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Air Quality Index: What AQI Colors and Numbers Mean

The U.S. Air Quality Index converts pollutant concentrations to a common 0–500 health scale. Green is Good, yellow Moderate, orange Unhealthy for Sensitive Groups, red Unhealthy, purple Very Unhealthy and maroon Hazardous.

Timeline

  1. Before an outdoor plan: Check the forecast AQI, the expected main pollutant and the pollutant-specific guidance for people in the group.
  2. During the day: Check the NowCast because ozone and particle pollution can change with smoke, weather and emissions.
  3. When AQI rises: Reduce exposure by changing time, place, duration or intensity according to the category and personal health plan.

The U.S. Air Quality Index, or AQI, translates measured or forecast concentrations of common outdoor pollutants into a shared health-communication scale. EPA defines AQI values for ground-level ozone, particle pollution (PM2.5 and PM10), carbon monoxide, sulfur dioxide and nitrogen dioxide. A higher number means more pollution and greater health concern. AQI is an index, not the concentration itself, so a value of 100 does not mean 100 units of every pollutant. [1][2][3]

The six categories are consistent across the scale: green, 0–50, is Good; yellow, 51–100, Moderate; orange, 101–150, Unhealthy for Sensitive Groups; red, 151–200, Unhealthy; purple, 201–300, Very Unhealthy; and maroon, 301–500, Hazardous. Above 500 is beyond the AQI scale. The boundaries communicate increasing concern; they do not imply that risk suddenly appears at one exact digit or that everybody responds identically. [1][2][4]

Each pollutant concentration is converted to its own sub-index using pollutant-specific averaging times and breakpoints. The overall daily AQI is the highest of the individual pollutant AQI values, and that pollutant is reported as the main pollutant. This makes the headline conservative and easy to compare, but it can hide a second pollutant that also matters. Open the detail view rather than assuming the same pollutant drives every orange or red day. [2][3][5]

“Sensitive groups” depend on the pollutant. For particle pollution, EPA guidance highlights people with heart or lung disease, older adults, children and teenagers, outdoor workers and some other populations. For ozone, people with lung disease, children, older adults and people active outdoors can be at greater risk. At orange, those groups should reduce prolonged or heavy exertion; at red and above, advice expands because risk increasingly includes everyone. [6][7][8]

Current AQI and forecast AQI answer different questions. State and local forecasters predict the next day using monitoring, weather, satellite and pollution models. AirNow’s NowCast uses several recent hours of monitor data to express the latest available ozone or particle conditions on the AQI scale, weighting shorter periods when air quality changes rapidly. Use the forecast to plan, then check the NowCast before and during an activity, especially near wildfire smoke. [4][9]

AQI breakpoints can change when EPA updates the health evidence and standards. In 2024 EPA revised PM2.5 breakpoints: for example, Good now covers a 24-hour concentration of 0.0–9.0 micrograms per cubic meter and Moderate starts at 9.1. The AQI category numbers and colors stayed the same. That means an old app, screenshot or article using earlier particle breakpoints may disagree with a current official display even when the measured concentration is identical. [3][10]

Use AQI to reduce dose, not to diagnose illness. Exposure can often be lowered by choosing a cleaner time or location, shortening activity, reducing intensity or moving into a building with cleaner air. Follow an asthma or heart-health plan and seek medical advice for concerning symptoms. Local smoke, indoor air, heat and distance from a monitor can change personal exposure, so consult AirNow and state, tribal or local air agencies for current instructions. [6][7][9]

Sources

  1. AirNow — AQI Basics
  2. EPA — Communicating Air Quality Conditions: The Air Quality Index
  3. EPA Air Quality System — Current AQI Breakpoints
  4. AirNow — National Maps and AQI Scale
  5. EPA — About AirData Reports
  6. AirNow — Air Quality Guide for Particle Pollution
  7. AirNow — Your Health
  8. AirNow — Action Days
  9. AirNow — Using the Air Quality Index
  10. EPA — Final Updates to the AQI for Particulate Matter

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