Carbon Dioxide

Lore & Background

In the cross-section world of Oxygen Not Included, CO₂ is the invisible tax on every living thing. The planet's atmosphere already carries a trace of it, and the moment a Duplicant draws in oxygen and exhales, the gas begins its slow, dense descent toward the floor of whatever chamber it occupies. Because CO₂ is heavier than the oxygen Duplicants need, it does not mix evenly; it seeps downward, pooling in basements, crawl-spaces, and the lowest floors of a colony. A Duplicant who wanders into a sealed lower level can find a thick, invisible layer of CO₂ waiting at ankle height, and without a gas mask or a well-ventilated route upward, suffocation is swift.

The game's designers made CO₂ a double-edged resource precisely because it mirrors real-world chemistry. It is not merely a hazard to be dumped into the void; it is a fuel that can be burned for energy, a reagent that reacts with water to yield carbonic acid, and a component in other chemical pathways. Colonists who learn to capture and channel CO₂ rather than simply vent it can turn their waste into a productive input, but the engineering required—gas scrubbers, pumps, sealed processing lines—demands careful planning. A single leak in a CO₂ pipeline can flood a living quarter faster than the Duplicants can react.

Over the course of a colony's story, CO₂ becomes a narrative thread. Early on it is a nuisance; mid-game it is a fuel source that powers engines and heaters; late-game it is a reagent in more complex chemistry. The tension between needing it and fearing it is one of the game's quiet, recurring tensions.

In Their Own Story

The alarm chirped at 03:47, a thin electronic wail that cut through the hum of the water pump. Kira, a Duplicant in a faded maintenance jumpsuit, pressed her palm to the cold glass of the observation window. Below the floor she stood on, in the sealed sub-basement where the colony's fuel storage sat, a pale grey fog had crept up to the level of the shelving units. It had been there for minutes, maybe an hour—she couldn't say. The CO₂ from a slow leak in the combustion line had settled like water, filling the lowest point first, then rising centimetre by centimetre. She could see the meniscus where the dense gas met the cleaner air above, a faint shimmer in the lamplight. She pulled her gas mask tight, checked the seal, and descended the ladder into the grey. The air tasted like nothing. That was the worst part. There was no smell, no sting. Just the slow, creeping certainty that the oxygen was gone and the carbon dioxide was where it should be, at the bottom, where heavy things go.

Reader's Guide

CO₂ is a gas that occupies the lowest available space in any enclosed volume because it is denser than O₂. In a typical cross-section, it will pool in the bottom-most floors, basements, and any sealed chamber below the Duplicants' main living levels. Its presence is invisible to the naked eye but visible in the game's gas-layer view as a distinct tint.

Physical properties to keep in mind: CO₂ is toxic to Duplicants, meaning any Duplicant breathing a CO₂-rich mixture will begin to suffocate and take damage. It is combustible, so it can be fed into a gas burner or similar fuel-consuming machine to generate energy. As a chemical reagent, it can be combined with water to produce carbonic acid, opening a small but useful chemistry pathway.

Hazards are primarily respiratory. Because CO₂ sinks, the greatest risk is in low-lying sealed rooms. Duplicants working in basements, fuel storage, or combustion chambers are most vulnerable. A single unvented combustion event in a small room can quickly push O₂ levels below survivable thresholds. Always pair combustion with active ventilation or a gas scrubber, and ensure Duplicants have a clear, ventilated path to higher ground. Gas masks provide individual protection but do not solve a room-wide buildup.

In practice, treat CO₂ as a fluid you must pump, filter, or burn. Let it accumulate and it will find the lowest Duplicant in the colony.

Did You Know?

• Because CO₂ is denser than O₂ in the game, it behaves like a liquid that fills the bottom of a container first—so a sealed basement will fill with CO₂ from the floor up, while the air above remains breathable.
• Every Duplicant's breathing cycle produces CO₂ as an output, meaning a colony of eight Duplicants generates a continuous, non-stop stream of toxic gas that must be managed 24/7.
• CO₂ is not just waste: it is a valid fuel input in the game, so the gas that poisons your Duplicants can also be burned to power their heaters and engines.
• A single small leak in a CO₂ pipeline can flood an entire lower floor with toxic gas faster than a Duplicant can react, making sealed gas lines one of the most dangerous infrastructure choices in a colony.