Gasses that can be present on a planet habitable by humans

[capslfern]

TL;DR Summary: what kinds of gasses can be in the atmosphere of a planet without killing them or not being naturally possible

I have been creating a fictional star system and ran into an issue, most of the planets atmospheres are not all the interesting, feels wrong to have most be oxygen, nitrogen, and some noble gas, what gasses could be in a planets atmosphere that aren't too rare in nature or will kill humans, or is just oxygen-nitrogen fine. I would rather they at least be somewhat possible to occur naturally on a planet

 

[Baluncore]

Welcome to PF.

Only oxygen is needed by humans.

Any reactive gasses will be removed from the atmosphere by the excess oxygen and starlight. That includes hydrogen, methane, ...

Oxygen concentration will become stable when the planet's surface cannot be oxidised further. Water and most rocks are oxides.

 

[capslfern]

so no reactive gasses, what gasses could exist, and how many would be toxic?

so anything that can be oxidized with oxygen in the atmosphere pretty much has been

 

[Baluncore]

so no reactive gasses, what gasses could exist, and how many would be toxic?

Probably the most toxic would be CO₂, that will dissolve in your blood, making it acidic with carbonic acid, which will kill you.
https://en.wikipedia.org/wiki/Hypercapnia#Tolerance

Vegetation will spontaneously burn in oxygen, which releases mostly CO₂ and water. The CO₂ gradually dissolves in water, (sea water), and slowly precipitates as carbonates such as CaCO₃, limestone.

CO₂ is used by plants, so you will need vegetation on the planet for a long period before you get there, that should remove most of the remaining toxic CO₂ from the atmosphere.

The oxygen concentration of the atmosphere must be less than about 25% or spontaneous fires will break out, all over your body. That will require "inert" filler gasses, such as argon or nitrogen, to stabilise the atmospheric pressure.
https://en.wikipedia.org/wiki/Apollo_1

Without sufficient atmospheric pressure, water will boil from your body at your body temperature and you will dehydrate.
https://en.wikipedia.org/wiki/Armstrong_limit

You will need water dissolved in the air, or breathing through your lungs will dry out your body. The atmosphere must be cooler than your body, or you will suffer heat stroke. The warmer air you breathe out will contain more water than that you breathe in, so you will need to drink water.