AQI
Also called: Air Quality Index
The Air Quality Index (AQI) is the U.S. Environmental Protection Agency's color-coded scale for reporting air pollution. It has six categories, from Good (0 to 50) to Hazardous (301 and higher). Values of 101 and above are unhealthy, first for sensitive groups and then, as the number rises, for everyone.
Why It Matters
The AQI turns pollutant concentrations into one number and one color, so you can decide at a glance whether to exercise outdoors, keep windows closed, or move plans inside. EPA sets an AQI for five pollutants: particulate matter, ozone, carbon monoxide, nitrogen dioxide, and sulfur dioxide.
U.S. AQI Categories, with the Matching PM2.5 Levels
| AQI | Category | Color | PM2.5, 24-hour average (µg/m³) |
|---|---|---|---|
| 0 to 50 | Good | Green | 0.0 to 9.0 |
| 51 to 100 | Moderate | Yellow | 9.1 to 35.4 |
| 101 to 150 | Unhealthy for Sensitive Groups | Orange | 35.5 to 55.4 |
| 151 to 200 | Unhealthy | Red | 55.5 to 125.4 |
| 201 to 300 | Very Unhealthy | Purple | 125.5 to 225.4 |
| 301 and higher | Hazardous | Maroon | 225.5 and higher |
PM2.5 breakpoints are those in effect since May 6, 2024. EPA lowered the top of the Good range from 12.0 to 9.0 µg/m³ and lowered the breakpoints for the top three categories. The 35.5 to 55.4 range did not change.
Source: U.S. Environmental Protection Agency: Final Updates to the Air Quality Index for Particulate Matter
How to Read It
- Green and yellow are fine for most people. At orange, people in sensitive groups may feel effects. At red, some members of the general public may too. Purple is a health alert for everyone, and maroon is an emergency condition.
- AirNow's current readings are the NowCast AQI, EPA's near-real-time estimate built from hourly monitor data.
- The U.S. AQI is one of several national scales. A number on the European or Canadian scale does not mean the same thing.
Frequently Asked Questions
What is a good AQI?
0 to 50, shown in green. Air quality is satisfactory and air pollution poses little or no risk. 51 to 100, yellow, is acceptable for most people.
At what AQI is the air unhealthy?
From 101. At 101 to 150, members of sensitive groups may experience health effects. At 151 to 200, some members of the general public may as well. 201 to 300 is a health alert with increased risk for everyone, and 301 or higher is a health warning of emergency conditions.
What changed in the AQI in 2024?
On May 6, 2024, EPA updated the PM2.5 breakpoints. The Good category now ends at 9.0 micrograms per cubic meter instead of 12.0, matching the new annual standard, and the breakpoints for Unhealthy, Very Unhealthy, and Hazardous were lowered. More days now fall in the Moderate category as a result.
Is the AQI the same as PM2.5?
No. PM2.5 is a pollutant, measured as a concentration in micrograms per cubic meter. The AQI is an index that converts a pollutant's concentration into a single number and a color, so that different pollutants can be read on one scale.
Related Terms
PM2.5
PM2.5 is fine particulate matter: airborne particles 2.5 micrometers or smaller in diameter. An average human hair is about 30 times wider than the largest of them. The particles are small enough to reach deep into the lungs, and some enter the bloodstream, which makes PM2.5 the particle pollution with the greatest health risk.
Red Flag Warning
A Red Flag Warning is a National Weather Service alert that weather conditions could cause extreme fire behavior. It is issued when those conditions are occurring or expected within 24 hours. Typical criteria combine dry fuels with sustained winds of 15 mph or more and relative humidity of 25% or less, and exact thresholds vary by state and local office.
Temperature Inversion
A layer of the atmosphere where temperature increases with altitude instead of decreasing. Inversions trap pollutants and moisture near the surface, causing fog, smog, and haze. They also cap thunderstorm development.
UV Index
The UV index is a forecast of the intensity of skin-damaging ultraviolet radiation expected to reach the ground at solar noon, when the sun is highest. It is 0 at night and rises to 11 or higher, and the higher the number, the faster unprotected skin and eyes are harmed. Sun protection is recommended at 3 and above.
Visibility
The horizontal distance at which objects can be seen and identified. U.S. aviation reports express prevailing visibility in statute miles. Below 3 statute miles falls in the IFR or LIFR weather category, but legal visual-flight minimums vary with airspace, altitude, and day or night under 14 CFR 91.155.
AQI: Guides and Explainers
These explainers connect this definition to real-world forecasting and decision-making.
Helpful Tools and Hubs
Use these pages to apply the concept, compare conditions, or follow related weather workflows.
Tool
AQI Exposure Planner
A free tool for working this out with your own numbers.
Feature
Air Quality
Track AQI, wildfire smoke, pollen, and pollutant breakdowns for your saved places.
Feature
Wildfire Tracking
Follow active wildfires, smoke forecasts, and fire weather conditions.
Feature
Conversational AI
Ask about rain timing, alerts, and what the forecast means in plain language.
Sources
Explore Other Terms
Tornado
A tornado is a violently rotating column of air in contact with the ground, extending from a thunderstorm. Tornadoes are rated from EF0 to EF5 on the Enhanced Fujita scale by the damage they cause. Most last less than 10 minutes, but the strongest can stay on the ground for more than an hour.
Snow Squall
An intense, short-lived burst of heavy snow accompanied by strong winds and often causing a sudden drop in visibility to near zero. Snow squalls can create extremely dangerous driving conditions despite lasting only 30-60 minutes.
Humidity
The amount of water vapor present in the air. Absolute humidity measures the actual water content, while relative humidity expresses how close the air is to saturation at its current temperature, shown as a percentage.
Velocity
Doppler radar measurement of how fast precipitation is moving toward or away from the radar. Velocity data reveals wind patterns, rotation in thunderstorms, and potential tornado signatures that reflectivity alone cannot show.
Zulu Time
Zulu time, written with a Z, is the name used in aviation and meteorology for Coordinated Universal Time (UTC), the time at the Greenwich prime meridian. Weather observations, model runs, and aviation reports all use it so that a time means the same thing everywhere. 12Z is 8 in the morning Eastern Daylight Time and 7 in the morning Eastern Standard Time.
METAR
Meteorological Aerodrome Report, a standardized format for reporting current weather conditions at airports. METARs are issued hourly (or more frequently during significant changes) and include wind, visibility, clouds, temperature, and pressure.
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In the WeatherAI App
WeatherAI shows air quality for the U.S. and Canada on the free tier and worldwide with Standard or Pro. It supports the U.S. and EU AQI scales, with Canada's AQHI handled separately.
Availability, coverage, update frequency, and alert delivery vary by data source, region, subscription plan, device permissions, and service conditions.