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Your weather app shows an air quality number, green or amber or red. Most people read the colour and ignore the number.
An air quality index number is:
Because the pollutants are different substances measured in different units, and nobody can hold five of those in their head.
One scale makes them comparable.
Several pollutants are measured, each converted separately, and the index shows the highest.
So the number is about one pollutant at a time, not a blend.
Two identical numbers can mean different things — one driven by wildfire smoke, another by traffic ozone.
The apps usually say which, in smaller text.
The colours mark where advice changes: fine for everyone, then a caution for sensitive groups, then for everyone, and so on upward.
People with asthma or other lung conditions, heart conditions, older people, young children, and people who are pregnant.
That is a large slice of any street.
On a bad air day, which actually helps?
Tap an item, then a bucket
Exertion multiplies how much air you move through your lungs, and deeper breathing carries particles further in.
So the same air is a different dose depending on what you are doing.
The one usually driving the number is fine particulate — small enough to reach the deepest part of the lung and cross into the blood.
Different countries use different formulas and different band edges. A number from one system does not mean the same as the same number from another.
Compare within a system, not across.
It is a converted risk score, not a measurement, and it reports the single worst pollutant. The colours tell you when advice changes. And what you are doing outside matters as much as the number.