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I've been re-watching Enterprise and I have the same question I had the first time.

In episode 12 of season 4, Babel One, Tucker and Reed are running low on air while in environmental suits, and Tucker uses 100% pure oxygen to refill the suits. Is there something I've missed about the suits that makes this non lethal?

I'm not interested in the biology as that is certain death, but more about the suits from any series or movie, there must be something to prevent the oxygen being lethal - I'm looking for an explanation on that.

Do they somehow manufacture nitrogen or another inert gas and if so why don't they just manufacture the oxygen, why didn't they die?

All the questions are linked so answering any of them answers them all.

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    More of a comment than an answer, but even in the real world breathing 100% oxygen is not "certain death". It can be tolerated for a certain length of time under certain conditions. en.wikipedia.org/wiki/Oxygen_toxicity
    – Mark Beadles
    Commented Aug 12, 2019 at 0:18
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    @MarkBeadles they were on that ship a lot longer than 96 minutes, even the low 6-10 min of that article suggests they shouldn't even be walking, so it doesn't change the question of what the suits do to protect people from that. They must do something or they would at least be on the floor
    – Matt
    Commented Aug 12, 2019 at 0:30
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    It's the partial pressure that is fatal, not the relative concentration.
    – OrangeDog
    Commented Aug 12, 2019 at 10:11
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    @Matt - That "96 minutes" (if you are referring to the table in the wp article) is at a depth of 90' (27m) , which equals a pressure of 2.7 atmospheres. 100% oxygen at atmospheric pressure is not particularly dangerous (it's what fire-fighters breath). If you look at the graph further down, you'll lose about 10% of your lung capacity if you breath pure oxygen for a day.
    – Martin Bonner supports Monica
    Commented Aug 12, 2019 at 10:32
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    Matt, in short you can breathe pure O2 if the pressure is low.
    – Fattie
    Commented Aug 12, 2019 at 18:11

2 Answers 2

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Current NASA spacesuits are filled with 100% oxygen at around 4psi so filling a spacesuit with pure oxygen is no issue assuming the pressure isn't too high. reference

The actual percentage of gas doesn't matter when it comes to toxicity, what matters is the partial pressure of the gas which you get by multiplying the pressure by the percent. 20% atmospheric oxygen is equivalent to 100% oxygen at one fifth atmospheric pressure.

Spacesuits are not pressurised to full atmospheric pressure for this reason, so they only need to bring oxygen and it increases mobility to have less of a pressure difference between the inside and outside of the suit.

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    The difference between full vacuum and full atmospheric pressure is only one atmosphere, divers are subjected to many times this (1atm every 30 feet) which is why decompression sickness is an issue. Being experienced spacers they would be well equipped to handle differences of one atmosphere, additionally, the treatment for decompression sickness is to be put in a 100% oxygen atmosphere so they already have a head start.
    – John Meacham
    Commented Aug 12, 2019 at 1:27
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    @Kai This doc from Hamilton Standard describes the Apollo-era life support system and mentions using pure oxygen at 3.9 psi. Note also the Oxygen Purge System on pg. 4, which is designed to provide emergency life support by flushing the system with pure oxygen in an open loop.
    – Cadence
    Commented Aug 12, 2019 at 1:30
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    @JohnMeacham I wasn't asking about biology but the suits instead, your answer is that nothing is needed other than to lower the pressure (which does render the whole question mute), it seems my question was based on a faulty premise,, I was assuming full oxygen at normal pressure. I will accept this if no-one can give any better. I usually wait 24 hours before accepting. Thanks
    – Matt
    Commented Aug 12, 2019 at 2:37
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    There's no need on this site for citations, for basic, everyday science.
    – Fattie
    Commented Aug 12, 2019 at 18:12
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    Moot. Not mute.
    – Chris Johnson
    Commented Aug 13, 2019 at 0:30
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There are two main things to consider here.

  1. Oxygen toxicity depends on the partial pressure of oxygen. In a low pressure environment you need 100% oxygen to achive a breathable suit atmosphere without making the suit unusablly stiff. As I understand it at 1 earth atmosphere you can breathe pure oxygen for many hours, at higher pressures the safe exposure time drops dramatically.
  2. The human body consumes a lot of oxygen. Nitrogen (or other dilutent gas) on the other hand is not directly consumed by the body.

Given the relatively low bulk of the suits we can assume they use a rebreather system rather than an open-cycle system.

If the suits are intended for short duration use at pressures of 1 earth atmosphere or less then it is likely a pure oxygen system would suffice and would be the simplest and most reliable system. This is how real world spacesuits work.

On other other hand if the suits are designed to work at pressures significantly above 1 earth atmosphere or to work at 1 atmosphere for long durations they would need to have a mixing system. Sensors would be needed to keep track of the gas mixture inside the suit and adjust accordingly. This is done today in diving rebreathers, so it's not something that needs magical future technology. The oxygen and the dilutent gas would presumablly be stored seperately by the suit.

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    "As I understand it at 1 earth atmosphere you can breathe pure oxygen for many hours, at higher pressures the safe exposure time drops dramatically." This is wrong. O2 at 1 atmosphere == instant death. O2 at 1/5th atmosphere == all good.
    – Fattie
    Commented Aug 12, 2019 at 18:14
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    All the sources that my searches are turning up say at 1 ATM you have hours (though they disagree on exactly how many hours). Your "instant death" claim doesn't pass the sniff test given the use of pure oxygen in some types of diving and the use of pure oxygen throughout the apollo missions (in space it was low pressure pure O2, but in the runup to launch it was at atmospheric pressure).
    – Peter Green
    Commented Aug 12, 2019 at 18:31
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    @Fattie As a scuba diver who's breathed pure O2 for a time at 2 ATM to decompress, I can guarantee O2 at 1 ATM is not instant death. Pure O2 is used in tons of breathing applications at 1 ATM for short durations without adverse effects.
    – Mwr247
    Commented Aug 12, 2019 at 19:12
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    @Fattie "100% oxygen can be tolerated at sea level for about 24-48 hours without any severe tissue damage."
    – Kevin
    Commented Aug 12, 2019 at 19:17
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    You can go to oxygen bars at trendy clubs and breath pure oxygen for ten bucks without any issues. Even at high pressure it isn't "instant death".
    – John Meacham
    Commented Aug 12, 2019 at 20:13

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