Dangerous gases and breathable atmosphere are a binary, they’re either met or not, CC cost 2 or 0. This is unrealistic: you can supplement a pure nitrogen atmo with minimal oxygen in an open nose feed, which would be much easier (e.g. lower CC) than working in full vacuum. More importantly from an Aurora perspective, that’s totally different than temperature, hydro extent, and pressure, which are gradients.
I propose making the CC from breath gas and dangerous gases a gradient (whatever shape, linear or nonlinear, that makes sense to Steve). I’d also suggest making breath gas a gradient between 0 and 1 CC, since it’s a simple problem to solve if you already have an atmosphere at a decent pressure without dangerous gasses.