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Let's say a Mars outpost has an inflatable greenhouse. First, it is likely to be high-CO2, as CO2 is easy to acquire and increases plant growth. Second, it may be low-pressure, as plants don't need as much atmosphere as humans, and this would reduce the necessary strength and therefore thickness of the greenhouse walls.

What would these mean for a greenhouse that needs to be easily accessible for humans? Namely, regarding CO2, what is the safe limit for inhalation, and is there any limit if the crew uses breather masks? Regarding pressure, what is a reasonable limit for quick pressure changes? The primary habitat would likely be lower-pressure as well. Let's say Skylab's 5 psi if specific numbers are required. Thanks!

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    $\begingroup$ Some numbers in case they help - stealing figures from The Case for Mars, hab pressure is 5 psi (3.5 O2, 1.5 N2). TCfM says CO2 inside hab should be under 0.4 mbar, and 7 mbar (Mars ambient) could be used for greenhouses. It also says plants can tolerate 0.7 psi, and 2.5 psi should allow entrance without a spacesuit. $\endgroup$ – Deimophobia May 31 '17 at 20:50
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    $\begingroup$ To clarify, my question about pressure is, what's the lowest pressure the greenhouse could be kept at while allowing someone from a 5 psi habitat to pass in without having to go through decompression? $\endgroup$ – Deimophobia May 31 '17 at 23:48
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The question regarding the lowest pressure required for plant growth was asked on the biology stack exchange The answer given there reports an experiment where vascular plants seemed to be at about their survival limit at 0.1 bar. Unfortunately the quoted paper is paywalled. This does not appear to be a particularly well researched area.

Regarding CO2 concentrations. The problem with raised levels of CO2 is that the body needs to dump it's waste CO2 into exhaled air. In order to do this there needs to be a lower partial pressure of CO2 in the inspired air than the partial pressure of CO2 in the blood. This is so that the gas will exchange out of the blood and into the air. If the breathing gas has elevated CO2, the body cannot rid itself, and problems rapidly arise.

There is an article in Wikipedia - Hypercapnia which contains a table of CO2 percentage verses time of exposure. About 6% being the upper limit for even short exposures. But remember this was at 1 bar pressure, so this represents an upper partial pressure limit of about 0.06bar.

Regarding decompression. Decompression is required to allow time for dissolved gases (usually N2) to come out of the blood during respiration. If this time is not adequate, the gas will come out of solution in the blood forming bubbles, these can then block blood vessels causing a whole suite of problems depending where the bubbles end up.

The requirement for decompression therefore depends on the pressure change together with the mixture and duration of the gasses being breathed. So there is no simple answer to your question.

In SCUBA diving (ie breathing normal air at all stages) it is considered safe to halve your absolute pressure without decompression and without any time limit at depth. A depth of 10m gives 2 bar pressure, and is the depth limit for unlimited time without the requirement to decompress before surfacing. Underwater habitats can exploit this. A habitat at 10m (2 bar absolute pressure) would allow its inhabitants to work in pressures up to 4 bar (30m) for unlimited time and still return to the hab without decompression. However, they cannot return to surface without decompression.

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  • $\begingroup$ The "halve your absolute pressure" seems reasonable, as I found Zubrin saying 2.5 psi would not require spacesuits to enter from a 5 psi habitat. $\endgroup$ – Deimophobia Jun 5 '17 at 18:29

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