How to Read the Air Quality Index

The AQI is the number you see in every weather app and on every air quality monitor. Knowing what it means takes about two minutes and changes the decisions you make on bad air days.

Why it matters

The EPA's Air Quality Index runs from 0 to 500. It rolls five regulated pollutants (ozone, fine particles, carbon monoxide, nitrogen dioxide, sulfur dioxide) into one number so you can compare today's air to yesterday's at a glance. An AQI of 100 corresponds to the National Ambient Air Quality Standard for the pollutant being reported. Anything above 100 means at least one pollutant is exceeding what the EPA considers safe.

The six categories

Each category has a color, a number range, and a plain-English description.

Good (0 to 50, green): Air is fine. No precautions needed.
Moderate (51 to 100, yellow): Most people are fine. Unusually sensitive people (severe asthma, advanced lung disease) may want to take it easier outside.
Unhealthy for Sensitive Groups (101 to 150, orange): Kids, older adults, pregnant people, and anyone with heart or lung disease should cut back on long or intense outdoor activity.
Unhealthy (151 to 200, red): Everyone may start to feel effects. Sensitive groups should limit outdoor exertion. General population should reduce it.
Very Unhealthy (201 to 300, purple): Health alert. Avoid prolonged outdoor activity. Sensitive groups stay indoors.
Hazardous (301 and up, maroon): Emergency conditions. Everyone stays indoors with windows shut. Run a filter.

What to do at each level

The rough rule: at 100, sensitive groups start paying attention. At 150, everyone does. At 200, outdoor activity is a bad idea for most people. At 300, treat it like a smoke emergency.

If you exercise outside, scale intensity to AQI. A run that's fine at AQI 40 is a bad call at AQI 130, because you're pulling 10x more air through your lungs than at rest. Walk instead, or move it indoors.

What to avoid

Don't average the day's AQI in your head and assume a "moderate" day is uniformly moderate. AQI often spikes in afternoon (ozone) or overnight (wood smoke inversion).
Don't rely only on the headline number. Check which pollutant is driving it. Wildfire smoke days are PM2.5 driven and respond well to N95s and HEPA filters. Ozone days don't, and a mask won't help.
Don't compare AQI across countries directly. China and India use different scales with different breakpoints.

Quick checklist

Check AQI before any outdoor workout.
Know which two categories matter most for you (your baseline and your bad-day threshold).
For sensitive household members, set a personal action threshold 50 points lower than the general guidance.
On smoke days, watch PM2.5 specifically, not just the overall AQI.
Recheck every few hours during wildfire events. Conditions shift fast.

Sources

This article is for educational purposes only. Canairy does not provide medical advice, diagnosis, or treatment. Talk to a qualified health professional about your specific situation.