While it is possible for humans to breathe pure oxygen, a pure oxygen atmosphere was implicated in the Apollo 1 fire. As such, Mars habitats may have a need for additional gases. One possibility is to take nitrogen and argon from the atmosphere of Mars; however, they are hard to separate from each other.[19] As a result, a Mars habitat may use 40% argon, 40% nitrogen, and 20% oxygen.[19] See also Argox, for the argon breathing gas mixture used in scuba diving