Humans are terrestrial, air-breathing mammals, and our natural ability to interact with the underwater environment is severely limited by our respiratory system. Unlike fish, we cannot extract oxygen from water, leading to the development of specific technologies and techniques to extend our stay beneath the surface. These solutions range from simple devices that connect us to the atmosphere to complex, self-contained systems that manage high-pressure air delivery and the physiological effects of depth. Exploring the methods humans use to breathe underwater requires an understanding of the fundamental biological constraints and the engineering developed to overcome them.
The Physiological Barrier
The limitation for humans underwater is that our lungs are designed for gas exchange with air, not water. Water contains oxygen, but this oxygen is either chemically bound to hydrogen in the water molecule or is present as dissolved oxygen gas in a much lower concentration than in the atmosphere. The atmosphere provides approximately 21% oxygen by volume, while even oxygen-rich water holds less than 1% of the gas required for human metabolism.
Fish use gills, which are highly efficient organs featuring a massive surface area that draws dissolved oxygen from the water flowing over them. Human lungs, with their small, delicate air sacs called alveoli, do not possess the necessary surface area to extract sufficient oxygen from the dense, low-concentration medium of water. Furthermore, the human body is warm-blooded and requires a higher metabolic rate than cold-blooded fish, demanding an oxygen intake that water cannot supply. The brief time a person can spend submerged is through breath-holding, which is an adaptation, not a true method of underwater breathing.
Surface-Assisted Air Supply
The simplest technological solution for breathing underwater involves maintaining a connection to the surface atmosphere. The snorkel is the most common example, allowing a person to float face-down and breathe air through a tube. This method is constrained by two factors: dead air space and hydrostatic pressure. Dead air space is the volume of exhaled, carbon dioxide-rich air trapped within the tube that is subsequently re-inhaled, leading to a buildup of carbon dioxide in the body.
The maximum effective depth is limited to about 1 to 2 feet (0.3 to 0.5 meters) due to water pressure. At this shallow depth, the surrounding water exerts so much pressure on the chest cavity that the respiratory muscles cannot overcome it to inflate the lungs. A more advanced surface-supplied method is the “hookah” diving system, which uses a floating compressor or air tank to pump low-pressure air through a long hose to a diver’s regulator. This system allows for extended shallow dives, typically limited to a few dozen feet, but it still maintains a physical connection to the surface.
Self-Contained Underwater Breathing Apparatus (SCUBA)
The Self-Contained Underwater Breathing Apparatus, or SCUBA, is the method used for underwater mobility and prolonged activity. This technology utilizes a high-pressure cylinder of compressed air, which is made breathable by a two-stage regulator system. The compressed air in the tank is stored at high pressure, often exceeding 3,000 pounds per square inch (psi).
The first stage of the regulator attaches directly to the tank valve and reduces this pressure to an intermediate pressure, typically around 140 to 150 psi above the surrounding water pressure. This air then travels through a hose to the second stage regulator, which is the mouthpiece. The second stage is a demand valve that only delivers air when the diver inhales, supplying the air at a pressure that perfectly matches the ambient water pressure.
This pressure equalization allows the diver to inhale effortlessly, regardless of the depth or the force of the water on their body. The system also includes a Buoyancy Control Device (BCD) to manage ascent and descent, and gauges to monitor tank pressure and depth. Due to the dangers of breathing compressed gas at depth, formal training and certification are necessary before a person can safely use SCUBA equipment.
Understanding Pressure-Related Hazards
Breathing compressed gas at depth introduces physiological hazards governed by the physics of pressure. One danger is barotrauma, which refers to injuries caused by the expansion or contraction of gas in enclosed body spaces, such as the sinuses, middle ears, and lungs. During descent, the diver must actively add air to these spaces to equalize the internal pressure with the external water pressure, preventing squeeze injuries.
Two systemic hazards relate to nitrogen, which makes up nearly 79% of the air mixture. Nitrogen narcosis is a reversible altered state of consciousness that occurs at depth due to the anesthetic effect of nitrogen under high partial pressure. This condition, sometimes compared to alcohol intoxication, typically begins to affect cognitive function at depths exceeding 100 feet and limits the safe depth for air diving.
Decompression Sickness (DCS), commonly known as “the bends,” is a serious condition caused by nitrogen dissolving into the body’s tissues under pressure. If a diver ascends too quickly, the surrounding water pressure drops rapidly, causing the dissolved nitrogen to form bubbles within the blood and tissues, similar to opening a carbonated drink. These bubbles can block blood flow and damage tissue, requiring a slow, controlled ascent, often guided by dive computers or tables, to allow the nitrogen to be safely exhaled.