Delhi’s September AQI was lowest in 5 years. So why did PM2.5 and PM10 rise?

Although Delhi recorded its lowest September average Air Quality Index (AQI) in five years at 102, individual pollutant levels like PM2.5 and PM10 actually went up during the month.

While this seems contradictory, it comes down to how the overall AQI is calculated—which tracks the single worst pollutant on any given day. So, even if PM2.5 or PM10 rose slightly, they were not high enough to drive up the overall daily index, resulting in a lower composite AQI score overall despite the increase in particulate matter.

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