In the movie, After Earth:

Humans have been away from Earth for over a thousand years living on other planets outside of our solar system. It is mentioned Humanity was under attack by an alien race, but it did not mention terraforming or any particular change of the Earth's ecosystem.

  • Why did Humans who were not native to Earth appear to be unable to breathe on Earth without technology?

  • I don't remember anyone explaining why humans couldn't breathe there. Was there something I missed or is this a plot hole we were expected to hand-wave away?

  • John, that was so wrong. At a number of levels. FUNNY, but oh so wrong. Let's pretend there was no underlying Scientological (is that a word?) underpinnings... – Thaddeus Howze Jun 3 '13 at 19:18
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    This io9 post has a good chunk of backstory from a companion book that was published. This bit isn't really explained further than DVK's answer. I'd heard elsewhere that it was due to pollution, but I can't find a source. – user1027 Jun 4 '13 at 19:44
  • Thanks Keen. I just finished that and am no more enlightened regarding why "the air isn't the air" any more. I just keep thinking we could write better movies... – Thaddeus Howze Jun 4 '13 at 19:57
  • Are we talking about the big-budget Will Smith movie or the low-budget Adrian Paul movie? In the Adrian Paul one, it was more like 300,000 years - not just 1000. But the humans that landed could breathe after a while, the air was just a lot heavier than normal. – Omegacron Sep 22 '14 at 19:22

As per Forbes review by Carol Pinchefsky:

In a phone interview, screenwriter Gary Whitta told me, “We started from the premise that this is less of an extinction and more of an eviction. Earth basically decides to get rid of us…through changes in atmospheric conditions.”

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    So um, yeah. Mother Geia decided to anthropomorphize itself and made it so humans can't breathe (but other species that evolved for the same atmosphere, can). No, I'm not making this up - that was the point made by the screenwriter in subsequent explanations. Welcome to Animal Liberation Front Philosophy Movie. – DVK-on-Ahch-To Jun 4 '13 at 1:31
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    actually we didn't see quite any of today's fauna in the movie. The biosphere normally adapts to any change, just leave a different composition – Balog Pal Jun 16 '13 at 13:37

There were clearly ferns in that movie, so we do have some modern flora. From the young boy's reaction to the air, I would assume the oxygen was too high; he showed signs of exasperation and shortness of breath, which would be caused by too much oxygen. Oxygen by itself, or in high quantities, is too volatile for our systems to consume.

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    Wouldn't shortness of breath be caused by too little oxygen? Too much oxygen would eventually cause fluid in the lungs and other damage that could lead to wheezing and coughing, but that'd take at least a couple of days. – Martha Jul 15 '13 at 13:57

They were used to the air on Nova wotsit where they live. Had adapted to that atomsphere. More important is why Will didn't give his son a few back up vials, instead of chancing it on the EXACT no. needed to make it.

  • Will's character is all about survival and honour etc. as that appears to be the theme as to how humans have/had to have survived. So giving him just the exact amount of vials seems very consistent with the setting and storyline. – SaltySub2 Sep 29 '18 at 15:33

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