Explainer: What does an AQI of 450 mean? Is it just a winter thing?

At 450, the concentration of pollutants in the air "affects healthy people and seriously impacts those with existing diseases"

The air quality index (AQI) in Delhi reached a high of 450 on November 3, and has been topping the 300 mark almost consistently between October 27 and November 8 this year. Yesterday, for the first time in 15 days, the AQI was below 300, at 260.

While Delhi's air quality, on which mainstream media focuses, improved slightly, smaller towns--Ambala, Khanna, Kurukshetra and Ludhiana--had severely polluted air (AQI over 400) on November 9, according to the air quality index bulletin published by the Central Pollution Control Board (CPCB).

But what does having an AQI over 400 really mean?

For one,