Air Pressure


Feeling the pressure…

Recently, I’ve been attempting to re-read a book. I first read it back in ’90s, and didn’t get that much from it. I thought I might enjoy it more now – unfortunately, though, I’m actually liking it even less. Without naming the guilty, it’s sort-of a space colonisation story. You could sum the plot up as Rugged Libertarian Authority Figure(tm) is forced to Take Control and show a bunch of useless strawman liberals what’s good for them. (The authors do not appear to notice the inherent irony of a libertarian authority figure, either.) Few of the characters are at all sympathetic – the only one who is has severe brain damage and thus the IQ of a 10-year old. Also, we’re repeatedly assured that the colonists are a special elite, selected for their inherent genetic superior-ness to ordinary, scummy, untermensch humans. (That is, you and me. Just so we’re clear on this.)

Given my current politics, I found all of this utterly irritating.

I don’t think I’ll bother to finish the book. But there’s actually worse, though. You see, it’s supposedly hard SF. Now, I like hard SF. But what I don’t like is schoolboy research errors. (For instance, in this book, the authors seem to think that Tau Ceti is an F-class star – it’s really, really not). Also, the book is supposedly about the impact of an alien environment on the colonists. (I say ‘suppposedly’ what it actually seems to be about is an excuse for their uber-manly Heinleinian poster-boy to flex his manly Marty Stuish pecs at the readers.) However, the environment makes a surprisingly small impact on the characters. In particular, two critical world-building factors are entirely ignored.

These are surface gravity and air pressure.

I’ll deal with gravity some other time, but let’s talk about air pressure.


For any hypothetical colony on another hypothetical terrestrial planet, air pressure is going to be a very, very significant factor in the colony’s viability. Air pressure has huge human-health implications and is not simply something that can be waved away.

Where does air pressure come from? The answer is weight – the atmosphere around you feels gravity, just like you do. Imagine a column of air, extending up from the surface of the Earth out into space. The ‘pressure’ is the weight across the area at the bottom of that column. And it’s quite high – the average number for sea-level air pressure on Earth is 101,325 Newtons per square metre*. (101,325 Pa = 1013.25 millibars, the standard sea-level atmospheric pressure.) This is equivalent to the weight of about 10 tons of rock, spread over 1 square metre.

That’s rather a lot. So, why don’t you feel a horrible sense of being squeezed?

Because your body continually has an equal and opposite counter-pressure. You push outwards at 101,325 Pa, thus resulting in a net force of zero. So you don’t feel anything.

But that’s true at sea level. What happens in other conditions?

The limiting case for pressure is hard vacuum, a pressure of effectively zero Pa. If you’re exposed to this, the consequence is called ‘explosive decompression’. Now, contrary to popular myth, explosive decompression doesn’t actually quite make people pop – the human body has a lot of internal strength. However, it certainly isn’t healthy. Explosive decompression will almost certainly kill you, even on a very short exposure.

Now, if we’re talking colonisable planets, then we’re presumably not talking high vacuum surfaces. So, what about intermediate air pressures?

Perhaps surprisingly, there is some flexibility.

Now, as well as mechanical pressure, there’s another issue. With declining air pressure, the availability of oxygen will also decline. Each lungful of air scoops up less and less air as the pressure falls, and this reduces the amount of oxygen available for the metabolism. This is believed to be one of the factors underlying altitude sickness.

In most people, altitude sickness doesn’t present until an altitude of at least 2,000 or so metres. This corresponds to an air pressure of about 794 millibars, or about 78% of sea-level. (However, it should be noted that some particularly-sensitive people may start to feel it even lower down.)

At 2,400 metres, or about 748 mb, altitude sickness becomes more likely to appear. It doesn’t necessarily occur immediately – symptoms are often delayed as much as six to ten hours after reaching altitude. Also, not everyone will be affected at this level – in fact, perhaps most people won’t.

Now, let’s go up further, to say 4,200 metres, or an air pressure of about 600 mb. Up here, as many as half of people will experience some sort of altitude sickness.

Now let’s keep on going up. The higher up you go, the more likely you are to have altitude sickness, and the worse it will be. But when you reach about 8,000 metres, or 356 mb, you hit the so-called ‘Death Zone’. The Death Zone is the region in which acclimatisation is impossible – a lungful of air simply cannot supply oxygen faster than your body burns it off. A prolonged stay in the Death Zone is guaranteed to end in death. Singificantly, this region on Mount Everest is – quite literally – littered with the corpses of people who’ve succumbed to it.

So we can definitely set 356 mb as the absolute lower limit. A planet with a lower air pressure absolutely can’t support exposed human life. The only way such a world could ever be colonised would be through some sort of pressure-dome system.

But this is an over-simplification. Some people can adapt to ~400 mb, maybe, but probably most can’t. So, if we’re considering permanent human habitation, realistically we’re looking at a lower altitude.

The highest city in the world, according to the Guiness Book of Records**, is Wenzhuan in China, at an altitude of 5,100 metres. (Or possibly La Rinconada in peru – there seems to be some disagreement over it.) These are places where large populations of people exist for extended periods. The air pressure at this level would be about 550 mb. This tends to suggest that ~500 millibars represents a ‘hard’ lower limit.

But this is all quite dry and technical. What does altitude actually feel like? Let’s finish with a little qualitative description of what altitude feels like.

The answer is, not nice.

My personal ‘altitude ceiling’ seems to be somewhere above 3000 metres. I’ve been observing at the WHT in La Palma (altitude 2,500 or so metres) and it was pretty much business as normal. I felt a bit light-headed for the first hour or so, but other than that, I didn’t notice any difference. (Except when I stupidly ran up four flights of stairs … but that was a pretty stupid thing to do, and I’d be quite knackered even at sea-level anyway.)

Last year, I went to the Mauna Kea Observatory in Hawaii. At Hale Pohaku, the observatory residency, I was pretty much fine. I felt a little out of breath after vigorous walking, but other than that things were pretty much as normal. (This corresponded to about 720 mb.)

But the observatory itself is at 4,200 metres, or about 618 mb. And there was a real difference there. The first few hours of the nights at the telescope were fine, but after that, a sense of lethargy set in. With it came headaches and a vague feeling of nausea. It felt like either a bad cold, or an intermediate hangover. It really wasn’t much fun, and I wasn’t any use to anyone for either night. Frankly, I wouldn’t really want to spend more than a couple of days up there.

So, based on my own experience, I could imagine a human society existing happily enough at 750 mb (La Palma), but not so much at 600 mb (Mauna Kea). And I wouldn’t even want to find out what 500 mb feels like!
_____________
*The unit for pressure is the Pascal, which is defined as one Newton of weight spread over one square metre. So the pressure for the Earth’s atmosphere can also be written as 101,325 Pa, or indeed 101.325 kPa.
**Via source.

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2 Responses to “Air Pressure”

  1. One of the things I quite like about Mass Effect is that it seems to be relatively hard sci-fi, once you accept the ‘science fairy’ of Element Zero, anyway. I’m not sold on their explanation of FTL, but other than that, space combat adheres to newtonian/einsteinian physics, and various background fluff includes reference to such things as surface gravity, air pressure and environmental allergens as factors in colonisation/resource exploitation efforts. There’s even one bit of fluff in ME2 which comments on something most sci-fi with FTL ignores: A planet is mysteriously destroyed at the height of a war, so some enterprising so-and-so’s travel out to the edge of the light-cone of the explosion, to try and see what happened.

    • Oh yes, the good old light-cone trick! There’s a thing like that in (I think I recall)a Jack McDevitt novel – I think it was Slow Lightning – where they have some missing signals or something. (It’s been ages since I read it, so I’m probably garbling this horribly…) Anyway, they retrieve them by doing much the same thing.

      I actually have Mass Effect (1, that is), although I bought it just before I had the big hard drive blow-up last year, so I didn’t get so far into it. I really need to dust it off at some point, as I’ve heard lots of good things about it.

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