Oxygen

Oxygen is a primary breathable gas in the game and one of the lightest atmospheric gases. It is required by Duplicants for survival and is produced and consumed by numerous in-game systems and critters. Oxygen naturally rises above heavier gases such as Carbon Dioxide, which influences how atmospheres stratify and how gas-based systems should be arranged. Breathability is determined by per-tile mass: anything below 50 g/tile is unbreathable and between 50–300 g/tile is breathable but will cause stress; Duplicants typically consume about 100 g/s of Oxygen (this rate is modified by traits such as Mouth Breather, Diver’s Lungs, and Deeper Diver’s Lungs).

Early-game Oxygen supply can be a major constraint depending on the map. Some starting biomes contain little accessible water for electrolysis, forcing players to dig or to process remote Polluted Water or Salt Water from other biomes. Plants such as Oxyfern convert Carbon Dioxide into Oxygen and are water-efficient: domesticated Oxyferns convert about 98% of water to Oxygen (more efficient than the Electrolyzer’s ~88%), but they require Dirt and irrigation. Oxyferns multiply the mass of their input gas by 50, producing 50 g Oxygen per 1 g CO2 consumed; this makes pressure management important because unchecked plant farms can overpressurize sealed rooms.

Practical usage and interactions:

• Gas behavior: Oxygen is light and will rise above CO2; design ventilation and gas pumps with vertical layering in mind. Good diffusion prevents local overpressure from plant farms and allows electrolyzers and diffusers to keep functioning.
• Breathability thresholds: Maintain tile oxygen above 50 g/tile to avoid unbreathable areas and aim for comfortable levels to prevent stress. Watch Duplicant oxygen consumption modifiers to budget supply.
• Production options: Electrolysis of water (requiring sufficient water access), Oxyferns (domesticated or wild), and Oxylite processing/creatures are common sources. Oxyferns require appropriate temperature (0–40 °C), CO2 atmosphere, pressure within growth limits (25 g to 10 kg per tile), Dirt and irrigation when domesticated, and they grow much faster when domesticated than wild.
• Pressure and sealing: Because Oxyferns greatly increase gas mass, sealed rooms can reach high pressures; the maximum safe initial CO2 in a sealed box without eventual overpressure is 200 g CO2/tile, which converts to roughly 10 kg/tile oxygen. Ensure pumping out excess oxygen or supply balancing to prevent growth shutdown and to avoid stopping other gas-dependent devices (e.g., electrolyzers producing hydrogen).
• Cooling and heat interactions: Oxyferns can provide meaningful heat deletion when fed very hot CO2 and kept cold, converting smaller masses of hot CO2 into larger masses of cooler oxygen. Oxygen has 20% higher specific heat than CO2, making it suitable for slow heat-exchange circuits where mixing with ambient oxygen is acceptable. Liquid Oxygen (when cooled) is an effective rocket oxidizer for Petroleum and Hydrogen Engines.
• Uses and consumers: Oxygen is needed to refill Atmo Suit, Jet Suit, Oxygen Mask, and Lead Suit docks; the Telescope requires Oxygen; the Oxylite Refinery converts Oxygen + Gold into Oxylite; certain critters (Puft Princes, Dense Pufts) breathe Oxygen and produce Oxylite; some critters (Longhair, Slicksters) consume oxygen-containing resources and can provide decor or other outputs.
• Availability and strategy: Oxygen is generally more available than many alternative gases and is forgiving for piping maintenance because it can mix with outside oxygen if a circuit is opened. On maps with scarce surface water (e.g., sandy starts), prioritize water access or transport Polluted/Salt Water from other biomes to enable electrolysis or plant-based oxygen production.

Thermal note: historic unit references in older documentation used watts and W/m/°C for thermal conductivity; current heat units are expressed as DTU/s, and 1 watt equals 1 DTU/s for conversion of legacy values.