This answer explains that from the time they suited up "in the Suit Lab before launch" until the time the capsule started depressurizing during ascent, the Apollo astronauts were breathing 100% oxygen at slightly above 1 bar (1 atmosphere).

100% Oxygen at 1 bar (100 kPa) partial pressure is toxic, or at least bad for you if you breathe it long enough. Presumably you'll be okay if it's not too long, so there must have been some time limit.


  • How long were the Apollo astronauts allowed to continuously breathe 100% oxygen at 1 bar?
  • Where there ever situations where the time was exceeded, and they literally had to "take a breather" and breathe something with a substantially lower oxygen partial pressure?

2 Answers 2


There are two different effects of oxygen toxicity, the Lorrain-Smith-effect and the Paul-Bert-effect. See Wikipedia.

The Lorrain-Smith-effect may occur at a partial oxygen pressure above 0.5 bar for more than about 24 hours. It is a lung toxicity.

The Paul-Bert-effect may occur at a partial pressure above 1.6 bar for minutes to a few hours. It is a central nervous system toxicity.

The time to onset of symptoms is highly variable but most individuals can tolerate 12-16 hours of oxygen at 1.0 ATA, 8-14 hours at 1.5 ATA, and 3-6 hours at 2.0 ATA before developing mild symptoms.

10 hours at a partial oxygen pressure of 1 bar may cause a 2 % reduction of the vital capacity.


For the Apollo astronauts the time when breathing pure oxygen at a pressure above 0.5 bar was limited to a few hours.

But to reduce the probability of decompression sickness extended prebreathing of pure oxygen over 3 to 4 hours is helpful. See 1, 2, 3.

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    $\begingroup$ What is "ATA"? That's a new one to me. $\endgroup$ Jul 19, 2019 at 14:55
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    $\begingroup$ @OrganicMarble ATA or ata is the absolut pressure in kp/cm^2, outdated for over forty years. Just replace it by bar, the error is small and negligible. $\endgroup$
    – Uwe
    Jul 19, 2019 at 15:06
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    $\begingroup$ Thanks! I learned something. By the way, the suit pressure control was stated in "inches of water", that's one I hadn't heard in a long time. $\endgroup$ Jul 19, 2019 at 15:08
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    $\begingroup$ Interesting! U-2 and SR-71 pilots both breathed 100% oxygen during their flights. I guess they also didn't exceed the 12-16 hour mark. $\endgroup$
    – Erin Anne
    Jul 20, 2019 at 4:27
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    $\begingroup$ @ErinAnne The U-2 and SR-71 pilots breathed 100% oxygen at a pressure substantially lower than 1 bar or 1 ATA, so they could exceed 12 to 16 hours without problems just like the astronauts during flight. $\endgroup$
    – Uwe
    Jul 20, 2019 at 8:26

The Crew Systems Division post-flight report for Apollo 11 states that:

Suiting was completed at 06:17 with an O2 concentration check at 06:21 indicating 100% O2 in the suit.

Launch was at 08:32 and the crew removed their helmets at ~08:45.

  • 1
    $\begingroup$ Could well be. I'll look for other CSD postflight reports. Or maybe there's a flight rule. $\endgroup$ Jul 19, 2019 at 14:39
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    $\begingroup$ Lots of Apollo "Flight Mission Rule Documents" (FMRD) out there but can't find a "Launch Mission Rule Document" (LMRD). Will ask a question. $\endgroup$ Jul 19, 2019 at 15:06
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    $\begingroup$ @uhoh There was a launch delay of nearly three hours during the Mecury-Redstone 3 mission with Alan Shepard. $\endgroup$
    – Uwe
    Jul 19, 2019 at 16:20
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    $\begingroup$ @uhoh Apollo launch delays were limited by the launch windows, the longest window for Apollo 8 at 4 hours and 41 minutes. Most launch windows were shorter than 4 hours. $\endgroup$
    – Uwe
    Jul 19, 2019 at 16:54
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    $\begingroup$ Launch delay for Apollo 17 was 2 hours and 40 minutes, so the time with 100 % O2 at 1 bar was substantially longer. $\endgroup$
    – Uwe
    Jul 20, 2019 at 17:43

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