AQI Levels Explained
The Air Quality Index tells you how clean or polluted the air is on a scale from 0 to 500. Here's what each level means and when you should limit time outdoors.
The Six AQI Categories
0–50: Good (Green)
Air quality is satisfactory. No health risk. Enjoy outdoor activities normally.
51–100: Moderate (Yellow)
Acceptable for most people. Unusually sensitive individuals (severe asthma, COPD) may notice symptoms during prolonged outdoor exertion.
101–150: Unhealthy for Sensitive Groups (Orange)
Children, elderly, people with asthma or heart disease should reduce prolonged outdoor exertion. Everyone else is generally fine.
151–200: Unhealthy (Red)
Everyone may begin to experience health effects. Sensitive groups should avoid prolonged outdoor exertion. Limit outdoor exercise.
201–300: Very Unhealthy (Purple)
Health alert: everyone is at risk. Avoid outdoor exertion. Keep windows closed. Run air purifiers if you have them. Sensitive groups should stay indoors entirely.
301–500: Hazardous (Maroon)
Health emergency. Everyone should stay indoors. Close all windows and doors. If you must go outside, wear an N95 mask. This level is rare and usually caused by nearby wildfire smoke.
What Causes Poor Air Quality?
Wildfire Smoke
The most common cause of hazardous AQI in the US. Smoke can travel hundreds of miles and cause dangerous air quality far from the fire itself. The 2023 Canadian wildfire season pushed AQI above 400 in New York City.
Ground-Level Ozone
Formed when sunlight reacts with vehicle exhaust and industrial emissions. Worst on hot, sunny, low-wind summer days. Common in cities and downwind suburban areas.
Particulate Matter (PM2.5)
Tiny particles from combustion, construction, and industrial processes. PM2.5 is small enough to enter your bloodstream through your lungs. This is what the AQI primarily measures.
Who's Most at Risk?
Some people are more sensitive to air pollution than others:
- Children — lungs are still developing, they breathe faster, and they spend more time outdoors
- Elderly — higher rates of heart and lung disease
- People with asthma or COPD — air pollution triggers attacks and worsens symptoms
- People with heart disease — PM2.5 increases risk of heart attack and stroke
- Outdoor workers — prolonged exposure during construction, landscaping, farming, or delivery work
Get AQI Alerts on Your Phone
StormCast monitors the Air Quality Index at your location and sends a push notification when AQI reaches unhealthy levels (101+). You also see the current AQI on your dashboard at all times, so you always know before heading outside.
Download StormCast Free on Google Play