While entering the atmosphere of LV-226, the captain is informed that the moon's atmosphere is comprised of 3/4 hydrogen and the remaining is oxygen. So a human can't breath for more than 2 minutes.

Also while entering the temple of Engineers, Dr. Halloway say that the sun is heating up the water making the air breathable for humans.

If engineers are the ancestors of humans, we can assume that they might have breathed oxygen.

Question 1: In that case, how did they survived on that moon if they breathe oxygen?

Question 2: If they didn't breathe oxygen, how did they survive inside the temple?

  • 12
    "If engineers are the ancestors of humans, we can assume that they might have breathed oxygen" - we cannot assume that, as all we know is they 'seeded' earth with their own DNA. The entire evolutionary process after that may have adapted the resulting species to the local atmosphere.
    – Moo
    Commented Aug 13, 2014 at 10:19
  • Related: scifi.stackexchange.com/questions/24878/…
    – user8719
    Commented Aug 13, 2014 at 12:43
  • 6
    The moon wasn't their home planet, just a base they were using, so I don't see why they couldn't have survived the same way as the humans in the movie: by having a breathable atmosphere in the temples and ships, and wearing environmental suits (like the elephant-looking helmet) when they went outside.
    – Hypnosifl
    Commented Aug 13, 2014 at 13:04
  • 6
    3/4 Hydrogen, 1/4 Oxygen - no smoking! youtube.com/watch?v=7SLsFAAHVZs
    – Rob Church
    Commented Aug 13, 2014 at 14:33
  • 1
    Coffee. Oh - you mean in the movie.
    – RSmith
    Commented Oct 29, 2015 at 14:29

1 Answer 1


You've mishead; it's not hydrogen, it's nitrogen that is dominant in the atmosphere, which means that the nitrogren and oxygen components are quite similar to Earth (78.09% nitrogen, 20.95% oxygen).

The stated reason in the movie for why humans can't breathe is the high carbon dioxide content, which at over 3% is significantly higher than that of Earth (0.039%, so it's almost 100 times higher):

Janek: What is the atmosphere?

Ravel: Atmosphere is 71 percent nitrogen, 21 percent oxygen, traces of argon gas.

Janek: Whoa, now, that's weather.

Charlie Holloway: Just like home.

Ford: Only if you're breathing through an exhaust pipe. CO2 is over 3 percent. Two minutes without a suit, you're dead.

The Engineers obviously therefore breathe a similar nitrogen/oxygen mix to that which we do, but have higher tolerance for carbon dioxide.

On an interesting sidenote, and as noted in this question, carbon dioxide in the movie is an obvious error: carbon dioxide in that concentration is not so poisonous, and nor is it in any event produced by an exhaust; carbon monoxide is what it obviously should be.

That the Engineers might have a biological requirement for breathing such a high level of carbon monoxide (or dioxide) may be discounted. The atmosphere inside their ship was breathable by humans (as the scene in the control room - where the humans are without their helmets - confirms), so evidently their preferred atmosphere mix is something that is breathable by humans, i.e non-lethal to us. However, the surviving Engineer apparently made it from his ship to the lifeboat without a helmet, so they appear to have a higher tolerance for a mixture that would be fatal to humans.

  • 5
    Why can't we consider a requirement for high CO2? They may have a hybrid repository system which works both like an animals and like a plants... This is scifi, anything can happen.
    – Moo
    Commented Aug 13, 2014 at 11:15
  • 2
    I'd love a citation on that, because as I remember it there was a link between the DNA, but it was never said it was completely identical.
    – Moo
    Commented Aug 13, 2014 at 12:45
  • 2
    And anyhow, there are enough other differences between the Engineers and humans physically (height, strength, build etc etc) that even with "identical DNA" there could be others, such as a difference in respiratory systems.
    – Moo
    Commented Aug 13, 2014 at 12:46
  • 3
    @Hypnosifl - IIRC the Engineer got from the crashed ship to the lifeboat without a helmet. It's equally possible of course that he was just able to hold his breath for long enough.
    – user8719
    Commented Aug 13, 2014 at 14:41
  • 4
    @Hypnosifl its worth noting in the scene in the life boat, the interior of the life boat is exposed to the exterior atmosphere for the entire duration of the Engineers confrontation of Shaw (he busts in through the airlock), the subsequent fight with the alien, the implantation and Shaws escape. Shaw is wearing her full suit including helmet during this scene. Later on, the Engineer is seen waking up after the implantation, none the worse for wear for being exposed to the high CO2 atmosphere for the duration. Of course the alien may have assisted in his breathing, but thats speculation...
    – Moo
    Commented Aug 13, 2014 at 15:11

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