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According to an official safety bulletin from the U.S. Chemical Safety and Hazard Investigation Board:

"Breathing an oxygen deficient atmosphere can have serious and immediate effects, including unconsciousness after only one or two breaths."

Most people can hold their breath for at least 30 seconds and sometimes up to several minutes; clearly, breathing an oxygen-deficient inert gas will affect a person far sooner than simply holding one's breath.

An inert gas like nitrogen is not inherently toxic, so why does breathing a concentrated inert gas cause unconsciousness almost immediately? Does it replace the oxygen that would otherwise remain in the body while holding one's breath?

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    $\begingroup$ Truth: it doesn't. That's why Wikipedia is a terrible source for scientific information. $\endgroup$
    – anongoodnurse
    Jul 27 '21 at 23:01
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    $\begingroup$ @anongoodnurse The Wikipedia article is merely quoting an official safety bulletin from the U.S. Chemical Safety and Hazard Investigation Board, which is why you should actually check the citations in an article. I even included their citation in my question. $\endgroup$
    – user45623
    Jul 29 '21 at 0:29
  • $\begingroup$ I did check Wikipedia, and the article (it's curious why people assume commenters don't read the citations.) They do not cite a study to support the statement (not rodent studies or other; not even solid observations.) I've been reading scientific literature all of my adult life; I'm also a physician. That statement is flat out incorrect. There are substances/gasses that can kill in a few breaths, but inert gasses in normal circumstances are not as dangerous as the statement makes them appear. If they were, people would die from recreational helium use (they don't.) $\endgroup$
    – anongoodnurse
    Jul 29 '21 at 8:45
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One reason you can hold your breath for 30 or more seconds is that you are not denying your body oxygen during that time. Wikipedia says:

After exhaling, adult human lungs still contain 2.5–3 L of air, their functional residual capacity or FRC. On inhalation, only about 350 mL of new, warm, moistened atmospheric air is brought in and is well mixed with the FRC. Consequently, the gas composition of the FRC changes very little during the breathing cycle.

Hence you are refreshing your lungs' air with only 10-15% fresh air with each breath. Furthermore, normal exhaled air has reduced the oxygen to about 16% from the 21% of fresh air. So there is a significant amount of oxygen remaining in the lungs at the end of a normal breath. When holding your breath, much of this remaining oxygen can also be used, albeit at a declining rate due to the reduced amount of oxygen. Note also that when trying to hold one's breath it is typical to fill the lung with more air than usual, thus providing a greater amount of oxygen.

When breathing an inert gas instead of air, the situation is radically different. There is no period where you are receiving any level of replacement oxygen. Normally your blood has some oxygen in it at all times, even when returning through the veins to pick up more oxygen from the lungs. However if the lungs have only inert gases in them, the remaining oxygen in the blood will be diffusing out of the blood and into inert gas in the lungs, making the lack of oxygen more severe. This means that highly-deoxygenated blood will flow out to the body's tissues, where if there is any oxygen present it will diffuse out of the tissues into the blood. This will quickly impact the tissues, and brain tissue has one of the highest requirements for oxygen.

The problem is not that the inert gas replaces the oxygen in the blood as such, it's just that the oxygen isn't available. All gases diffuse between the blood and the gas in the lungs. The inert gas nitrogen (79% of air) normally diffuses this way and is in balance between the blood and the atmosphere. Oxygen and carbon dioxide diffuse too, of course, but they are also helped along by the hemoglobin in red blood cells. The hemoglobin grabs either carbon dioxide or oxygen, preferring the latter. The gas molecules so grabbed had already diffused into the blood. The lack of oxygen in the inert gases in the lungs means carbon dioxide will build up, which is a negative thing but minor compared to the lack of new oxygen for the tissues.

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    $\begingroup$ This answer is wrong. Lung/plasma O2 % cannot be diluted to ~0% in one or two breaths (look at helium). The quote is an oft repeated one which has no scientific source (google it in quotes. You'll see it stated on a number of websites without one study to back it up.) Also, CO is not an inert gas, it is preferred by Hgb, yet death does not follow one or two breaths. Even cutting off all O2 to the brain (e.g. effective manual strangulation) is not immediate. The quote is nonsensical BS, which you've treated as fact. Note that there's no source on Wikipedia, which is always a bad sign. $\endgroup$
    – anongoodnurse
    Jul 27 '21 at 23:03
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    $\begingroup$ No, the Wiki citation is for accidental deaths from nitrogen inhalation over a period of decades, not specifying "number of breaths" (i.e. it does not have a source for that statement.) Wikipedia editors often misinterpret scientific literature, which is why I think it's a terrible source for scientific information. Writers of publications sometimes make inaccurate statements, which make it into the lay press. This is one of them. For whatever reason, there's a lot of BS floating around as "facts". $\endgroup$
    – anongoodnurse
    Jul 29 '21 at 8:49
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    $\begingroup$ @anongoodnurse And of course if an article on Wikipedia is factually incorrect, you should edit the article to correct the mistake. $\endgroup$
    – user45623
    Jul 30 '21 at 16:23
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    $\begingroup$ @anongoodnurse My apologies; I was not questioning your knowledge or credentials. It sounded like you were saying Wikipedia had no citation for the quote, and I was saying "yes, they have a citation, it's in that safety bulletin". As far as the accuracy of the safety bulletin, I have no opinion on the subject. My question was originally prompted by a recent news article about 6 workers dying from nitrogen exposure in a poultry facility; one source said they died instantly, but specifics are light. If you'd like to share your knowledge as an answer, please do so. $\endgroup$
    – user45623
    Jul 30 '21 at 18:25
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    $\begingroup$ @anongoodnurse When a question is framed incorrectly, just acknowledge this in your answer. If I wrote a question with some ridiculous premise like, "How come humans are immune to harmful effects of radiation?", it would be perfectly valid for you to post an answer explaining that humans are not immune to radiation but rather are vulnerable to radiation sickness, cancer, and other effects. $\endgroup$
    – user45623
    Aug 4 '21 at 22:11

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