In the article on Apollo command module design, I read:

  • The cabin atmosphere at launch was changed to 60% oxygen and 40% nitrogen at sea-level pressure: 14.7 psi (10.1 N/cm2). During ascent the cabin rapidly vented down to 5 psi (3.4 N/cm2), releasing approximately 2/3 of the gas originally present at launch. The vent then closed and the environmental control system maintained a nominal cabin pressure of 5 psi (3.4 N/cm2) as the spacecraft continued into vacuum. The cabin was then very slowly purged (vented to space and simultaneously replaced with 100% oxygen), so the nitrogen concentration fell asymptotically to zero over the next day. Although the new cabin launch atmosphere was significantly safer than 100% oxygen, it still contained almost three times the amount of oxygen present in ordinary sea level air (20.9% oxygen). This was necessary to ensure a sufficient partial pressure of oxygen when the astronauts removed their helmets after reaching orbit. (60% of 5 psi is 3 psi, compared to 20.9% of 14.7 psi (10.1 N/cm2), or 3.07 psi (2.12 N/cm2) in sea-level air.)

  • The environment within the astronauts' pressure suits was not changed. Because of the rapid drop in cabin (and suit) pressures during ascent, decompression sickness was likely unless the nitrogen had been purged from the astronauts' tissues prior to launch. They would still breathe pure oxygen, starting several hours before launch, until they removed their helmets on orbit. Avoiding the "bends" was considered worth the residual risk of an oxygen-accelerated fire within a suit.

(emphasis mine)

The article on Oxygen Toxicity has a following table: Oxygen poisoning at 90 feet (27 m) in the dry in 36 subjects in order of performance

"Several hours" seem like much longer than the 96 minutes the most persistent test subject withstood, of exposure to pure oxygen at atmospheric pressure, and risk of oxygen-accelerated fire within a suit seems like the least of worries the astronauts would have.

How were they dealing with oxygen toxicity before and during the launch, before the pressure was vented to 5 psi?

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    $\begingroup$ Isn't there a factor of 5 between the table (oxygen at 3 atmospheres pressure, i.e. 15 times more oxygen than in normal air) and the environment in the command module (max. 0.6 atmospheres oxygen partial pressure)? $\endgroup$
    – asdfex
    Jan 11, 2016 at 19:39
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    $\begingroup$ @asdfex: Oh, I had assumed 27m altitude (near sea level pressure), not depth... Still, the factor would be closer to factor of 3 for the few hours before launch - the command module is only vented in space. (other than that it would be more, the command module was launched at 60% oxygen content and refilled to 100% over the first day in space) $\endgroup$
    – SF.
    Jan 11, 2016 at 21:57
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    $\begingroup$ Your chart is for a substantial depth in the ocean. At sea level the safe exposure period is a lot longer (I want to say 16 hours but I won't swear to it) than they would need before the pressure drops to safe levels. $\endgroup$ Jan 12, 2016 at 0:41
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    $\begingroup$ @SF. Looks like my memory was wrong. Safety limit is 5 hours: decodoppler.files.wordpress.com/2009/02/noaa-tables.jpg $\endgroup$ Jan 12, 2016 at 20:21
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    $\begingroup$ @SF The limits for 1bar pure oxygen is given in : en.wikipedia.org/wiki/Oxygen_toxicity#Signs_and_symptoms "One or two days of exposure without oxygen breaks are needed to cause such damage". So 5 to 16 hours as safety limit are possible depending on the safety margin. $\endgroup$
    – Uwe
    Sep 30, 2017 at 21:20

2 Answers 2


Reading the listed Wikipedia article, it is evident that these experiments were done at pressure, the same pressure that would be experienced at 90 feet of water. I've been trying to find a better source, and I looked at the source article for that chart. It contains the following:

... were safe breathing oxygen ... for at least three hours at 3 atmospheres

It seems the toxicity only really happens once one gets above 3 atmospheres, the safe exposure for 4 atm is around 30 minutes.

While it isn't recommended for everyone, there seems to be no adverse effects to breathing pure oxygen at atmospheric pressures for several hours, as was done for the Apollo program.

  • 1
    $\begingroup$ 90' of (fresh) water is roughly 4 Atm, assuming there's 1 atm above the water. The symptoms listed for 6-10 minutes aren't life-threatening and probably aren't even completely debilitating, but I wouldn't really call them "safe" for anybody who needs to be doing anything, especially in what could easily become and emergency situation. $\endgroup$
    – CBHacking
    Jan 17, 2016 at 11:37
  • $\begingroup$ In diving the limit for breathing pure oxygen is 1.6 bar or a depth of 6 m water. At 1.7 bar or more there may be convulsions. "at least three hours at 3 atmospheres" is considered unsafe in the water and should be used in recompression chambers only with supervision personel breathing no pure oxygen but compressed air. $\endgroup$
    – Uwe
    Jul 11, 2018 at 20:38
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    $\begingroup$ It seems the article I used was from 1947, so it might be a bit out of date... $\endgroup$
    – PearsonArtPhoto
    Jul 11, 2018 at 21:19
  • $\begingroup$ For open water diving we assume normally that pure O2 becomes toxic at a partial pressure of 160kPa (that would be at about 6m/20ft depth). But for recreational diving there are very wide margins for safety included. So even when they pressurized the capsule for Apollo 1 toxicity of O2 was not an issue at all (it was still not a very smart move) $\endgroup$
    – TrySCE2AUX
    Dec 20, 2022 at 9:06

Answer: There is no concern for lung toxicity for the first 12 hours breathing 100% O2 at one atmosphere..

Pulmonary oxygen toxicity is a time/partial pressure effect. From https://en.wikipedia.org/wiki/Oxygen_toxicity : enter image description here

“Lambertsen concluded in 1987 that 0.5 bar (50 kPa, 50%O2) could be tolerated indefinitely.”

Evidence of decline in lung function as measured by pulmonary function testing can occur as quickly as 24 hours of continuous exposure to 100% oxygen under normobaric conditions

The decrease in lung function breathing 100% O2 under normobaric conditions may be partially due to the absence of nitrogen which allows alveolar atelectasis (collapse). The addition of an inert gas (without changing the ppO2) will prevent O2 toxicity.

US Navy hyperbaric tables for treating decompression sickness use 100% O2 at 2.3 - 3.0 bar for 4 ½ hours (with brief “air breathers”) https://en.wikipedia.org/wiki/Hyperbaric_treatment_schedules

In clinical practice, There is no concern for lung toxicity for the first 12 hours breathing 100% O2. This leaves a wide safety margin since the only way to approach 100% O2 is in an unconscious, intubated patient on a ventilator. Conscious, spontaneously breathing patients with a face mask have significant dilution from dead space effect. https://www.ncbi.nlm.nih.gov/books/NBK482501


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