Any citizen or Raider on EVA will have only eight minutes' worth of oxygen in their spacesuit.
Each Oxygen Recycler provides just enough oxygen for three citizens. However, it is inadvisable to keep your oxygen level at capacity, as the saturation of each room will drop to ~70% and lower your citizens' Morale.
Construct one or two more recyclers than you need. This will keep your citizens happy and allow for some recyclers to be destroyed without your citizens running out of air.
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