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.