Compared to average outdoor levels.
"Significant" is subjective here, but it needs to be a large enough effect that people would actually care to fix their air quality. (For example, asbestos has a "significant" health risk.)
Resolves once there's a well-established scientific consensus.
fwiw it's sometimes 2k ppm inside my house and it's not obviously super bad although I would guess it's noticeably stuffier. My unconfident guess is that noise pollution is worse for working (which is why I currently sometimes close my windows at high CO2 ppm despite open windows being helpful for reducing CO2)
@NoaNabeshima I notice that you think 4% productivity drop is significant in another market. I think noise pollution probably drops productivity by more than 4%, so shrug