On days when pollution is off the charts, if I leave the purifier on level 2, the Air Quality (AQ) easily hits 200-250. I can't stand the Mi purifiers with their lousy sensors (and who knows if Philips is any better), but I guess it's better to have them, even if they're just giving me a ballpark figure. I've noticed that even cranking the purifiers to level 2 doesn't always do the trick – sometimes, the pollution is still too high, and I have to play detective to find out where the nasty air is creeping in from. Don't get me wrong, fresh air is a must, but if I'm stuck in bad air for too long, it's like the purifier is only doing half its job. Like, there are times purifiers are chilling at level 2 while the AQ is hitting 250 and above.