AQI is based on Particulate Matter, Ozone, NOx, SO2, and CO.
Because there are multiple pollutants measured, with different hazardous concentrations, reporting the raw ppm isn’t super useful. Additionally, on an acute basis there is no cumulative impact for multiple pollutants.
What this means is that the AQI is based on the worst case pollutant at a given time. There’s an index that is, through some reasonably sophisticated statistical calculation, normalized across the hazard levels for all pollutants. Basically a 150 AQI means a similarly hazardous level of Ozone OR Particulate Matter etc.
So it’s basically “what pollutant is worse right now?” Followed by “what is the relative level of the specific pollutant”
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