The Air Quality Index is a scale from 0 to 500 that translates raw pollution measurements into a single number you can act on. The lower the number, the cleaner the air. It’s divided into six color-coded categories, each tied to specific health risks, so you can glance at a number or color and immediately know whether it’s safe to go for a run, open your windows, or keep your kids inside.
The Six AQI Categories
Every AQI reading falls into one of six tiers. Each has a color, a label, and a range of numbers:
- Green (0 to 50): Good. Air quality is satisfactory and poses little or no risk. Go about your day normally.
- Yellow (51 to 100): Moderate. Air quality is acceptable for most people, though unusually sensitive individuals may notice mild effects.
- Orange (101 to 150): Unhealthy for Sensitive Groups. People with asthma, heart disease, or other risk factors may start experiencing symptoms. Most healthy adults won’t notice anything.
- Red (151 to 200): Unhealthy. Everyone can begin to feel effects. Sensitive groups face more serious risk.
- Purple (201 to 300): Very Unhealthy. A health alert for the entire population. Prolonged outdoor activity becomes risky for anyone.
- Maroon (301 and higher): Hazardous. Emergency conditions. Everyone is likely to be affected.
Most weather apps and websites display the color alongside the number, so even a quick glance gives you useful information. If you see orange or above, it’s time to pay closer attention.
What Pollutants the AQI Tracks
The AQI doesn’t measure “air quality” as a single thing. It tracks six specific pollutants set by the EPA: fine particulate matter (PM2.5), coarse particulate matter (PM10), ground-level ozone, carbon monoxide, sulfur dioxide, and nitrogen dioxide. Each pollutant gets its own AQI value based on its measured concentration, and the highest individual value becomes the overall AQI you see reported.
This matters because the dominant pollutant changes depending on where you live and the time of year. In summer, ground-level ozone often drives the number up, especially in cities with heavy traffic. During wildfire season, PM2.5 (the tiny particles that penetrate deep into your lungs) is usually the culprit. Knowing which pollutant is elevated helps you take the right precautions. Most AQI apps and the AirNow website tell you which pollutant is responsible for the day’s reading.
Who Counts as a “Sensitive Group”
You’ll see “sensitive groups” referenced constantly in AQI guidance, starting at the orange tier. This isn’t a vague category. The EPA defines it to include people with heart or lung disease, people with diabetes, older adults (generally 65 and up, though susceptibility increases gradually with age), and children under 18. People with lower socioeconomic status also face elevated risk, partly because of higher baseline exposure and less access to filtered indoor air.
Children qualify not because they’re fragile but because they breathe faster relative to their body size and spend more time outdoors. Older adults face increased risk due to the gradual decline in the body’s defenses and higher rates of pre-existing heart and lung conditions. If you or someone in your household falls into any of these categories, the orange tier is your signal to start limiting outdoor exertion, not the red one.
What to Do at Each Level
At green and yellow levels, no special precautions are needed for most people. You can exercise outdoors, open windows, and let kids play outside freely.
Once the AQI hits orange (101 to 150), sensitive groups should reduce prolonged or heavy outdoor exertion. That means shortening your jog, moving a workout indoors, or keeping recess activities low-key for kids with asthma. Healthy adults can still be active but should pay attention to any unusual coughing, tightness, or shortness of breath.
At red (151 to 200), everyone should scale back. Move exercise indoors, close windows, and run air purifiers or HVAC systems with good filters if you have them. Sensitive groups should avoid outdoor exertion altogether.
At purple and maroon levels (201 and above), stay indoors as much as possible. If you must go outside, an N95 respirator offers meaningful protection against fine particles like wildfire smoke. Experts recommend wearing one any time the AQI exceeds 100, though it becomes especially important above 150. Standard cloth or surgical masks don’t filter fine particulate matter effectively. N100 or P100 respirators filter even more (99.97% of fine particles) but can make breathing noticeably harder.
Where to Check and Which Data to Trust
The most reliable source for AQI data in the United States is AirNow.gov, which pulls readings from regulatory-grade monitors. These stations are placed according to EPA guidelines, maintained by trained technicians, and follow strict quality protocols. Most weather apps pull from the same data.
Consumer air sensors, like PurpleAir, are popular because they fill in geographic gaps, especially in rural areas or neighborhoods far from official monitors. But they come with tradeoffs. EPA scientists have found that PurpleAir sensors consistently overestimate fine particle concentrations, particularly in humid conditions. Nobody verifies where private sensors are placed, how close they are to pollution sources like a grill or a busy road, or whether they’re properly maintained.
AirNow does incorporate sensor data into its Fire and Smoke Map, but only after applying a correction formula to reduce bias, averaging the data to hourly intervals, and filtering out sensors that appear to be malfunctioning or placed indoors. This corrected sensor data is used only for the Fire and Smoke Map, not for the official AQI dial on the main site. If you’re comparing PurpleAir directly to AirNow and the numbers look very different, that correction process is usually why.
For day-to-day decisions, AirNow’s official readings are your most trustworthy source. During fast-moving events like wildfires, nearby consumer sensors can give you a useful early warning, but treat them as rough estimates rather than precise measurements.
Why AQI Numbers Vary Between Countries
If you’ve traveled internationally or compared readings across different apps, you may have noticed the same air producing different AQI numbers. That’s because countries set their own scales and thresholds. The U.S. AQI is based on EPA standards. For example, the EPA’s annual standard for PM2.5 was tightened in February 2024 from 12.0 to 9.0 micrograms per cubic meter. Other countries and the World Health Organization use different breakpoints, which means an AQI of 80 in the U.S. system might translate to a different number on India’s or China’s scale, even though the actual air is identical.
When using international air quality apps, check which scale they’re reporting on. Apps like IQAir often let you toggle between the U.S. EPA scale and local standards, which helps avoid confusion.