What Air Quality Index Numbers Mean
AQI scales compress pollutant readings into one number from 0 to 500. The number alone is meaningless without the scale, dominant pollutant, and local breakpoints.
When a weather app shows “AQI 142,” the number is doing a lot of compression. It folds together up to six pollutants — particulate matter, ozone, carbon monoxide, sulfur dioxide, nitrogen dioxide, and sometimes lead — and reports the worst of them on a single scale. The result is convenient but easy to misread.
To use AQI well you need three things: the scale, the breakpoints, and the dominant pollutant.
The basic idea
An Air Quality Index is a piecewise-linear function from a pollutant concentration to a number on a fixed scale (often 0 to 500). The function is calibrated so that the number bands map onto health categories.
The U.S. EPA scale, the most widely cited:
| AQI | Category | Color |
|---|---|---|
| 0–50 | Good | Green |
| 51–100 | Moderate | Yellow |
| 101–150 | Unhealthy for sensitive groups | Orange |
| 151–200 | Unhealthy | Red |
| 201–300 | Very unhealthy | Purple |
| 301–500 | Hazardous | Maroon |
The number you see is computed for each tracked pollutant separately, and the headline AQI is the maximum across all pollutants. So a city with PM2.5 at 95 and ozone at 130 will report AQI 130, with ozone listed as the limiting pollutant.
The pollutants that drive most readings
Globally, two pollutants account for the large majority of high AQI values:
- PM2.5 — fine particulate matter under 2.5 µm. Comes from combustion (vehicles, wood fires, industrial sources) and from secondary aerosol formation. Penetrates deep into the lungs and crosses into the bloodstream.
- Ozone (O₃) — formed when sunlight reacts with nitrogen oxides and volatile organic compounds. Causes acute respiratory irritation and worsens asthma.
PM2.5 dominates winter readings (heating and inversion-trapped pollution); ozone dominates summer afternoons in sunny cities.
PM10, NO₂, SO₂, and CO play smaller roles. They can be the limiting pollutant near specific sources (a coal plant for SO₂, a busy intersection for NO₂) but rarely drive the headline AQI for an entire city.
Why scales differ between countries
The same pollutant concentration can produce very different AQI numbers depending on the system.
- U.S. EPA uses 24-hour averages for PM2.5 and 8-hour averages for ozone. Breakpoints are based on the U.S. NAAQS health standards.
- European EAQI uses a 1–5 scale tied to EU air quality standards.
- Chinese MEP uses a 0–500 scale with breakpoints tightened from the U.S. version.
- U.K. Daily Air Quality Index uses a 1–10 scale.
A U.S. AQI of 100 is not the same as a U.K. AQI of 5 or a Chinese AQI of 100. They were designed for different regulatory contexts. When you compare AQI across borders, you are mostly comparing scales.
How to read AQI safely
Three habits avoid the most common mistakes.
Look at the dominant pollutant
A reading of “AQI 130, dominant: ozone” tells you to limit outdoor exercise on a sunny afternoon. A reading of “AQI 130, dominant: PM2.5” tells you that even indoor air quality may be elevated and a mask might help outdoors. The category is the same; the right response is not.
Pay attention to averaging windows
AQI values are usually reported as 24-hour averages or 8-hour averages. A spike from a passing wildfire smoke plume might briefly hit an instantaneous concentration well above the 24-hour average. The displayed number may lag the air you are actually breathing by hours.
Treat the headline number as a category, not a precise score
A jump from AQI 95 to AQI 105 changes the band from “moderate” to “unhealthy for sensitive groups,” but the underlying pollutant change is small. Use AQI to decide which category you are in; don’t try to interpret a single-point change as meaningful.
Sensitive groups
AQI categories above 100 explicitly call out sensitive groups. The list is fairly stable across health agencies:
- Children and adolescents (developing lungs).
- Adults over 65.
- People with asthma, COPD, or other chronic respiratory conditions.
- People with cardiovascular disease.
- Pregnant people.
- Outdoor workers and athletes (higher exposure due to elevated breathing rate).
For these groups, the threshold for limiting outdoor activity is typically AQI 100, not 150. The “for sensitive groups” warning is the actionable boundary.
A practical reading workflow
- Note the AQI value and the band (good, moderate, unhealthy for sensitive groups, etc.).
- Note the dominant pollutant — this changes which mask, filter, or behavioral response is appropriate.
- Note the averaging window so you know if a recent spike is reflected.
- Cross-check with a second source if AQI is above 150 — different stations and different scales can disagree by 20+ points.
Air quality is one of the most-improved monitoring areas of the last decade. Most cities now publish hourly readings from dozens of stations, and consumer-grade sensors can supplement official data. The number is more useful when you remember it is a compressed summary, not a precise quantity.