A chemical oxygen generator is a device that produces breathable oxygen through an exothermic chemical reaction, most commonly the thermal decomposition of sodium chlorate or potassium perchlorate. Unlike an oxygen tank, which simply stores compressed gas, a chemical oxygen generator creates oxygen on demand when triggered, and it does so with no moving parts, no electricity, and no external power source. That self-contained simplicity is what makes these devices ubiquitous in aviation, mining, submarines, and spacecraft, but the intense heat they produce during operation has also made them a serious safety concern when mishandled.
How the Chemistry Works
The core reaction is straightforward. When sodium chlorate is heated past a critical temperature, it breaks apart into sodium chloride (ordinary table salt) and oxygen gas. The reaction releases energy as heat, which is why the process is called exothermic. Calorimetry measurements put the heat of decomposition at about 46 kilojoules per mole of sodium chlorate at 25 °C.1PubMed Central. Heat of Decomposition of Sodium and Potassium Chlorate That released energy is not a side effect to be managed away; it is what keeps the reaction going. Once initiated, the heat from one layer of decomposing chlorate ignites the next layer, and the reaction propagates through the canister like a slow-burning fuse.
Getting the reaction started, and keeping it burning steadily, requires some help. Most chemical oxygen generators include a small amount of metal fuel, typically iron or tin powder, and a catalyst such as cobalt oxide. The metal fuel burns in the newly released oxygen, adding extra heat to sustain the reaction front. The catalyst lowers the temperature at which decomposition kicks in. Research on these additives has shown that cobalt oxide is particularly effective at reducing both the onset and final temperatures of decomposition, and its catalytic effect is strong enough to mask the separate contributions of the metal fuels.2Combustion Science and Technology. On feasibility of decreasing metal fuel content in chemical oxygen generators The practical result is a self-sustaining chemical reaction that, once a user pulls a pin or activates an igniter, burns for a predictable duration and delivers a controlled flow of oxygen.
Where You Have Already Encountered One
If you have ever listened to a flight attendant explain the overhead oxygen masks, you have been briefed on a chemical oxygen generator without knowing it. The emergency oxygen system on most commercial aircraft uses small chlorate candles mounted above each row of seats. When cabin pressure drops, the masks fall and pulling one toward your face tugs a lanyard that fires a percussion cap, which ignites the generator. That is why flight attendants say “oxygen will be flowing even if the bag does not inflate.” There is no tank to pressurize the bag; oxygen simply seeps out of the generator as the reaction burns. Each unit typically produces enough oxygen for roughly 12 to 22 minutes, depending on design, which gives pilots time to descend to an altitude where passengers can breathe ambient air.
The same basic technology appears in a range of other settings. Mining refuge chambers use chemical oxygen generators to supply air in sealed underground shelters after an explosion or collapse. Submarines have historically relied on chlorate candles as one component of their atmosphere management systems, alongside electrolysis units that split seawater into hydrogen and oxygen.3SAE International. U.S. Navy Submarine Life Support Systems Space stations and military escape systems have used variants as well. The appeal in every case is the same: chemical oxygen generators are compact, stable during long storage, and require no infrastructure to operate. You light them and they work.
Why They Get So Hot
The single most important thing to understand about chemical oxygen generators is that they become dangerously hot during operation. The exothermic reaction can push the surface temperature of the canister well above 260 °C (500 °F), and the core temperature can be considerably higher. In an aircraft, this is managed by placing the generators inside insulated housings with heat shields. The passenger never touches the canister; they touch the mask and the tubing. But outside that engineered enclosure, a bare activated generator can ignite nearby materials on contact.
This heat is not a flaw that engineers are trying to eliminate. It is intrinsic to how the device works. The reaction sustains itself by generating enough heat to decompose the next layer of chlorate, and the metal fuel additive increases the thermal output further. Design improvements can moderate the burn rate and manage the heat flow, but they cannot make the process cool. Any oxygen generator based on chlorate decomposition will be hot when activated and will remain hot for minutes after the oxygen flow stops.
The ValuJet Disaster and Its Regulatory Aftermath
The deadliest incident linked to chemical oxygen generators was the crash of ValuJet Flight 592 in May 1996. Expired oxygen generators removed from another aircraft were improperly packaged in cardboard boxes and placed in the cargo hold of the DC-9. The generators had not been fitted with their safety caps, which cover the firing pins. During the flight, at least one generator activated, likely from jostling or from the firing pin being inadvertently struck. The intense heat ignited surrounding materials in the cargo hold, and the fire grew rapidly in an environment being fed fresh oxygen by the burning generators. The aircraft crashed into the Florida Everglades, killing all 110 people on board.
The disaster changed how the aviation industry handles these devices. The Federal Aviation Administration subsequently required fire detection and suppression systems in the cargo holds of all passenger aircraft. Rules for transporting expired or spent generators tightened dramatically. Generators must now have their initiators removed or their firing mechanisms capped and physically secured before they can be shipped. The accident underscored a hazard that engineers had always known about in the abstract: a chemical oxygen generator is both an oxygen source and a potent incendiary device, and treating spent generators as ordinary scrap can be catastrophic.
How They Compare to Other Oxygen Sources
Chemical oxygen generators occupy a specific niche in the broader landscape of oxygen supply technology. Other methods include cryogenic distillation (cooling air until oxygen liquefies), pressure swing adsorption (using molecular sieves to separate oxygen from nitrogen), membrane separation, water electrolysis, and superoxide-based systems. Each has different strengths depending on the application.4Wiley Online Library. Progress of Oxygen Generation Technologies in High‐Altitude and Emergency Rescue Conditions
Compressed oxygen cylinders, for instance, deliver a known volume of gas at adjustable flow rates and do not generate heat. But they are heavy relative to the oxygen they hold, they can become projectiles if the valve is damaged, and an empty cylinder is useless. A chemical oxygen generator, by contrast, stores oxygen in a solid chemical form that is stable for years and does not leak. The weight-to-oxygen ratio is favorable because you are not carrying the mass of a pressurized steel or aluminum vessel. For an aircraft designer trying to minimize weight above each row of seats, that difference matters.
Electrolysis systems, which split water into oxygen and hydrogen using electricity, are the preferred long-term solution aboard submarines and space stations because they can run continuously as long as water and power are available. But they need both of those inputs, making them useless in a blackout or an emergency where power is lost. Chemical oxygen generators serve as the backup: they sit inert on a shelf until the moment power fails or the primary system goes down, and they activate instantly with no infrastructure.
Superoxide-based systems, which use chemicals like potassium superoxide, have the interesting property of absorbing carbon dioxide while releasing oxygen. This dual function makes them attractive for enclosed spaces, but superoxide chemicals are highly reactive and more difficult to handle safely than sodium chlorate. They also tend to be more expensive. Chemical oxygen generators based on chlorate remain the workhorse for most applications because the chemistry is well understood, the raw materials are cheap, and the engineering is mature.
The “Chlorate Candle” Name
You will sometimes see chemical oxygen generators called “chlorate candles” or “oxygen candles,” especially in submarine and mining contexts. The name comes from the physical form of the device: a cylindrical block of sodium chlorate mixed with metal fuel and catalyst, pressed into the shape of a large candle. The term is slightly misleading because the device does not burn like a wax candle with an open flame. The reaction front moves through the solid core more like a smoldering ember, consuming the material from one end to the other. But the comparison is apt in one sense: once lit, it burns at its own pace until the fuel is consumed, and you cannot meaningfully speed it up or slow it down. The oxygen flow rate is set by the formulation and geometry of the candle, not by a valve the user adjusts.
This fixed-rate behavior has practical consequences. In an aircraft, the designer engineers the candle to produce enough oxygen for the number of masks connected to it over the expected descent time. The passenger cannot get more oxygen by breathing harder; the flow is what it is. In a mining refuge chamber, the flow rate must be matched to the number of occupants and the volume of the shelter, and getting this calculation wrong in either direction is dangerous. Too little oxygen and people suffocate. Too much oxygen and the shelter becomes an enriched-atmosphere fire hazard.
Shelf Life and Maintenance
One of the strongest selling points of chemical oxygen generators is their long shelf life. Because the active ingredient is a stable solid, properly sealed generators can sit for years before being activated. Aircraft oxygen generators typically have service lives on the order of 10 to 15 years, after which they are removed and replaced. The expiration is generally a conservative estimate driven by the possibility that moisture intrusion or seal degradation could affect performance, not because the sodium chlorate itself degrades significantly over time.
Maintenance during that shelf life is minimal. There is nothing to pressurize, no gas to top off, no valves to test. Periodic inspections check the physical condition of the housing and the integrity of the initiator, but the generator itself is a sealed unit. Compare that to a compressed oxygen cylinder, which must be hydrostatically tested on a regular schedule, checked for corrosion, and monitored for slow leaks. For airlines operating hundreds of aircraft with dozens of generators per plane, the maintenance simplicity of chemical generators translates directly into lower operating costs.
The flip side is that a chemical oxygen generator is a single-use device. Once activated, it runs to completion and is discarded. You cannot partially use one and save the rest for later. This makes them poorly suited for applications requiring repeated short-duration oxygen delivery, which is one reason they do not appear in medical settings where compressed cylinders or oxygen concentrators dominate.
Ongoing Research and Design Challenges
Although the basic technology dates back decades, researchers continue to refine chemical oxygen generators for new applications. One area of active work is reducing the metal fuel content while maintaining a reliable burn. Less metal fuel means less heat generation, which improves safety margins and reduces the risk of accidental ignition. Studies have explored various catalyst and fuel combinations to find formulations that decompose reliably at lower temperatures and produce less excess thermal energy.5Combustion Science and Technology. On feasibility of decreasing metal fuel content in chemical oxygen generators
Another direction involves tuning the oxygen generation rate. Standard generators produce oxygen at a rate dictated by their formulation, but some applications need a slower, steadier output over a longer period. Mining refuge chambers, for example, may need to sustain breathable air for 96 hours or more. Achieving a very low generation rate from a chlorate system without the reaction stalling out is a nontrivial engineering problem. If the reaction front moves too slowly, it can lose enough heat to its surroundings that it extinguishes, leaving the occupants with a half-spent generator and a dwindling air supply.
High-altitude emergency oxygen is another frontier. At extreme elevations, such as those encountered during mountaineering or in high-altitude military operations, the partial pressure of oxygen drops to levels that impair cognition and can cause acute altitude sickness. Portable chemical oxygen generators could offer a lightweight supplement, but existing designs are optimized for enclosed spaces like aircraft cabins, not for open-air use where much of the generated oxygen disperses. Adapting the technology for a personal breathing device that a climber could carry involves trade-offs in weight, duration, flow rate, and heat management that have not yet been fully solved.6Wiley Online Library. Progress of Oxygen Generation Technologies in High‐Altitude and Emergency Rescue Conditions
Handling Expired Generators Safely
For maintenance crews, the most dangerous phase of a chemical oxygen generator’s life is after it has been removed from service but before it has been properly disposed of. An expired but unfired generator still contains all of its sodium chlorate, metal fuel, and an intact initiator. It is, in every meaningful sense, a live pyrotechnic device. The ValuJet accident happened not because the generators were defective but because spent or expired units were treated as inert cargo.
Proper disposal typically involves either firing the generator in a controlled setting so the reaction consumes the chlorate, or removing the initiator assembly so the device cannot be accidentally triggered. Regulations now mandate that generators being transported must have their initiators physically removed or covered by protective caps that prevent inadvertent activation. Shipping them by passenger aircraft is prohibited unless they are verifiably spent, meaning they have been fired and the reaction has fully consumed the charge.
Even in non-aviation contexts, the disposal question matters. Mining operations cycling through generators in their refuge chambers face the same challenge: you are handling a device that can sustain temperatures high enough to ignite cardboard, wood, or fabric, and that simultaneously produces the oxygen an accidental fire would need to grow. Training and procedural discipline around these devices remain as important as the engineering inside them.