The US EPA Air Quality Index (AQI) runs from 0 to 500, with higher numbers meaning worse air. Six color-coded categories help you decide if it's safe to walk, run, cycle, or send children outside. The reported AQI is the highest sub-index across six pollutants, with the dominant pollutant named. Sensitive groups—including children, older adults, people with asthma or heart disease, and those who exercise outdoors—need to take extra precautions. This guide explains how to read the AQI, adjust your plans at each level, and includes a short pre-outing checklist.
What the AQI Number and Color Mean
The US EPA Air Quality Index (AQI) is a simple number from 0 to 500 that tells you how clean or polluted the air is. The higher the number, the greater the health concern. The index is divided into six categories, each with a color and a health message.
| AQI Range | Color | Health Concern |
|---|---|---|
| 0–50 | Green | Good: Air quality is satisfactory, and air pollution poses little or no risk. |
| 51–100 | Yellow | Moderate: Air quality is acceptable. However, there may be a risk for some people, particularly those who are unusually sensitive to air pollution. |
| 101–150 | Orange | Unhealthy for Sensitive Groups: Members of sensitive groups may experience health effects. The general public is less likely to be affected. |
| 151–200 | Red | Unhealthy: Everyone may begin to experience health effects. Members of sensitive groups may experience more serious effects. |
| 201–300 | Purple | Very Unhealthy: Health alert: everyone may experience more serious health effects. |
| 301–500 | Maroon | Hazardous: Health warnings of emergency conditions. The entire population is likely to be affected. |
The AQI is calculated separately for each of the six major pollutants: fine particles (PM2.5), coarse particles (PM10), ground-level ozone (O3), nitrogen dioxide (NO2), sulfur dioxide (SO2), and carbon monoxide (CO). The reported AQI is the highest of those sub-indices, and that pollutant is called the "main" or "dominant" pollutant. So if you see an AQI of 120 with "Ozone" as the dominant pollutant, it means ozone is driving the number.
In 2024, the EPA tightened the PM2.5 scale. A 24-hour PM2.5 reading of 9.0 micrograms per cubic meter (ug/m3) is now the top of the Good band, down from 12.0 ug/m3 under the old rules. That means older apps or websites may show slightly different colors for the same air. The World Health Organization's 2021 guideline for PM2.5 is even stricter: an annual mean of 5 ug/m3 and a 24-hour mean of 15 ug/m3.
How to Find the Dominant Pollutant
Most air quality apps and websites show the AQI number and color, plus the name of the dominant pollutant. Look for a line that says "Main Pollutant" or "Dominant Pollutant." This matters because different pollutants behave differently throughout the day.
- Ozone (O3): Peaks on sunny afternoons, especially in warm weather. If ozone is the main pollutant, plan outdoor activities for the morning or evening.
- Fine particles (PM2.5): Often higher in the morning and evening, especially during temperature inversions, wildfire smoke, dust events, and heavy traffic. If PM2.5 is the main pollutant, try to avoid being outside during rush hour or when smoke is present.
- Coarse particles (PM10): Can be high during dry, windy conditions or near construction sites.
- Nitrogen dioxide (NO2), sulfur dioxide (SO2), carbon monoxide (CO): Usually higher near traffic or industrial sources.
Why the Time of Day Matters
Air quality is not constant. Ozone builds up during the day as sunlight drives chemical reactions, so it's often worst in the late afternoon. Particle pollution can spike during morning and evening commutes, or when a wildfire or dust storm moves through. Many services show a "NowCast" or hourly figure that reacts faster than a 24-hour average. Checking the hourly forecast can help you choose the cleanest window for your walk, run, or bike ride.
How to Adjust Plans at Each Level
Here is a clear guide for what to do at each AQI level. The advice applies to everyone, but sensitive groups should follow the stricter recommendations.
| AQI Range | Color | What to Do |
|---|---|---|
| 0–50 | Green | It's a great day to be outside. Go for a walk, run, or bike ride. Send the kids out to play. No restrictions needed. |
| 51–100 | Yellow | Air quality is acceptable for most people. If you are unusually sensitive to air pollution, consider reducing prolonged or heavy outdoor exertion. For everyone else, it's fine to be active. |
| 101–150 | Orange | Sensitive groups should reduce prolonged or heavy outdoor exertion. Take more breaks during activity. The general public is less likely to be affected, but if you notice symptoms like coughing or shortness of breath, move indoors. |
| 151–200 | Red | Everyone should reduce prolonged or heavy outdoor exertion. Sensitive groups should avoid prolonged or heavy exertion outdoors altogether. Consider moving workouts indoors or rescheduling for a cleaner day. |
| 201–300 | Purple | Sensitive groups should avoid all outdoor physical activity, and everyone else should avoid prolonged or heavy exertion. Keep windows closed. If you must go out, keep it brief and wear a well-fitted N95 or FFP2 respirator if you have one. |
| 301–500 | Maroon | Stay indoors. Avoid any outdoor exertion. Use a HEPA air purifier if available. If you must go outside, wear a well-fitted N95 or FFP2 respirator and keep the trip very short. |
Special Advice for Sensitive Groups
Sensitive groups include people with asthma or other lung disease, people with heart disease, children and teenagers, older adults, pregnant people, and people who work or exercise hard outdoors. If you fall into any of these categories, you need to be more cautious.
- Children and teenagers: Their lungs are still developing, and they breathe more air per pound of body weight than adults. On Orange or higher days, keep outdoor play short and avoid peak pollution hours.
- People with asthma: Check the AQI before going out. On Orange or higher days, carry your rescue inhaler and consider staying indoors. Ozone and particles can trigger attacks.
- Older adults: Age can make the heart and lungs more vulnerable. On Red or higher days, avoid outdoor exertion entirely.
- Pregnant people: Air pollution can affect fetal development. On Orange or higher days, reduce time outdoors and avoid heavy activity.
- Outdoor workers and athletes: You breathe harder and longer, so you take in more pollution. On Orange or higher days, take frequent breaks, and consider rescheduling or moving indoors.
Short Pre-Outing Checklist
Before you step outside, run through this quick checklist:
- Check the AQI number and color. Use a reliable app or website that shows the NowCast or hourly forecast.
- Identify the dominant pollutant. Is it ozone (peak afternoon) or particles (peak morning/evening)? Adjust your timing.
- Consider your sensitivity. Are you or anyone in your group in a sensitive category? If yes, follow the stricter advice.
- Plan the activity. If the AQI is Orange or higher, reduce the duration and intensity of outdoor exertion. Consider a walk instead of a run, or a short bike ride instead of a long one.
- Have a backup plan. If the air is poor, move your workout indoors, use a HEPA purifier, or reschedule for a cleaner day.
- Wear protection if needed. For very poor air (Purple or Maroon), a well-fitted N95 or FFP2 respirator filters fine particles far better than cloth masks.
Frequently Asked Questions
What does the AQI number actually measure?
The AQI measures the concentration of six pollutants in the air: fine particles (PM2.5), coarse particles (PM10), ground-level ozone (O3), nitrogen dioxide (NO2), sulfur dioxide (SO2), and carbon monoxide (CO). The reported AQI is the highest of the six sub-indices, so it reflects the worst pollutant at that moment.
Why does the same air get a different AQI in other countries?
Other countries use their own air quality scales. For example, the European Air Quality Index, the UK Daily Air Quality Index (which runs from 1 to 10), and India's National AQI all use different breakpoints and colors. So the same air can get a different number and color depending on where you are. Always use the local index for your location.
How can I protect myself indoors during poor air quality?
Keep windows and doors closed. Use a HEPA air purifier to reduce indoor particle levels. Avoid activities that generate indoor pollution, such as burning candles, using gas stoves without ventilation, or vacuuming without a HEPA filter. If you have central air conditioning, set it to recirculate mode.
Is it safe to exercise outdoors when the AQI is Yellow or Orange?
For most people, Yellow is fine for exercise. For Orange, sensitive groups should reduce prolonged or heavy exertion. The general public can still exercise but may want to take more breaks. If you notice any symptoms like coughing, wheezing, or chest tightness, move indoors.
What should I do if I have to go outside during a Purple or Maroon day?
Keep your time outdoors as short as possible. Wear a well-fitted N95 or FFP2 respirator, which filters fine particles far better than cloth masks. Avoid any physical exertion. If you have a car, use recirculated air mode. Once inside, remove your shoes and outer clothing to avoid tracking pollutants indoors.