What If Humans Could Breathe Underwater?

Why gills would struggle to keep a person alive

Air is 20.95% oxygen, which works out at about 280 mg of oxygen in every litre at 20 °C. Seawater at the same temperature, fully saturated, holds about 7.4 mg per litre, nearly 40 times less. A resting adult of 70 kg uses about 245 ml of oxygen a minute (3.5 ml per kg, the standard resting rate), which is about 350 mg. Getting that from seawater means passing about 47 litres over the gills every minute, even if the gill took out every last milligram. Fish manage because they are cold-blooded and use far less oxygen for their size. Any exercise, such as swimming, multiplies a human’s need several times over.

What depth does to the body, gills or not

Pressure rises by about one atmosphere for every 10 m of seawater, so at 10 m you are under 2 atmospheres and at 30 m under 4. Any air inside you shrinks in step (Boyle’s law): the air in your lungs, sinuses and middle ears halves in volume by 10 m and is a quarter of its surface volume by 30 m. That squeeze is what makes ears hurt on the way down. Deep-sea animals cope with crushing pressure, around 200 atmospheres at 2,000 m, mainly because they have no air spaces at all. A human with gills but still with lungs, sinuses and ears would not have that advantage.

The bends would be a different story

Scuba divers get decompression sickness because they breathe compressed air. Extra nitrogen dissolves into their tissues at depth, and if they come up too fast it forms bubbles. Gills would take oxygen from gas already dissolved in the water, at roughly the levels set by the air at the surface, so in principle a gilled swimmer would not soak up that extra nitrogen. Divers breathing air also start to feel nitrogen narcosis below about 35 m, which gills would avoid for the same reason. Pressure on air spaces would remain the limit.

Liquid breathing has been done, in a lab

In 1966 researchers kept mice alive while they breathed an oxygen-rich fluorocarbon liquid. In 1996 doctors treated 13 very ill premature babies with partial liquid ventilation, filling part of the lungs with a similar liquid. A lung full of liquid barely compresses, because liquid hardly squashes, so in theory it would sidestep the lung squeeze at depth. The catch is effort: pushing a thick liquid in and out of the lungs is hard work, and it has stayed a specialist hospital and research technique rather than a way to dive.

Worked example: gills for a resting adult in 20 °C seawater

1. Oxygen used at rest by a 70 kg adult: 3.5 ml per kg per minute × 70 kg = 245 ml a minute.
2. As a weight: 245 ml × 1.429 mg per ml ≈ 350 mg of oxygen a minute.
3. Fully saturated seawater at 20 °C holds about 7.4 mg of oxygen per litre.
4. 350 mg ÷ 7.4 mg per litre ≈ 47 litres of seawater a minute, or about 2,840 litres an hour, assuming the gill takes out every milligram.
5. The same 350 mg of oxygen is in about 1.25 litres of air (350 ÷ 280).

A gilled human sitting still would need to move nearly 40 times more fluid than an air-breather, and that fluid is hundreds of times heavier than air. That is why large warm-blooded sea animals, such as whales and dolphins, still breathe air. A Cuvier’s beaked whale has been tracked diving to 2,992 m on one breath.

What people get wrong

“Fish get oxygen by splitting water.”

They do not break up H₂O. Gills absorb oxygen gas that is dissolved in the water, which is why fish struggle in warm, still or polluted water that holds little oxygen.

“Deeper water has more oxygen because it is under more pressure.”

How much oxygen water holds depends mainly on temperature, salt and contact with air. Cold water holds more: about 14.6 mg per litre at 0 °C against 7.6 mg at 30 °C in fresh water.

“Breathing liquid is pure science fiction.”

Mice breathed an oxygen-rich liquid in a 1966 experiment, and premature babies were treated with partial liquid ventilation in a 1996 study. It remains a specialist research technique, not something people can use to dive.

“You could breathe liquid oxygen.”

Liquid oxygen boils at about −183 °C, so it would freeze tissue instantly. Liquid breathing uses a different liquid that carries dissolved oxygen at body temperature.

Common questions

How we calculated this

Absolute pressure is 1 atm plus 1 atm per 10 m of seawater. Air spaces shrink to 1 ÷ absolute pressure of their surface volume (Boyle’s law). Seawater needed by gills = resting oxygen use (70 kg × 3.5 ml/kg/min × 1.429 mg/ml ≈ 350 mg a minute) ÷ 7.4 mg of oxygen per litre of saturated seawater at 20 °C, assuming every milligram is extracted. Time at depth multiplies that flow. There is no decompression calculation.

Assumptions and limitations

Sources

• NOAA Ocean Exploration — how pressure affects ocean animals

  Pressure rises about one atmosphere every 10 m; about 200 atmospheres at 2,000 m; animals without air spaces are less affected.

• USGS — DOTABLES dissolved oxygen solubility

  Oxygen solubility by temperature and salinity: about 9.1 mg per litre in fresh water and 7.4 mg in seawater at 20 °C.

• USGS Technical Memorandum 2011.03 — oxygen solubility

  The Benson and Krause equations behind the solubility figures, and 1.429 mg of oxygen per ml.

• NASA — Earth fact sheet

  Dry air is 20.95% oxygen by volume.

• Compendium of Physical Activities

  One MET, the oxygen cost of sitting quietly, is 3.5 ml per kg per minute.

• CDC Yellow Book — scuba diving and decompression illness

  Breathing air under pressure dissolves extra nitrogen into tissues, which can form bubbles on ascent.

• OSHA — commercial diving health effects

  Nitrogen narcosis below about 35 m (120 ft) and Boyle’s law squeeze on air spaces.

• Schorr et al. (2014) — Cuvier’s beaked whale dives, PLOS ONE

  Dives to 2,992 m lasting up to 137.5 minutes, mammal records.

• Clark and Gollan (1966) — Survival of mammals breathing organic liquids, Science

  Mice survived breathing an oxygenated fluorocarbon liquid.

• Leach et al. (1996) — Partial liquid ventilation in premature infants, NEJM

  A study of 13 premature babies with severe breathing problems.

• Royal Society of Chemistry — Oxygen

  Boiling point of oxygen: −182.962 °C.

All IfSketch sources · Methodology