What the AQI represents
The Air Quality Index, or AQI, converts pollutant measurements into a shared scale from 0 to 500. It is not a weather forecast and it is not a measure of how much smoke you can see. It is a public-health tool summarizing how concerning outdoor air may be at a particular place and time. In the United States, AirNow publishes current and forecast values using monitoring networks operated by federal, state, local and Tribal agencies.
The scale uses colors and six categories. Zero to 50, green, is considered good. Fifty-one to 100, yellow, is moderate. Between 101 and 150, orange, air may be unhealthy for sensitive groups. From 151 to 200, red, it is considered unhealthy for everyone. Values from 201 to 300 are very unhealthy, and those above 300 are hazardous. The displayed value reflects the pollutant creating the greatest risk at that time, often ozone or fine-particle pollution.
Who should pay closer attention
Pollution can affect anyone, but risk is not evenly distributed. Children, older adults, pregnant people, outdoor workers, and people with asthma, lung disease or heart disease may experience effects sooner. An orange category does not automatically mean everyone must stay indoors. It means people at greater risk should reduce prolonged or heavy exposure and monitor symptoms.
How to use the number
Check the AQI for your location instead of relying on a screenshot circulating online. Compare the current reading with the forecast because smoke, wind and ozone can change during the day. When levels rise, shorten or reduce the intensity of outdoor exercise, choose a time when air is better, and keep doors and windows closed when practical. Appropriate filtration can reduce indoor particles. Follow any personal action plan provided by a health professional.
Low-cost neighborhood sensors can reveal useful trends, but they may differ from regulatory monitors because of placement, calibration, humidity or maintenance. EPA recommends AirNow for health information. A single sensor should not replace official data or medical care.
Common interpretation mistakes
A clear sky does not guarantee healthy air, and the smell of smoke cannot tell you the particle concentration. Numbers from different countries may also use different scales. AQI communicates population-level risk; it does not diagnose illness. Seek medical help for breathing difficulty, chest pain or other severe symptoms.