How to Understand Air Quality
Updated 2026-01-10
What the index measures
An Air Quality Index condenses measurements of several pollutants into a single scale so it's easy to judge air quality at a glance. Lower numbers mean cleaner air; as the index rises, health risk increases, first for sensitive groups and eventually for the general population at very high levels.
The pollutants behind the number
The index is typically driven by whichever pollutant is currently highest relative to health guidelines — commonly fine particulate matter (PM2.5 and PM10), ground-level ozone, nitrogen dioxide, sulfur dioxide or carbon monoxide. PM2.5 in particular is small enough to be inhaled deep into the lungs, which is why it's weighted heavily in most air quality calculations.
Who should pay closer attention
People with asthma or other respiratory conditions, older adults, children and pregnant people are generally more sensitive to elevated pollution levels and may want to reduce prolonged outdoor exertion sooner than the general population, even at "moderate" readings.