I have 5 mil. atmosphere, and it's slowly going up, as well as an option to build a paste boiler, which "Leaks atmosphere into the sky" but I'm holding off on building because, well, I don't know what atmosphere does.
After 5 million it allows for terraforming of your planet (wood, grass and livestock) and can be converted to water via atmospheric condenser. However it should not exceed over 15 million, due to it then becoming toxic to your colonists.
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Though the fact that being over 15m makes colonists sick hasnt been implemented yet, so it does nothing if you exceed the limit. But it may come in the future so I recommand controlling that.
I'm a newer user and I'm confused on atmosphere as well. I have a human world which has declared independence. I just built a star port but it apparently requires zero atmosphere. If I reduce my atmosphere to zero (not going to be easy!), won't that kill all my colonists?
Sorry, never mind. I figured it out - apparently it was somehow deactivated.
It won't kill your colonists. You can reach 15m to unlock all techs and go back to 0.