Your weather app shows an air quality number, green or amber or red. Most people read the colour and ignore the number.
Sources
Air quality index methodology: raw pollutant concentrations converted onto a common scale, with the index reporting the highest sub-index among the pollutants measured, and bands set where health advice changes
Sensitive groups as defined by air quality agencies: people with asthma or other lung disease, heart conditions, older adults, children, and pregnant people
That index formulas and band edges differ between countries, so the same number does not carry the same meaning across systems