The scale
What the Air Quality Index actually measures
The AQI is a 0 to 500 scale that converts pollutant concentrations into one number a person can act on. It is not a measure of how much pollution is in the air. It is a measure of how much health effect that pollution is expected to produce, which is why 100 means roughly the same thing whether the pollutant that day was ozone or fine particles.
An AQI of 100 corresponds to the level of the national ambient air quality standard for that pollutant. Below 100 is generally considered acceptable; above it, the concern widens from sensitive groups to everyone as the number climbs.
The six categories
What each band means
| Category | Index | What it means |
|---|---|---|
| Good | 0-50 | Air quality is satisfactory and air pollution poses little or no risk. |
| Moderate | 51-100 | Acceptable for most, but there may be a risk for people unusually sensitive to air pollution. |
| Unhealthy for sensitive groups | 101-150 | People with lung disease, older adults and children are at greater risk from ozone; people with lung or heart disease, older adults and children are at greater risk from particles. |
| Unhealthy | 151-200 | Some members of the general public may experience health effects; sensitive groups may experience more serious effects. |
| Very unhealthy | 201-300 | Health alert: the risk of health effects is increased for everyone. |
| Hazardous | 301-500 | Health warning of emergency conditions: everyone is more likely to be affected. |
Behind the number
The five pollutants the index tracks
Fine particles (PM2.5)
Particles under 2.5 micrometres across - small enough to reach deep into the lung and pass into the bloodstream. Comes from combustion: vehicles, power generation, wood burning and, increasingly, wildfire smoke.
The pollutant behind most unhealthy days in the US today.
Ground-level ozone
Not emitted directly. Formed when nitrogen oxides and volatile organic compounds react in sunlight, which is why it peaks on hot, still, sunny afternoons and in summer rather than winter.
An irritant that inflames airways; the classic summer smog pollutant.
Coarse particles (PM10)
Particles up to 10 micrometres, including dust, pollen and mould spores. Deposits higher in the airway than PM2.5.
Dominant in dust-storm and agricultural regions.
Carbon monoxide
A product of incomplete combustion. Displaces oxygen in the blood. Outdoor levels have fallen sharply since catalytic converters became standard.
Rarely the driving pollutant outdoors now.
Sulphur dioxide and nitrogen dioxide
Both from combustion, sulphur dioxide mainly from coal and industrial processes, nitrogen dioxide mainly from traffic. Both irritate the airway and both contribute to particle formation downwind.
Usually local and industrial rather than regional.
Reading a county page
Why the 90th percentile, not the maximum
County pages here lead with the 90th percentile AQI rather than the annual maximum. The maximum is one reading: a single wildfire day, an inversion, a refinery event. It tells you the worst thing that happened, not what living there is like.
The 90th percentile describes a day worse than nine out of ten - a bad day of the kind that recurs. Together with the count of days above moderate, it gives a much better picture of a year than either the maximum or the average would alone.
Both matter for different reasons. If you have asthma or COPD, the count of days above moderate is the number that describes how often you will notice.
Categories, breakpoints and pollutant coverage from the EPA Technical Assistance Document for the Reporting of Daily Air Quality. County figures on this site come from EPA Air Quality System annual summaries. Lung.com publishes environmental and public-health data. It is not medical advice, diagnosis or treatment, and it is not a substitute for a clinician.
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