Atmospheric issues.

[SMJB]

Let's say we have a planet with an earth-like atmosphere, but thicker. Much thicker. About how thick can it get before humans can no longer inhabit the surface full-time for whatever reason?

What I'm looking for, really, is a situation where people can survive on the surface for a while if needs must, but would much rather return to the higher atmosphere as soon as possible.

[Borgholio]

Well if you're talking normal breathing without a respirator, about 2 atmospheres I think. Beyond that you need to do what Scuba divers do and start adding helium to the mix and decreasing the amount of oxygen.

If you're allowing the inhabitants of the planet to wear gas masks, you could theoretically survive dozens of atmospheres...so long as you were prepared to use a decompression chamber before you went back to Earth normal pressures.

[Napoleon the Clown]

Looking at stuff on Google suggests that one can avoid oxygen and nitrogen toxicity for a surprising number of atmospheres of pressure. This hypothetical world could get to some really impressive degrees if the gasses present. Every 10 meters is an additional atmosphere of pressure, and oxygen becomes toxic at about the 63m mark, if what I'm reading is accurate.

So up to four atmosphere would be pretty safe, though this number may change based off of activity level. The pressure wouldn't really make it harder to breathe since the air in your lungs would be equally adjusted. The human body is surprisingly good at adapting to changes in the atmosphere around it. Hell, it's possible to be in a hard vacuum for over a minute and not die, though you need to go into it right. (Protip: Don't try to hold your breath)

[krakonfour]

The gravity of the surface determines how many meters of atmosphere it can retain.
Take the maximum amount of meters, the corresponding gravity, and work out how much heavier the planet is.
Divide that mass by the planet's average density; you'll get volume.
From the volume, use the sphere forumla to work out a radius.

[Terralthra]

The gravity of the surface determines how many meters of atmosphere it can retain.
Take the maximum amount of meters, the corresponding gravity, and work out how much heavier the planet is.
Divide that mass by the planet's average density; you'll get volume.
From the volume, use the sphere forumla to work out a radius.

That doesn't even begin to answer the question.

[krakonfour]

About how thick can it get before humans can no longer inhabit the surface full-time for whatever reason?

Maybe not.

[Irbis]

The gravity of the surface determines how many meters of atmosphere it can retain.
Take the maximum amount of meters, the corresponding gravity, and work out how much heavier the planet is.
Divide that mass by the planet's average density; you'll get volume.
From the volume, use the sphere forumla to work out a radius.

Which is why Titan can retain atmosphere 1.25 times as massive as Earth's (with 1.5x surface pressure) despite having grand total of 0.066 Earth's volume?

[krakonfour]

The gravity of the surface determines how many meters of atmosphere it can retain.
Take the maximum amount of meters, the corresponding gravity, and work out how much heavier the planet is.
Divide that mass by the planet's average density; you'll get volume.
From the volume, use the sphere forumla to work out a radius.

Which is why Titan can retain atmosphere 1.25 times as massive as Earth's (with 1.5x surface pressure) despite having grand total of 0.066 Earth's volume?

Because its density is much higher than earth? It's much colder, so the gas weighs more per meter cubed. The actual density is 1.45 that of Earth.
Also, there is about 20% gas in total than on Earth, because tectonic activity induced by Saturn causes volcanoes to erupt spontaneously and release a lot of gas, more than on Earth.

[SMJB]

Venus has something like eight times the atmospheric thickness of Earth.

Somehow, I hadn't even considered SCUBA-like breath masks. That seems to open up some possibilities--only the air at the altitude where people are going to live needs to be Earthlike. What if the ground levels are saturated with, like, way too much carbon dioxide? I heard that's a relatively heavy gas, and it's obviously not going to be harmful to the planet's vegetation.

[Bedlam]

Venus has something like eight times the atmospheric thickness of Earth.

Somehow, I hadn't even considered SCUBA-like breath masks. That seems to open up some possibilities--only the air at the altitude where people are going to live needs to be Earthlike. What if the ground levels are saturated with, like, way too much carbon dioxide? I heard that's a relatively heavy gas, and it's obviously not going to be harmful to the planet's vegetation.

Actually at a high enough concentration it might be, plants need to respire as well as photosynthesise and they need oxygen for that. Over time they should normally produce more oxygen than they require but over short periods they may need to 'import' oxygen.

Of course the plants on this planet may just be more effective as scavenging it or storing it until needed.

You would also need to think of the greenhouse effect of increased Carbon Dioxide levels.

[energiewende]

The system as a whole is a bit too complicated for an easy answer.

But simplified, a denser atmosphere would be beneficial for humans, since the oxygen concentration would be higher. We would be able to metabolise sugars more quickly, expending more energy in movement, but we don't run fast enough to be mainly limited by air resistance, so we could run faster. Survivable pressures are quite high; realistically it will be limited by the temperature of the atmosphere long before the pressure (and they're related, in a complex way).

If you're counting external mechanical aids, well, people have survived at the bottom of the Mariana Trench at 10,000m and a pressure of 110MPa (~1,000x atmospheric pressure). Theoretical capability would be even higher.

Which is why Titan can retain atmosphere 1.25 times as massive as Earth's (with 1.5x surface pressure) despite having grand total of 0.066 Earth's volume?

His methodology is a bit confused but he's approximately right in terms of retaining an atmosphere with no external sources. Titan's atmosphere is believed to be constantly replenishing from methane vents. There is also a dependence on the temperature of the atmosphere (hotter is worse for retention) and the molecular mass of the gas in question (lower is worse for retention).

[Irbis]

But simplified, a denser atmosphere would be beneficial for humans, since the oxygen concentration would be higher. We would be able to metabolise sugars more quickly, expending more energy in movement, but we don't run fast enough to be mainly limited by air resistance, so we could run faster.

And we also would die really fast, too!

In case you don't know: oxygen in high concentrations is toxic. In high concentrations plus high pressure it only does such nice things as oxidation damage to cells, causes collapse of cavities in your lungs, and literally takes apart your eyes as it turns out they're built for normal pressure.

If you're counting external mechanical aids, well, people have survived at the bottom of the Mariana Trench at 10,000m and a pressure of 110MPa (~1,000x atmospheric pressure). Theoretical capability would be even higher.

Which says about zero about human capabilities, as they did it in craft comparable in complexity to spaceships. Real humans (and we're talking professional deep divers with excellent condition) never crossed 500-700 metres of depth, and that in simulations.

[SMJB]

Actually at a high enough concentration it might be, plants need to respire as well as photosynthesise and they need oxygen for that. Over time they should normally produce more oxygen than they require but over short periods they may need to 'import' oxygen.

Of course the plants on this planet may just be more effective as scavenging it or storing it until needed.

You would also need to think of the greenhouse effect of increased Carbon Dioxide levels.

The CO2 levels only need to be high enough to be dangerous to humans.

Real humans (and we're talking professional deep divers with excellent condition) never crossed 500-700 metres of depth, and that in simulations.

Which is still 50-70 atmospheres, according to the 10m = 1atm rule. Which seems impractical; I'd better go with the heavy gasses thing.

[krakonfour]

I'm unsure of something: Will higher pressures change the composition of the atmosphere? How?

[Guardsman Bass]

Wikipedia is saying that Nitrogen Narcosis starts to become noticeable above 2 Bars of pressure. It's not really bad until you get up to and beyond 4-6 bars of pressure, but it would put a limit on where you could go with a normal nitrogen-oxygen mix.

[SMJB]

Wikipedia is saying that Nitrogen Narcosis starts to become noticeable above 2 Bars of pressure. It's not really bad until you get up to and beyond 4-6 bars of pressure, but it would put a limit on where you could go with a normal nitrogen-oxygen mix.

See, now that sounds doable. About how long can one survive at, say, 3 bars of normal atmosphere? What are the effects of nitrogen narcosis and the timeline? How does strenuous activity effect this?

[krakonfour]

I was trying to prove through my question that for the OP's scenario to be valid, we have to throw out the assumption of an Earth-like atmosphere, since the increased pressure would change its composition.

Sorry for interrupting.

The effects of 3 bar nitrogen are not fatal, and do not increase (worsen over time). According to wiki,
2–4 bars
Mild impairment of performance of unpracticed tasks
Mildly impaired reasoning
Mild euphoria possible

Only from 10 bars can we possibly have death and unconsciousness certainly occurring. I think oxygen toxicity is a bigger danger before that.