Most oxygen tanks contain exactly what the name suggests: oxygen gas, compressed to very high pressure inside a metal cylinder. But “oxygen” does not always mean pure oxygen, and “tank” covers a surprisingly wide range of containers used in hospitals, dive boats, aircraft, and spacecraft. A medical oxygen cylinder holds gas that is roughly 99.5% pure, while the tank strapped to a scuba diver’s back almost certainly does not contain pure oxygen at all. What is inside depends entirely on what the gas is for and who is breathing it.
Medical Oxygen Tanks
When people picture an oxygen tank, they usually imagine the green or white cylinders found in hospitals and ambulances. These contain medical-grade oxygen, which in most countries must meet a purity standard of at least 99% to 99.5% oxygen by volume. The remaining fraction of a percent consists of trace amounts of nitrogen, argon, carbon dioxide, and water vapor left over from the production process. A simulation of cryogenic air separation, one of the main methods for producing this gas, demonstrated output purities of about 99.5% liquid oxygen.1PubMed Central. Novel Study on Cryogenic Distillation Process and Application by Using CHEMCAD Simulation That tiny impurity margin matters more than you might think: for a patient breathing supplemental oxygen through a nasal cannula for hours or days, even small contamination with oils, particulates, or reactive gases could irritate the lungs or worse.
The gas inside a standard medical cylinder is stored at pressures that can exceed 2,000 pounds per square inch (psi) when full. At that pressure, enough oxygen to last a patient several hours fits inside a cylinder small enough to wheel down a hallway. The cylinder itself is typically made of steel or aluminum alloy, and it incorporates several safety features including color coding, a pin-index system that prevents attaching the wrong regulator, a pressure-relief device to vent gas if the cylinder overheats, and a label identifying its contents.2PubMed Central. Anaesthesia gas supply: gas cylinders The pin-index system is worth knowing about: small metal pins on the valve only align with a regulator designed for that specific gas. You physically cannot connect an oxygen regulator to a nitrous oxide tank, for example, because the pins are in different positions.
How the Oxygen Gets In
Oxygen does not come from some chemical formula mixed in a factory. It comes from air. The atmosphere is roughly 21% oxygen, 78% nitrogen, and about 1% argon with traces of other gases. Separating the oxygen from everything else is the entire challenge, and industry uses two main approaches.
The first and oldest large-scale method is cryogenic distillation. Air is filtered, compressed, and cooled until it becomes a liquid at extremely low temperatures (around minus 183°C for oxygen). Because nitrogen and argon have slightly different boiling points, the liquefied air can be separated into its components in a distillation column, much like alcohol is distilled from a fermented mixture. This process feeds the bulk of the world’s medical and industrial oxygen supply and can achieve very high purity.1PubMed Central. Novel Study on Cryogenic Distillation Process and Application by Using CHEMCAD Simulation Large hospitals and industrial users often receive their oxygen as a cryogenic liquid delivered by tanker truck, then stored in insulated bulk tanks on site and allowed to warm back into gas as needed.
The second method is pressure swing adsorption, or PSA, which works on a completely different principle. Instead of cooling air into a liquid, PSA pushes compressed air through beds of a material called zeolite, a type of mineral with a crystal structure full of tiny molecular-sized pores. Zeolite preferentially grabs nitrogen molecules and lets oxygen pass through.3PubMed Central. Quadrupolar Interaction with Zeolite and Pressure Swing Adsorption in Portable Medical Oxygen Concentrators for Breathing of Covid-19 and COPD Patients The system alternates between two beds: while one bed is adsorbing nitrogen under pressure, the other is being flushed clean so it can take the next cycle. This back-and-forth continues nonstop, producing a steady stream of oxygen-enriched gas.4Results in Engineering. High purity oxygen production by pressure vacuum swing adsorption using natural zeolite
PSA is the technology inside portable oxygen concentrators, the suitcase-sized devices that let patients with chronic lung disease move around without dragging a cylinder. These concentrators typically produce oxygen in the range of 90% to 96% purity rather than the 99.5% of a cryogenic plant. That lower purity is acceptable for supplemental breathing because the patient is blending the output with room air anyway. But it means the gas from a home concentrator is not identical to the gas in a hospital cylinder. It contains a higher proportion of argon, which is harmless but technically makes the output “oxygen-enriched air” rather than pure oxygen.
Scuba Tanks Are Not Oxygen Tanks
One of the most persistent misconceptions about compressed gas is that scuba divers breathe pure oxygen. They almost never do. A standard recreational scuba tank is filled with regular compressed air: about 21% oxygen and 79% nitrogen, just like the air you are breathing right now, but squeezed into a cylinder at around 3,000 psi. The reason is straightforward: pure oxygen becomes dangerous at depth.
When you descend underwater, the surrounding water pressure increases. That higher ambient pressure raises the partial pressure of every gas you inhale. Breathing pure oxygen below about 6 meters (roughly 20 feet) pushes the oxygen partial pressure high enough to risk central nervous system toxicity, which can trigger seizures with little warning.5PubMed Central. OXYGEN TOXICITY A seizure underwater is frequently fatal, so divers limit the oxygen fraction in their breathing gas based on how deep they plan to go.
For recreational diving, some divers use enriched air nitrox (often called EANx or just nitrox), which bumps the oxygen fraction up to around 32% to 36% while reducing the nitrogen fraction. The advantage is less nitrogen absorption, which means fewer decompression bubbles forming in the body after a dive. A randomized trial found that breathing enriched air nitrox markedly reduced venous gas bubbles at every time point measured after decompression compared to standard air.6PLOS ONE. Enriched Air Nitrox Breathing Reduces Venous Gas Bubbles after Simulated SCUBA Diving: A Double-Blind Cross-Over Randomized Trial The trade-off is a shallower maximum depth, because the higher oxygen content hits the toxicity threshold sooner.
Technical divers who go deeper use trimix, a blend of oxygen, helium, and nitrogen. A typical trimix might be 21% oxygen, 35% helium, and 44% nitrogen.7PubMed Central. The impact of different gas mixtures on inflammatory responses in advanced recreational divers Helium replaces some of the nitrogen to reduce nitrogen narcosis, the intoxicating effect that nitrogen has at high partial pressures. Studies have collected physiological data from divers breathing air, nitrox, and trimix at depths ranging from about 10 meters to over 60 meters.8PubMed Central. Effect of Shallow and Deep SCUBA Dives on Heart Rate Variability At the deepest end, the oxygen fraction may be reduced to as low as 10% or even less, with helium making up the bulk of the mix. Calling these “oxygen tanks” would be actively misleading.
Why Pure Oxygen Can Be Dangerous
Oxygen is not just fuel for your cells. At high enough partial pressures, it becomes a poison. The body has antioxidant defenses to cope with the reactive byproducts of normal oxygen metabolism, but when oxygen pressure rises above what those defenses can handle, damage accumulates. Oxygen toxicity can show up in the lungs, the central nervous system, or the eyes, depending on the pressure and the duration of exposure.5PubMed Central. OXYGEN TOXICITY
Pulmonary oxygen toxicity develops more slowly, typically after hours of breathing oxygen at moderately elevated pressures. Symptoms start with chest tightness and a cough and can progress to serious lung inflammation. This is a concern in intensive care units where patients are sometimes maintained on high-fraction oxygen for extended periods, and it is one reason clinicians try to wean patients to the lowest effective oxygen level as quickly as possible.
Central nervous system toxicity is faster and scarier. At high partial pressures, such as those encountered during hyperbaric oxygen therapy or deep diving, seizures can occur without reliable warning signs. Research in animal models has identified a pattern of increased breathing rate that precedes seizures during hyperbaric oxygen exposure, but this signal appeared only minutes before the event.9PubMed. A potential early physiological marker for CNS oxygen toxicity: hyperoxic hyperpnea precedes seizure in unanesthetized rats breathing hyperbaric oxygen In practice, the narrow window makes prediction difficult, which is why strict depth and time limits exist for anyone breathing high-oxygen mixtures underwater or in a hyperbaric chamber.
Industrial Oxygen and the Grade Distinction
Not all oxygen tanks are filled for breathing. Industrial-grade oxygen is used in welding, metal cutting, glass blowing, and wastewater treatment. It can have the same nominal purity as medical-grade oxygen, but the key difference lies in quality control and contamination standards, not necessarily in the oxygen molecule itself. Medical oxygen must be produced, handled, stored, and distributed under pharmaceutical-grade conditions with traceable documentation, batch testing, and strict limits on moisture and particulates. Industrial oxygen does not carry those guarantees.
During the COVID-19 pandemic, many countries faced severe shortages of medical oxygen and debated whether industrial-grade supplies could be repurposed for patient use. In some cases, industrial oxygen was reclassified for medical use after additional testing and filtration, but the regulatory hurdle exists for good reason. An industrial cylinder may have been used to store other gases previously, and residues of oils, greases, or solvents inside the tank could be harmful if inhaled. Components intended for oxygen service must be cleaned to eliminate reactive contaminants, because oxygen aggressively supports combustion.10Alternatives to Chlorofluorocarbon Fluids in the Cleaning of Oxygen and Aerospace Systems and Components. A Selection and Evaluation Protocol for Alternatives to Halogenated Hydrocarbon Solvents for Oxygen Cleaning Applications A speck of grease inside a high-pressure oxygen fitting can ignite spontaneously, a phenomenon that has caused fatal accidents in both industrial and clinical settings.
The color of the tank is supposed to help with identification: green for oxygen in the United States, white in most of Europe, and black with a white shoulder in some other countries. But international color-coding standards are not fully harmonized, and there have been documented near-miss incidents where misleading or non-standard cylinder colors contributed to the wrong gas being connected to a patient.2PubMed Central. Anaesthesia gas supply: gas cylinders The pin-index safety system and the cylinder label remain the most reliable identifiers, not the paint.
Oxygen That Is Not in a Tank
Some environments cannot rely on refillable cylinders. Submarines, spacecraft, and mine-rescue shelters use chemical oxygen generators, sometimes called oxygen candles, which produce breathable oxygen through a controlled chemical reaction rather than from a stored supply of compressed gas. The most common formula uses sodium chlorate as the primary ingredient. When heated, sodium chlorate breaks down and releases oxygen. A typical oxygen candle composition tested in recent research consisted of about 86% sodium chlorate along with a metal fuel like manganese, a catalyst like cobalt oxide, and a small amount of kaite as a binder.11Journal of Chemistry. Coupling Effect of Metals and Oxides on the Oxygen Supply Performance of Sodium Chlorate Oxygen Candle That study achieved an average oxygen supply rate of about 1.6 liters per minute from a single candle, enough to support one person’s breathing at rest for a limited period.
Oxygen candles have a major advantage over tanks in confined or remote settings: they are solid, stable, and shelf-stable for years without pressure maintenance. You light them when you need oxygen and discard the spent slug afterward. The downside is that the reaction produces intense heat and cannot be stopped once started, which creates a fire risk. The 1997 fire aboard the Russian space station Mir was caused by a malfunctioning oxygen-generating canister, a reminder that chemical oxygen generation is not without hazard.
Aboard the International Space Station, oxygen is supplied through a combination of methods. High-pressure tanks of oxygen and nitrogen are carried to the station by cargo vehicles for atmosphere replenishment and crew breathing needs.12SAE International. Nitrogen Oxygen Recharge System (NORS) for the International Space Station The station also uses electrolysis, splitting water into hydrogen and oxygen, as a renewable onboard source. These high-pressure tanks operate at around 6,000 psi, roughly three times the pressure of a typical medical cylinder on Earth, because weight and volume are at an extreme premium when you are launching supplies into orbit.
Liquid Oxygen Versus Compressed Gas
Oxygen can be stored in tanks in two very different physical states. The small portable cylinders most people encounter hold compressed gas: oxygen in its gaseous form, squeezed into a small space. But large-scale users like hospitals, steel mills, and rocket launch sites store oxygen as a cryogenic liquid. Liquid oxygen (often abbreviated LOX) is cooled to about minus 183°C and kept in vacuum-insulated vessels called dewars. A liter of liquid oxygen expands into roughly 860 liters of gas at room temperature and atmospheric pressure, which is why liquid storage is so much more space-efficient for bulk supply.
Home patients who need continuous oxygen sometimes use small portable liquid oxygen systems. These consist of a stationary base reservoir that gets refilled by a delivery service and a smaller portable unit that the patient fills from the reservoir before leaving the house. The liquid slowly warms and converts to gas as the patient breathes it. These systems have become less common in recent years as portable oxygen concentrators have improved, but they remain in use for patients who need very high flow rates that concentrators cannot match.
Liquid oxygen is also the oxidizer in many rocket propulsion systems. The SpaceX Merlin engines, for instance, burn liquid oxygen with rocket-grade kerosene. In this context the oxygen is not for breathing at all. It is there to combust fuel at enormous rates in the vacuum of space, where there is no atmospheric oxygen to sustain combustion. The purity requirements for rocket LOX are stringent but optimized for combustion chemistry rather than respiratory safety.
What Happens Inside the Tank Over Time
A sealed oxygen cylinder does not go “bad” in the way food does, but it is not completely inert either. Over years, moisture inside the tank can promote corrosion of the cylinder walls, especially in steel tanks. Aluminum cylinders are more resistant to this but can develop stress cracks around the neck if mishandled. This is why gas cylinders are subject to periodic hydrostatic testing, where the tank is filled with water and pressurized to check for expansion, deformation, or leaks. In many jurisdictions, steel oxygen cylinders must be retested every five years and aluminum cylinders every five to ten years depending on the manufacturer’s specification.
The gas itself remains oxygen. It does not decompose or react with the cylinder walls under normal conditions. But a partially empty cylinder that has been opened and reclosed in a humid environment could theoretically accumulate trace moisture or even atmospheric nitrogen if the valve was left open and the tank pressure dropped below ambient. This is one reason medical gas suppliers prefer to refill cylinders before they are completely empty, maintaining a small positive residual pressure to keep contaminants out.
Regulators and valves also need maintenance. The O-ring seals between the cylinder valve and the regulator, sometimes called Bodok seals in medical systems, can crack or deform over time. A leaking seal means wasted oxygen at best and a potential ignition source at worst if the leak is near any hydrocarbon contamination. Anyone who has ever heard the hiss of a poorly seated regulator on a scuba tank knows the sound of money (and safety margin) escaping into the air.