Actually, my SO2 limit listed there is way too generous. Like, I probably dropped some zeroes somewhere. SO2 cannot be adapted to like CO2; it's like breathing acid. Chronic exposure to levels as low as 50 ppm seems to cause coughing and burning sensations in most people, and brief exposure to (unspecified) higher concentrations, even for just a few hours, can continue to cause life-threatening symptoms days later and respiratory complications that last for years.

5 ppm (0.000005 or 10[sup]-6[/sup] atm) is a more realistic upper limit for SO2, but even that might be pushing it. Most of this data comes from adults exposed for hours at a time, not infants and children breathing it from birth.

Speaking of SO2, have the devs ever addressed it? People here seem to agree that it's unrealistically common.

How livable is my custom planet?

i've neglected the forums a bit, so here's a breathable i found; unfortunately, it isn't very habitable...
as far as i know, this is the first 100% breathable atmosphere in 0.990 (low CO2, no SO2, little to no CO, above 0.15 atm partial pressure O2, etc...)

here's another breathable i found a couple weeks back. there's a bit too much CO2 for my liking, but i don't think i'll find anything better.

this is also pretty habitable, but there's still a few glaring flaws i want to fix in the future (aka i want to find a better planet)

Is the atmosphere of this planet breathable?
The Oxygen and H2O levels look too high but I'm not sure.

Whoops nevermind, I totally misread the SO2 levels.

Some of the best planets I've found so far.

Image 1: SO2 is actually within safe levels, but the O2 is too high, of all things. Might be more livable at high altitudes.
Image 2: More breathable, but much drier.