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
How AQI Is Actually Calculated
The Air Quality Index isn't a direct sensor reading — it's a standardized index the EPA calculates from measured pollutant concentrations, converting each one onto the same 0–500 scale so different pollutants can be compared on equal footing. Monitors track several pollutants separately: PM2.5, PM10, ground-level ozone, carbon monoxide, sulfur dioxide, and nitrogen dioxide. The reported AQI is whichever pollutant scores highest at that location and time — on a hot summer day that's usually ozone, during wildfire season it's almost always PM2.5. That's why the "cause" of a bad air quality day changes with the season even in the same city.
Frequently Asked Questions
Can AQI be different for the same city on the same day?
Yes — large cities often have multiple monitoring stations, and localized sources like a nearby highway or industrial site can push AQI higher in one neighborhood than another a few miles away.
Do air purifiers actually help indoors during bad AQI days?
Yes, a HEPA-filter purifier sized for the room can meaningfully reduce indoor PM2.5, especially during wildfire smoke events. Closing windows and running the purifier together is more effective than either alone.
Does a regular cloth or surgical mask protect against wildfire smoke?
Not well — smoke particles are small enough to pass through cloth and most surgical masks. An N95 or better respirator, properly fitted, is what's actually recommended for outdoor exposure during hazardous AQI.
Is AQI the same worldwide?
No. The US EPA's AQI scale (0–500) is used in this article and by StormCast, but other countries use their own indices with different scales and thresholds, so an AQI reading from an international source may not directly compare.
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.
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A note on this guide: This is general safety information, not medical or professional emergency advice, and specific criteria can vary by region. In an actual emergency, call 911 and follow instructions from local officials and the National Weather Service.