The Air Quality Index (AQI) is a scale from 0 to 500 that tells you how clean or polluted the air is right now and what health effects you might experience. The higher the number, the worse the air. The scale is divided into six color-coded categories, each tied to specific health guidance, so you can glance at a color or number and know whether it’s safe to go for a run or whether you should stay inside.
The Six AQI Categories
Each category covers a range of numbers and is assigned a color. Here’s what they mean in practice:
- Green (0–50): Good. Air quality poses little or no risk. Go enjoy the outdoors.
- Yellow (51–100): Moderate. Air quality is acceptable, but people who are unusually sensitive to pollution may notice symptoms like coughing or shortness of breath during prolonged activity. Most people are fine.
- Orange (101–150): Unhealthy for Sensitive Groups. If you have asthma, heart disease, or lung disease, or if you’re older, a child, or work outdoors, shorten and lighten your outdoor activities. Everyone else can continue as normal but should pay attention to symptoms.
- Red (151–200): Unhealthy. Everyone is affected at this level. Reduce prolonged or intense outdoor exercise. Sensitive groups should consider moving activities indoors or rescheduling.
- Purple (201–300): Very Unhealthy. Sensitive groups should avoid all outdoor physical activity. Everyone else should avoid long or intense exertion and consider staying indoors.
- Maroon (301–500): Hazardous. Everyone should avoid all outdoor physical activity. Sensitive groups should remain indoors and keep activity levels low.
Most days in most U.S. cities, you’ll see green or yellow. Orange and above typically show up during wildfire smoke events, high-ozone summer days, or in areas with heavy industrial emissions.
What the Number Actually Measures
The AQI isn’t measuring one single thing. It tracks five major air pollutants: ground-level ozone, particle pollution (fine particles called PM2.5 and coarser particles called PM10), carbon monoxide, sulfur dioxide, and nitrogen dioxide. Each pollutant gets its own AQI value, and the highest one becomes the number you see reported. So an AQI of 130 might be driven entirely by particle pollution from wildfire smoke while ozone and the other pollutants sit in the green range.
This matters because different pollutants affect different people. Ozone is especially problematic for people with lung disease, children, and older adults. Particle pollution hits people with heart or lung conditions hardest. Sulfur dioxide is most dangerous for people with asthma. If you’re checking AQI because of a specific health condition, it’s worth clicking through to see which pollutant is driving the number. Most AQI apps and the AirNow.gov website break this out.
How Real-Time AQI Is Calculated
The official daily AQI for particle pollution is based on a full 24-hour average, which means the “true” number can’t be calculated until the day is over. That’s not very useful if smoke just rolled into your city an hour ago. To solve this, the EPA uses a method called NowCast, which takes the past 12 hours of hourly readings and creates a weighted average. When air quality is changing rapidly, the most recent hours count more heavily. When conditions are stable, each hour is weighted about equally.
This is why you might see the AQI on your weather app jump around during a wildfire event. It’s responding to real changes in the air, not just random fluctuations. If you’re deciding whether to go outside in the next hour, the NowCast number is your best guide.
Who Counts as a “Sensitive Group”
You’ll see the phrase “sensitive groups” a lot in AQI guidance. It covers more people than you might expect. For particle pollution, sensitive groups include anyone with heart disease (including coronary artery disease and heart failure), lung disease (including asthma and COPD), older adults, and children. For ozone, the list adds people who are active outdoors for extended periods, like runners, cyclists, and outdoor workers.
Older adults are included even if they feel perfectly healthy, because they’re more likely to have undiagnosed heart or lung conditions. Children breathe faster relative to their body size and spend more time outdoors, which increases their exposure. If you fall into any of these categories, the orange “Unhealthy for Sensitive Groups” range is your signal to start adjusting your plans.
What to Do at Each Level
In the green and yellow range, you don’t need to change your routine. At yellow, the only people who might notice anything are those who are unusually reactive to air pollution, and even then, the advice is simply to consider shorter, less intense outdoor activity.
At orange (101–150), sensitive groups should take more breaks during outdoor exercise and keep activities shorter. People with asthma should have quick-relief medication handy. People with heart disease should watch for palpitations, unusual fatigue, or shortness of breath, as these could signal something more serious than just the air quality.
At red (151–200), everyone should cut back. Skip the long run; take a shorter walk instead, or move your workout indoors. Sensitive groups should strongly consider staying inside.
At purple (201–300) and maroon (301+), outdoor activity becomes a real health risk. At the purple level, sensitive groups should avoid all outdoor exertion, and everyone else should significantly limit theirs. At maroon, everyone should stay indoors. If you must go outside at AQI levels above 200, an N95 respirator provides meaningful protection against particle pollution. At levels above 250 to 300, respirator use shifts from optional to essential for anyone with prolonged outdoor exposure.
Reading AQI on Apps and Websites
The easiest way to check AQI is through AirNow.gov, which is run by the EPA, or through weather apps like Apple Weather, Google Maps, or dedicated air quality apps like IQAir and PurpleAir. Most display the color and number together, so you get both at a glance.
A few things to keep in mind when reading these numbers. First, the AQI can vary significantly across a single metro area. A monitoring station near a highway may show a different number than one in a residential neighborhood. If your app lets you see data from individual sensors nearby, that’s more useful than a citywide average. Second, some consumer-grade air sensors (like PurpleAir) report raw data that may not align perfectly with the EPA’s official AQI. Many apps apply a correction factor to bring these readings closer to the EPA scale, but the numbers still aren’t identical.
Third, the AQI you see on a weather forecast is a prediction, while the AQI on a current-conditions page reflects actual measurements. Forecasts are helpful for planning your day, but check the real-time reading before heading out, especially during wildfire season or high-ozone days when conditions can shift within hours.
Why the Same AQI Means Different Things Globally
If you travel internationally, be aware that not every country uses the same AQI scale. China, India, and the European Union each have their own systems with different breakpoints. An AQI of 100 in the U.S. doesn’t mean the same thing as an AQI of 100 in India. When comparing readings across countries, check which scale the app or website is using. Most international air quality apps let you toggle between local and U.S. EPA scales.
Even within the U.S., standards evolve. In 2024, the EPA tightened its annual standard for fine particle pollution (PM2.5), lowering the acceptable level from 12 to 9 micrograms per cubic meter. This change reflects growing evidence that lower concentrations of particle pollution still carry health risks, and it means areas that previously met the standard may now fall short.