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Xen’s atmosphere is not dissimilar to that of Earth’s,
When Gordon Freeman visits Xen, there is difference of gravity and he is required to wear a breathing apparatus for his suit. However when the Aliens invade earth they have no trouble in adapting to Earth’s atmosphere, as they are pretty much immune to the oxygen levels of the planet, so much so that they have no trouble killing anything in their path. Though it can be explained that they are under the Nihilanth or Combines orders to take over Earth and found Earth a perfect place to invade and enslave.
