De acordo com the100.wikia.com , eles instalaram purificadores de oxigênio em S04E13. A parte relevante desse wiki (a ênfase é minha):
"Then, Murphy, Emori, Harper, Monty, Raven, Bellamy, and Echo get onto the Ark, but Raven is low on oxygen. Murphy and Emori decide to share theirs along with everyone else. Meanwhile, Monty instructs Bellamy to connect the oxygen scrubber to the conduits on the Ark."