure-safety skills, and supervised progression through depth limits, consider the practical courses offered at manifreediver.ir — working with an experienced instructor in the water is the best way to make these concepts second nature.