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Air Quality Basics

Understanding the AQI: A Complete 2026 Guide to Air Quality Index Categories and Health Actions

Published: 2026-06-11 · 6 min read

The Air Quality Index (AQI) is the EPA's standardized way of translating raw pollution measurements into a single number from 0 to 500 — think of it like a thermometer for air pollution. The higher the number, the greater the health concern. But what should you actually do at each level? Here's a plain-language breakdown.

The AQI Scale at a Glance

0-50
Good
Air quality is satisfactory. Enjoy normal outdoor activities.
51-100
Moderate
Acceptable for most people. Unusually sensitive individuals should consider reducing prolonged or heavy outdoor exertion.
101-150
Unhealthy for Sensitive Groups
People with asthma, heart/lung disease, older adults, children, and pregnant individuals should reduce prolonged outdoor exertion.
151-200
Unhealthy
Everyone should begin to limit prolonged outdoor exertion. Sensitive groups should avoid it entirely.
201-300
Very Unhealthy
Health alert: everyone should avoid prolonged outdoor exertion. Sensitive groups should remain indoors.
301-500
Hazardous
Health emergency: everyone should avoid all outdoor exertion and remain indoors with air filtration where possible.

What Pollutants Make Up the AQI?

The US AQI is based on five pollutants regulated under the Clean Air Act:

  • Ground-level ozone — forms in sunlight from vehicle and industrial emissions, peaks on hot summer afternoons
  • PM2.5 (fine particulate matter) — the dominant pollutant during wildfire smoke events; small enough to penetrate deep into the lungs and bloodstream
  • PM10 (coarse particulate matter) — dust, pollen, and larger particles
  • Carbon monoxide — mostly from vehicle exhaust, more relevant in winter inversions
  • Sulfur dioxide and nitrogen dioxide — primarily industrial and vehicle emission sources

The AQI value reported for a location is based on whichever of these pollutants is currently highest — so the "responsible pollutant" can change day to day depending on conditions (e.g., ozone in summer, PM2.5 during a smoke event or winter inversion).

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Reading AQI Like a Local

A single AQI number is useful, but trends matter more for planning:

  • Check the forecast, not just "now." Ozone rises through the afternoon; smoke plumes can arrive within hours.
  • Compare to your area's baseline. An AQI of 60 might be a normal day in one city and an unusually bad day in another — our city pages show historical context so you can tell the difference.
  • Know your "responsible pollutant." If it's ozone, plan around afternoon hours. If it's PM2.5 from smoke, indoor filtration matters more than timing.

Quick Action Guide by Level

  1. 0-50 (Good): No restrictions — great day for outdoor activity.
  2. 51-100 (Moderate): Fine for almost everyone; sensitive individuals doing intense, prolonged outdoor activity might notice mild symptoms.
  3. 101-150 (Unhealthy for Sensitive Groups): If you have asthma, COPD, heart disease, are pregnant, elderly, or a child, shorten or move outdoor activity indoors.
  4. 151-200 (Unhealthy): Everyone should reduce prolonged outdoor exertion; sensitive groups should stay indoors.
  5. 201-300 (Very Unhealthy): Stay indoors, run a HEPA purifier, keep windows closed.
  6. 301+ (Hazardous): Treat as an emergency — avoid all outdoor exposure if possible.

Where to Check Your AQI

Use our city air quality pages for real-time AQI plus historical and seasonal trends, or check our rankings to see how your area compares nationally.

Related Reading

Note: AQI categories and health guidance summarized here follow EPA conventions. Always consult official sources and your healthcare provider for personalized health decisions.