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I'm looking into a suggestion I've seen mentioned occasionally, most recently in the comments for another question in Worldbuilding. The idea is that human respiration relies on having the correct partial pressure of oxygen, whereas a typical fire is less sensitive to pressure but needs a percentage of oxygen in the atmosphere. Therefore, if you reduce the %O2 and compensate by increasing the pressure, you can have a perfectly breathable atmosphere with a reduced fire risk.

I believe the first part is accurate: higher pressure increases the solubility of O2 in lung fluid, aiding absorption.

I'm not convinced by the second: higher pressure means a greater density of oxygen, which should allow a faster burn. A quick search confirms that at least some fires burn faster at higher pressures.

However, that doesn't rule out the possibility that the lung will "benefit" from increased pressure more than the fire.

To be specific, suppose I took a sample of Earth's atmosphere at sea level, replaced some of the oxygen with nitrogen, and increased the pressure so that it was safely breathable for humans. Would that atmosphere be as good for burning a wad of dry cloth as the typical sea-level atmosphere?

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    $\begingroup$ The way this is asked doesn't seem to fit well with the "hard-science" tag -- you might want to change that to "science-based". $\endgroup$
    – Zeiss Ikon
    Commented Oct 5, 2022 at 12:22
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    $\begingroup$ The concept you are missing is "buffer gas". The buffer gas (mostly nitrogen in Earth's air) controls (within limits) the rate of combustion at equal partial pressure of oxidizer. That is to say, stuff will burn more vividly and fire will start more easily in 100% oxygen at 0.2 atm than in 10% oxygen at 2 atm. (But stuff will burn in 10% oxygen at 2 atm, it just won't burn as vividly.) $\endgroup$
    – AlexP
    Commented Oct 5, 2022 at 12:23
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    $\begingroup$ @AlexP Please don't answer in comments -- make that an answer once this is answerable (i.e. no longer "hard-science"). $\endgroup$
    – Zeiss Ikon
    Commented Oct 5, 2022 at 13:32
  • $\begingroup$ @AlexP Oh, just noticed, the tag has been changed. $\endgroup$
    – Zeiss Ikon
    Commented Oct 5, 2022 at 13:37
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    $\begingroup$ Look at the tag wiki if in doubt. "Hard-science" requires citations and equations, and answers without them, however, correct, can be deleted by moderators without input from the asker. "Science-based" allows the good answers without requiring pasting in from a math/equation site, looking up scientific articles that are likely behind paywalls, etc. $\endgroup$
    – Zeiss Ikon
    Commented Oct 5, 2022 at 13:51

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It will work and it already has practical uses in real life

The principle in the question is sound and actually used - at least experimentally - for deep diving: a mix of hydrogen and a small quantity of oxygen is breathable under pressure but it's not explosive.

It is breathable because what matters for breathing is the amount of oxygen and, for example, a mixture with 1/50 of the percentage of oxygen in the atmosphere but under 50 atmospheres of pressure, has the same quantity of oxygen per volume unit than air at 1 atmosphere.

And the mixture is not explosive (nor flammable), even if it has oxygen and hydrogen, because there is not enough oxygen to sustain combustion. Combustion is a chain reaction. To be sustained, heat produced by the combination of one molecule of oxygen and two of hydrogen must heat another molecule of oxygen and two of hydrogen enough to allow them to start their own reaction, and when oxygen is just a very small percentage of the mixture, that heat is going to heat a lot of hydrogen molecules but it's unlikely to reach one oxygen molecule. Therefore, even if combustion starts, it's likely to decay soon.

If that happens with a highly flammable gas like hydrogen, you can be sure it can happen with any other gas like nitrogen. The only caveat is that nitrogen at pressure becomes toxic - in fact, all gases - so there is a limit on the pressure you can use - basically, a few atmospheres. Anyway, an atmosphere with more pressure and less percent of oxygen will keep being breathable but will hinder combustion making fires harder to start and maintain, and probably making nonflammable some materials that are flammable under usual conditions.

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    $\begingroup$ There is a funny video of a deep-water explorer in a high-pressure, hydrogen-rich atmosphere trying to light the candle on a birthday cake — it does not work at all, of course, and he complains about it in a voice made hilariously squeaky by the light-weight gas. $\endgroup$
    – Michael Lorton
    Commented Oct 6, 2022 at 0:18
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    $\begingroup$ Would love to see that video! Unfortunately my google searches yielded nothing. Please share a link! 🍿 $\endgroup$
    – micha
    Commented Oct 6, 2022 at 10:12
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Nitrogen is an antioxidant

By keeping the amount of oxygen in the air the same but increasing only the Nitrogen, you are not just reducing fires through pressure changes, you are also reducing them by the presence of the Nitrogen itself. The bond between dinitrogen molecules (N2) is very strong; so, when you add it to a fire, the molecules can absorb a lot of energy from the fire without taking on enough energy to trigger any reactions that could produce more energy.

So, nitrogen acts as an energy sink inhibiting oxygen based chain reactions. So, a more nitrogen rich atmosphere would be able to absorb more heat per molecule of oxygen giving you a similar effect as trying to burn damp wood.

That said, it is not without its health risks

While your O2 levels may remain high enough for respiration, heightened levels of Nitrogen in the atmosphere have been linked to increased levels of ammonia and ozone. So while the Nitrogen may be inert, it could lead to toxic levels of these other gasses. So, to make the air breathable for humans, you may need to introduce local flora or some other alien environmental factor that absorbs and/or breaks down these gases keeping them from building up to levels that would be toxic to humans.

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Don't bother increasing pressure.

A high percentage of humans in good health can acclimate to elevations as high as 4 km ASL -- which is equivalent to keeping the pressure at 1 bar while reducing oxygen percentage to below 15%. This will reduce the activity of fires by at least 25% compared to Earth normal (probably more, since there's more "inert" nitrogen taking away heat as well as less oxygen participating in producing it).

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    $\begingroup$ Reducing O2% and keeping pressure stable increases N2 by same % as O2 drops. But high pressure and lower % O2 seems like the circumstance that will provide the most inert nitrogen (or other gas) as regards total gas mass. $\endgroup$
    – Willk
    Commented Oct 5, 2022 at 14:04
  • $\begingroup$ Perhaps higher pressure and still reduced ppO2 -- but if you only reduce the O2 to the same partial pressure as on Earth, you get just a little damping from the higher nitrogen. Reducing ppO2 will have more effect, regardless of pressure. $\endgroup$
    – Zeiss Ikon
    Commented Oct 5, 2022 at 16:00
  • $\begingroup$ This will increase risk to smokers I think. If your lungs are full of tar and your-mom-knows-what-else, then they will have a harder time with what you're suggesting $\endgroup$
    – Omar and Lorraine
    Commented Oct 6, 2022 at 12:55
  • $\begingroup$ Why would you intentionally bring addicts to a place where you presumably won't waste cleared, terraformed arable land growing poison? $\endgroup$
    – Zeiss Ikon
    Commented Oct 6, 2022 at 12:56
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I'm a little hesitant to add this as the OP hasn't said what they are trying to keep from burning other than the example of a dry cloth. But .. increasing the moisture content of the air would help as well. This reduces the rate of evaporation of moisture from vegetation and other combustibles and can result in more frequent heavy dews and rain to keep them moist.

This also increases the amount of heat needed to sustain burning as the moisture needs to be driven into gas phase before the remaining material can ignite.

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Yes, you can suppress fire via reducing the percentage of O2 at 1atm, and continue to breath normally. At least according to the folk in this teaser-video for the UK's Royal Institution Christmas Lectures from 2012. The clip is only 2minutes 50seconds long, so provides some visuals rather than a full analysis. See https://www.youtube.com/watch?v=V-E1PLZpJG0

The video should hopefully remain up for quite some time as it's posted by the Royal Institution itself, but in summary - the company visited for the demo is using a 15% O2 environment at normal atmospheric pressure precisely for fire suppression. The company claim humans can remain and work within that environment indefinitely with no ill effects from the reduced O2 content. The presenter demonstrates the difficulty in trying to ignite a gas lighter, some common matches and (at ~2:20) shows the reduced O2 atmosphere extinguishing a previously-lit 'medieval torch' doused in petrol.

As to "could it be faked" and "did they forget anything obvious" - the video is a taster for the Royal Institution lectures, and they generally have a solid reputation as accurate-yet-accessible science for teenagers.

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