Running and the Air Quality Index: How to Read the AQI Before a Run
The U.S. Air Quality Index (AQI), published through the Environmental Protection Agency's AirNow site, rates outdoor air from 0 upward, and a value of 100 or below is generally considered satisfactory. Runners can check three versions of it: the daily AQI, the next-day forecast, and the hourly NowCast. This page uses AirNow's descriptions as published in October 2026.
1. What the AQI is
AirNow describes the AQI as EPA's tool for communicating about outdoor air quality and health. The higher the number, the more air pollution and the greater the health concern. According to AirNow:
- An AQI value of 50 or below represents good air quality.
- An AQI value over 300 represents hazardous air quality.
- For each pollutant, an AQI of 100 generally corresponds to the level of the short-term national air quality standard for protecting public health. Values at or below 100 are generally thought of as satisfactory.
- Above 100, air quality is unhealthy, first for certain sensitive groups, then for everyone as values rise.
2. The six color categories
| Color | Category | AQI values | What AirNow says |
|---|---|---|---|
| Green | Good | 0 to 50 | Air quality is satisfactory, and air pollution poses little or no risk |
| Yellow | Moderate | 51 to 100 | Acceptable, but there may be a risk for some people, particularly those unusually sensitive to air pollution |
| Orange | Unhealthy for Sensitive Groups | 101 to 150 | Members of sensitive groups may experience health effects; the general public is less likely to be affected |
| Red | Unhealthy | 151 to 200 | Some of the general public may experience health effects; sensitive groups may experience more serious effects |
| Purple | Very Unhealthy | 201 to 300 | Health alert: increased risk of health effects for everyone |
| Maroon | Hazardous | 301 and higher | Health warning of emergency conditions: everyone is more likely to be affected |
The colors are designed to make it easy to see at a glance when air quality is reaching unhealthy levels.
3. What it measures
AirNow says EPA sets an AQI for five major pollutants regulated by the Clean Air Act:
- ground-level ozone
- particle pollution (particulate matter, including PM2.5 and PM10)
- carbon monoxide
- sulfur dioxide
- nitrogen dioxide
Most state and local forecasters, AirNow notes, issue forecasts for ozone and particle pollution, the two most widespread pollutants in the US.
4. Three versions of the AQI, and when to use each
AirNow describes three ways the AQI is reported. Each answers a different question for a runner.
1. The daily AQI: "How was the air yesterday?" This is the official daily report. AirNow notes the time frame depends on the pollutant: the ozone AQI is an 8-hour index, and particle pollution is based on 24 hours. AirNow suggests checking the previous day's AQI to learn about air quality in your community.
2. The forecast: "Should I plan to run outside tomorrow?" AQI forecasts predict the next day's air quality. AirNow says they are usually issued in the afternoon for the next day by state and local forecasters. It compares them to a weather forecast: just as a weather forecast tells you whether to pack an umbrella, an air quality forecast tells you when you may want to change outdoor activities. Many forecasters also explain when pollution is expected to peak during the day.
3. The NowCast: "Is right now a good time?" The NowCast shows current air quality for the most recent hour available, calculated from several hours of monitor data. EPA calculates it for ozone and particle pollution only. AirNow says it uses longer averages when air quality is stable and shorter averages when it is changing quickly, such as during a wildfire. Its advice is the most useful line for runners: even on days when the AQI forecast isn't good, there may be times during the day when air quality is OK for outdoor activity.
5. A simple routine for runners
- The afternoon before, check the forecast for your area on AirNow.
- If the forecast is green or yellow, plan your run as usual.
- If it is orange or worse, read the forecast discussion, if your area provides one, to see when pollution should peak, and consider shifting your run to a better time or moving it indoors.
- Right before you go, check the NowCast for current conditions.
If you are in a sensitive group, the orange category is the one AirNow flags for you. AirNow's activity guides for ozone and particle pollution describe ways to protect your health when the AQI reaches unhealthy levels.
6. Where to find it
AirNow says metro areas with more than 350,000 people are required to report the daily AQI, and many smaller areas report it as a public service. You can find it on AirNow, on state and local agency websites, and in some local news reports.
7. Key takeaways
- The AQI runs from good (0 to 50) to hazardous (301 and up), with 100 roughly marking the health-based standard.
- It covers ozone, particle pollution, carbon monoxide, sulfur dioxide, and nitrogen dioxide.
- Use the forecast, usually issued the afternoon before, to plan tomorrow's run.
- Use the hourly NowCast just before you leave; even bad-forecast days can have better hours.
- Orange, "Unhealthy for Sensitive Groups," is the first level that flags health effects for some people.
This article is general information, not medical advice. If you have asthma, heart disease, or another condition affected by air pollution, ask your doctor how air quality should shape your exercise plans.