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Q&A

Could a high-pressure, low oxygen atmosphere reduce fire risk while still being breathable?

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Years ago I read a letter in a magazine suggesting a method for reducing the fire risk in an enclosed environment such as a space shuttle. The idea was to reduce the oxygen concentration to the point where most common materials won't readily combust, and compensate by increasing the pressure so that the partial pressure of oxygen was the same as it is in "normal" air at sea level, making it breathable.

The assertion was that the ppO2 is the deciding factor in oxygen-haemoglobin binding, so a person could live comfortably in such an atmosphere, but the increased pressure would not increase the fire risk.

I understand deep-sea divers sometimes use reduced oxygen concentrations to compensate for high pressure, so the first part seems reasonable. However it seems to me that a higher ppO2 would also make oxygen more readily available for other reactions, including fire, and at least some fires do burn faster at higher pressures. Hence I wouldn't expect much benefit, if any. I have looked for research on the topic but drawn a blank, which might be a clue about the likely efficacy.

Would using a high-pressure, low-oxygen atmosphere as described give any useful reduction in fire risk?

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Spacecraft design considerations (2 comments)

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Your intuition is correct. Let's suppose that at STP, a human needs to breathe 1L of O2 in 200 minutes. That's 1.43g of gaseous oxygen. If the pressure is raised to 5 bar, a human still needs 1.43g of O2 -- but it will only take up 0.2L of volume, and the other neutral gas mass will have to increase as well.

Similarly, a fire that consumes 143g of O2 and 90g of powdered aluminum will use those proportions regardless of whether the O2 is mixed in a higher or lower pressure of neutral gas -- the gating factor is how fast the oxygen can come in contact with the aluminum.

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Works for me (1 comment)

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