I was in Beijing a few weeks ago when the AQI was in the mid to high 200s. While it looked nasty, what really surprised me was that I couldn't really sense that I was breathing polluted air – it didn't smell bad and it wasn't difficult to inhale. The only indicator that I was breathing bad air was the cough I developed after a week in the city. I attributed it to a traveling cold but it quickly went away when I returned to the relatively clean Manhattan air.