Prometheus

Suitable atmospheric pressures for humans

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Another thread inspired by Artemis by Andy Weir (it's an OK book with some interesting near future sci-fi ideas).

In book's Lunar base the atmosphere is 100% oxygen but at 0.2 atms so that the partial pressure of oxygen is about the same as at sea-level Earth. First question: what economic/engineering or other pragmatic factors might be in play to consider a low pressure environment. The ISS maintains a 1 atm pressure.

However, my main interest is in speculating upon the long term health implications of living in such a system. 

For instance, i don't recall that the book mentions water vapour. With the low pressure this would surely lead to quite bad drying out of eyes/nasal cavities etc... it gets uncomfortable enough for some people in dry conditions on Earth. So water vapour would likely be added to the mix (to around 50% relative humidity), raising the overall pressure a bit. 

 

I know surfactant is the primary means by which to keep alveoli from collapsing during exhalation, however would the amount of surfactant required change in a low pressure environment? Perhaps this could lead to pulmonary disorders being more likely. I just imagine low pressure would mean easier collapse but i can't find any numbers to corroborate this.

 

Any other long or short term health effects you can imagine?

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They would add water to the oxygen- either deliberately or accidentally (people produce quite a bit  of water vapour).
But it wouldn't affect the overall pressure by much- the odd 1% or so depending on what humidity they chose.

One potential problem would be condensation on any "cold spots".

I'm not sure that the low pressure would actually make much difference

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I think there's a lot more data on higher-pressure. A problem with going with low pressure and 100% oxygen is that things tend to catch on fire more easily.

 

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1 hour ago, swansont said:

A problem with going with low pressure and 100% oxygen is that things tend to catch on fire more easily.

Basically what happened to the Apollo 1 disaster

What about when first moving to the low pressure environment: would this not lead to the bends if done suddenly as a person will still have residual nitrogen in their blood? This would perhaps require people to breathe a pure oxygen atmosphere at normal pressure first.

I guess the decision would be an economic one? Do the savings made by a low pressure near pure oxygen system (reduced mass, structural integrity, easier gas management) compensate for the increased risk of fire. In the moonbase scenario you wouldn't have the launch and reentry risks, which were what apparently led the Apollo missions to a two-gas system. Might make it viable?

 

2 hours ago, John Cuthber said:

I'm not sure that the low pressure would actually make much difference

What would the effect on thermal equilibrium be? With a lower pressure sweat would evaporate much quicker, taking less thermal energy with it. I assume radiative heat loss wouldn't change though? So over-heating might be a problem the moon colony needs to deal with (or just forces its occupants to endure), especially as they need to exercise to keep their bones healthy.

 

I imagine plants would find this atmosphere toxic, so no greenery either. Might impact the mental health of some people?

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Posted (edited)
8 minutes ago, Prometheus said:

With a lower pressure sweat would evaporate much quicker, taking less thermal energy with it.

I don't see how there's a change in the heat of evaporation of water.

The thermal conductivity is nearly independent of pressure (in this range).
A lack of CO2 would trouble plants, but the lack of atmospheric N2 shouldn't, except for plants that fix their own nitrogen.

Edited by John Cuthber

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Just now, John Cuthber said:

I don't see how there's a change in the heat of evaporation of water.

I'm basing this on the fact that water boils at lower temperatures at lower pressures. I imagine this translates to sweat evaporation too: sweat will require a lower amount of energy to evaporate, hence taking away less heat from the body?

2 hours ago, John Cuthber said:

One potential problem would be condensation on any "cold spots".

Why would these be problematic? Just because they take water vapour out of the atmosphere?

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Sweat doesn't usually boil- if it does then you are already in all sorts of trouble.

The energy needed to separate gas phase water molecules from the liquid  should be independent of  the presence of nitrogen.
So the cooling per gram of sweat should be the same.

 

However, in the relative absence of air, the rate of evaporation will be higher (A bit like a vacuum desiccator).

I'm not sure what effect that will have but I suspect that the body's feedback systems may well be able to deal with it.

 

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11 minutes ago, John Cuthber said:

A lack of CO2 would trouble plants, but the lack of atmospheric N2 shouldn't, except for plants that fix their own nitrogen.

I was wondering about that.

Plants don't fix/unfix nitrogen AFIK, but a lack of nitrogen fixing bacteria combined with nitrogen releasing bacteria would slowly convert nitrogen compounds to gaseous nitrogen and possibly have adverse effects on soil ecology.

The minimum gaseous nitrogen required for 'normal' soil may be anything from an insignificant partial pressure up to the usual 80%.

Supporting nitrogen fixing bacteria is energy intensive for plants and is only done in soils with inadequate fixed nitrogen.

 

 

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Posted (edited)
On 1/5/2019 at 4:57 PM, Prometheus said:

Any other long or short term health effects you can imagine?

Ears.

Edited by dimreepr

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1 minute ago, Carrock said:

Thanks for your reference confirming that plants don't fix/unfix nitrogen.

yw :rolleyes:

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2 hours ago, dimreepr said:

Ears.

Don't worry, you can still wiggle them in 0.2 bar oxygen.

 

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7 hours ago, John Cuthber said:

I'm not sure what effect that will have but I suspect that the body's feedback systems may well be able to deal with it.

I was conflating evaporation with boiling. 

I got a book from the library, Fundamentals of Aerospace Medicine 3rd ed. Talking about pressures used in past missions it lists an advantage of normal pressure systems is that the 'gas density is adequate for atmospheric cooling of people and electronic equipment'. This implies lower pressure systems could have over-heating problems, but it does not elaborate on how.

It also lists it's optimum for the efficient functioning for the human respiratory system, but again does not elaborate on how low pressure systems would be detrimental.

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