breathing on another planet (no mask)

[JonClarke]

Oh, btw, Oxygen is also a poison to the human organism, and you can only breathe pure O2 for a limited time before having some serious complications.

Only at high partial pressures.

I don't know what the record is but the Apollo astronauts brethed pure oxygen (at reduced pressure) for at to 12 days. On Sky lab they breathed a oxygen with minor nitrogen for 84 days. There was no specified amount CO2 in the breathing gases.