An Air Quality Index (AQI) above 100 is generally considered bad, meaning the air can start affecting people’s health. At 101, conditions become unhealthy for sensitive groups like children and older adults. At 151 and above, the air is unhealthy for everyone. The scale runs from 0 to 500, and the higher the number, the more dangerous it is to be outside.
How the AQI Scale Works
The EPA divides the AQI into six color-coded categories:
- Green (0 to 50): Good. Air pollution poses little or no risk.
- Yellow (51 to 100): Moderate. Air quality is acceptable, though people who are unusually sensitive to pollution may notice mild effects.
- Orange (101 to 150): Unhealthy for Sensitive Groups. People with heart or lung conditions, children, and older adults may start experiencing symptoms.
- Red (151 to 200): Unhealthy. Some otherwise healthy people may feel effects, and sensitive groups face more serious risk.
- Purple (201 to 300): Very Unhealthy. A health alert for the entire population.
- Maroon (301 to 500): Hazardous. Emergency conditions where everyone is likely to be affected.
The AQI is calculated from five major pollutants regulated under the Clean Air Act: ground-level ozone, particulate matter (tiny particles and droplets in the air), carbon monoxide, sulfur dioxide, and nitrogen dioxide. When you check a forecast, the AQI reflects whichever pollutant is highest that day. In most cities, the two biggest drivers are ozone in summer and particulate matter during wildfire season or winter inversions.
What “Bad” Air Actually Does to Your Body
Once the AQI crosses into the red zone (151 and above), the effects go beyond mild irritation. High concentrations of airborne particles can decrease lung function, worsen asthma, trigger flare-ups of chronic lung disease, and increase respiratory infections. The cardiovascular effects are just as real: elevated particle pollution can trigger heart attacks and increase hospitalizations for heart disease. These aren’t just long-term risks from years of exposure. They can happen within hours or days of breathing polluted air.
At the purple and maroon levels (201 and above), the effects broaden to the general population. Even young, healthy adults may experience coughing, throat irritation, chest tightness, or shortness of breath. People with pre-existing conditions face a much steeper risk curve at these levels.
Who Is Most at Risk
The EPA defines several “sensitive groups” who feel the effects of bad air earlier and more severely than the general population. These include people with heart or lung disease, people with diabetes, adults 65 and older, and children under 18. People with lower socioeconomic status also face higher risk, partly because of greater exposure and partly because of less access to protective measures like air filtration.
If you fall into any of these groups, the AQI starts mattering at 101, not 151. At that orange level, the recommendation is to cut back on prolonged or heavy outdoor activity. For healthy adults, the same guidance kicks in at 151.
What You Should Do at Each Level
The practical guidance shifts as the numbers climb. At the orange level (101 to 150), people with heart disease, asthma, or chronic lung disease should reduce prolonged or heavy outdoor exertion. Everyone else can generally continue normal activities.
At the red level (151 to 200), the general public should also start cutting back on extended outdoor exercise. People with heart or lung conditions should avoid heavy outdoor exertion entirely.
At the purple level (201 to 300), healthy adults should avoid prolonged outdoor exertion, and people with heart or lung conditions should avoid all outdoor physical activity. At the maroon level (301 and above), everyone should avoid outdoor activity. People with chronic conditions should stay indoors and keep activity levels low, even inside.
The distinction between “reduce,” “avoid prolonged,” and “avoid all” matters. A 20-minute walk to your car is different from a 45-minute jog. When the AQI is in the red range, short necessary trips outside are generally fine, but sustained heavy breathing outdoors, like running, cycling, or manual labor, dramatically increases how much pollution your lungs absorb.
Particulate Matter and the New Standard
Particulate matter, especially the smallest particles known as PM2.5, is the pollutant most closely linked to serious health outcomes. These particles are small enough to pass deep into your lungs and even enter your bloodstream. Wildfire smoke, vehicle exhaust, and industrial emissions are common sources.
In February 2024, the EPA tightened its annual standard for PM2.5 from 12.0 micrograms per cubic meter down to 9.0, reflecting growing evidence that even lower levels of particle pollution cause harm over time. This doesn’t change the AQI scale itself, but it signals that what was considered “safe” a few years ago may not be safe enough.
Checking the AQI in Your Area
The easiest way to check your local AQI is through AirNow.gov, which provides real-time readings and forecasts by ZIP code. Most weather apps also display AQI alongside temperature and humidity. If you live in a wildfire-prone area or a city with regular ozone alerts, checking the AQI before outdoor exercise is worth making a habit, the same way you’d check the weather before heading out.
Keep in mind that AQI can change quickly. A reading of 80 in the morning can spike well above 150 by afternoon, especially on hot days when ozone builds up or when wind shifts push wildfire smoke into your area. Forecasts help, but real-time data is more reliable for planning outdoor activities on borderline days.