When wildfire smoke was blowing into the US from Canada, you could feel your eyes stinging in seconds at like 250 AQI, and at 400 you could see/smell/taste/feel the smog even inside of buildings. It was supposedly like smoking a pack of cigarettes a day.
999 means move. There's no convenience that a city offers that is worth years and years of your life, and a lower quality of life during the many fewer years you do have. I'd opt for subsistence farming without electricity in the most remote village over living in pollution that bad.