The six categories
| Category | Colour | AQI | What it means |
|---|---|---|---|
| Good | Green | 0 to 50 | Air quality is satisfactory, and air pollution poses little or no risk |
| Moderate | Yellow | 51 to 100 | Acceptable, but there may be a risk for some people, particularly those unusually sensitive to air pollution |
| Unhealthy for sensitive groups | Orange | 101 to 150 | Members of sensitive groups may experience health effects; the general public is less likely to be affected |
| Unhealthy | Red | 151 to 200 | Some members of the general public may experience health effects; sensitive groups may experience more serious effects |
| Very unhealthy | Purple | 201 to 300 | Health alert: the risk of health effects is increased for everyone |
| Hazardous | Maroon | 301 and higher | Health warning of emergency conditions: everyone is more likely to be affected |
The categories are AirNow's, with its health messages in shortened form. The step that matters most is from yellow to orange at 100: below it, air quality is generally considered satisfactory; above it, sensitive groups first and then everyone may be affected.
Why 100 is the line
The index is anchored to a legal standard. AirNow explains that an AQI of 100 generally corresponds to the short-term national ambient air quality standard, the concentration the EPA sets to protect public health. The rest of the scale is built around that point, which is why 100 and not 50 is where the warnings begin.
The five pollutants
The EPA calculates an AQI for five major pollutants regulated under the Clean Air Act:
- Ground-level ozone
- Particle pollution, both fine particles (PM2.5) and coarser ones (PM10)
- Carbon monoxide
- Sulfur dioxide
- Nitrogen dioxide
Forecasts and maps usually name the pollutant behind the number, which tells you whether the day's problem is ozone, particles or another gas.
Reading a forecast
- Check the category, not just the number. The colour tells you at once whether the day falls above 100.
- Check the pollutant the forecast names, since the five behave differently.
- Check the time the value refers to, a day or an hour.
- Mind the scale. This is the US index. Other countries publish their own scales with different breakpoints.
Air quality is one of several numbers in a summer forecast. The heat index calculator shows how hot it feels, the dew point calculator how humid, and the UV index guide covers the third number in a summer forecast. The sunrise and sunset calculator helps plan outdoor time around the cooler early and late hours.
Frequently asked questions
What is the Air Quality Index?
A scale the US Environmental Protection Agency uses to report daily air quality, from 0 to above 300. It covers five major pollutants regulated under the Clean Air Act: ground-level ozone, particle pollution (PM2.5 and PM10), carbon monoxide, sulfur dioxide and nitrogen dioxide. The higher the number, the greater the health concern.
What does an AQI of 100 mean?
According to AirNow, an AQI of 100 generally corresponds to the level of the short-term national ambient air quality standard set to protect public health. Values at or below 100 are generally thought of as satisfactory; above 100, air quality is considered unhealthy, first for sensitive groups and then for everyone as the number rises.
What do the colours mean?
Each category has a colour so it can be read at a glance: green for good, yellow for moderate, orange for unhealthy for sensitive groups, red for unhealthy, purple for very unhealthy and maroon for hazardous.
Is the AQI the same in every country?
No. This guide describes the US EPA index used by AirNow. Other countries use their own scales with different breakpoints and colours, so a number from one country's index cannot be read on another's scale.
Sources
General information based on the sources above, not medical advice.