The Air Quality Index, or AQI, is a numbered scale from 0 to 500 that tells you how clean or polluted the air is in your area right now and what health effects you might experience. The higher the number, the worse the air quality. An AQI of 50 or below is considered good, while anything above 150 means the air is unhealthy for everyone, not just people with lung conditions.
The EPA calculates a separate AQI value for each major pollutant, including fine particulate matter (PM2.5), ground-level ozone, carbon monoxide, sulfur dioxide, and nitrogen dioxide. The highest individual value becomes the overall AQI reported for your area. So when your weather app shows “AQI 85,” that number reflects whichever pollutant is most elevated at that moment.
The Six AQI Categories
The scale is divided into six color-coded levels, each tied to a range of health concern:
- Green (0 to 50), Good: Air pollution poses little or no risk. This is the ideal range for outdoor activity.
- Yellow (51 to 100), Moderate: Air quality is acceptable, though people who are unusually sensitive to pollution may notice mild effects.
- Orange (101 to 150), Unhealthy for Sensitive Groups: People with asthma, heart disease, or other conditions may begin to experience symptoms. Most other people will feel fine.
- Red (151 to 200), Unhealthy: Some healthy adults may notice irritation, and sensitive groups face more serious effects.
- Purple (201 to 300), Very Unhealthy: Everyone is at increased risk of health effects. This is a health alert for the entire population.
- Maroon (301 and higher), Hazardous: Emergency conditions. Everyone is likely to be affected.
Most U.S. cities sit in the green or yellow range on a typical day. Orange and red readings are common during wildfire season, heat waves, or high-traffic periods when pollution builds up. Purple and maroon readings are rare but occur during major wildfire smoke events or industrial accidents.
Why PM2.5 Matters Most
Fine particulate matter, known as PM2.5, is the pollutant that drives unhealthy AQI readings most often. These particles are 30 times smaller than a human hair, small enough to pass through your lungs and enter your bloodstream. Once in circulation, they trigger inflammation that can spread throughout the body.
Short-term exposure to high PM2.5 levels irritates the airways and can worsen asthma or cause coughing, even in healthy people. The deeper concern is cardiovascular. PM2.5 particles promote blood clotting, damage blood vessel walls, and increase the risk of heart attack and stroke. Research has shown that platelet counts rise measurably with each small increase in PM2.5 concentration. The particles can also disrupt heart rhythm, raising the risk of arrhythmias during acute exposure. These effects explain why air pollution advisories emphasize heart disease alongside lung disease.
In February 2024, the EPA tightened the PM2.5 standards. The “Good” category now tops out at 9 micrograms per cubic meter of air, down from 12. This means air that previously would have registered as green on the scale now falls into yellow. The change was based on updated health science showing harm at lower concentrations than previously thought. The World Health Organization goes even further, recommending annual average PM2.5 levels stay below 5 micrograms per cubic meter.
How Ground-Level Ozone Forms
The other major AQI driver is ground-level ozone. Unlike PM2.5, ozone isn’t released directly from smokestacks or tailpipes. It forms when pollutants from cars, power plants, and industrial facilities react with sunlight. That’s why ozone levels peak on hot, sunny afternoons and tend to be lowest in the morning.
Breathing elevated ozone irritates and inflames the airways, similar to a mild chemical burn inside the lungs. People with asthma are especially vulnerable, but prolonged exposure on high-ozone days can cause chest tightness and reduced lung function in anyone. If you check the AQI on a summer afternoon and see orange or red values driven by ozone, morning is a better time for outdoor exercise.
Who Counts as a “Sensitive Group”
You’ll see the phrase “sensitive groups” in nearly every AQI forecast. This covers a broader range of people than you might expect. It includes children, infants, pregnant women, older adults (especially older women), and anyone with pre-existing heart disease, lung disease, asthma, or diabetes.
People who work outdoors also qualify, not because of a medical condition but because of prolonged exposure. And economically disadvantaged neighborhoods tend to face both higher pollution levels and greater health vulnerability, making the “sensitive groups” threshold relevant for a large portion of the population. If you or someone in your household falls into any of these categories, the orange AQI range (101 to 150) is your signal to reduce extended outdoor activity.
How to Protect Yourself on Bad Air Days
When the AQI climbs into unhealthy territory, the most effective step is reducing your time outdoors, particularly avoiding vigorous exercise outside. Keep windows closed and run air conditioning on recirculate if you have it. Portable air purifiers with HEPA filters can meaningfully reduce indoor PM2.5 levels in a single room.
If you need to be outside during poor air quality, mask choice matters. N95 and KN95 respirators filter 83 to 99% of fine particles when fitted properly. Surgical masks range from about 42 to 88% filtration, a decent option if an N95 isn’t available. Cloth masks, by contrast, only capture 16 to 23% of fine particles, and a bandana blocks roughly 9%. For wildfire smoke or sustained high-AQI events, an N95 is the clear choice.
Where to Check the AQI
AirNow.gov is the EPA’s official source, providing real-time AQI readings and forecasts by ZIP code. Most weather apps now display AQI alongside temperature and precipitation. The number you see is updated hourly at most monitoring stations, so it reflects current conditions rather than a daily average. During fast-changing events like approaching wildfire smoke, checking multiple times a day gives you a more accurate picture than relying on a morning forecast alone.