How close to Earth, in terms of atmosphere, would another planet have to be for humans to survive there relatively unaided?
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What do you mean by “how close”? That’s a subjective question. At minimum, it would have to have an atmospheric pressure of no less than 197 millibars (roughly 19.5% of Earth sea level pressure) **if** the atmosphere is pure oxygen. If it’s not pure oxygen, the pressure needs to be higher. If you’re ok with needed supplemental oxygen, you can get the total atmospheric pressure down to about 62.6 milibars, (about 6.1% of Earth sea level pressure) but anything less than that and you would just need a full space suit.
The atmospheric temperatures, at least on some part of the planet, would have to be pretty consistently be within a range that humans can tolerate, so roughly the same as here on Earth.
It would need to *not* have any gases, vapors, or other contaminants in high enough concentrations to be toxic, and there are far too many of those to list here.
You’d also need an ozone layer, otherwise you’d get fried by ultraviolet radiation.
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