You check the weather and see a clear sky, a comfortable 78°F (26°C), and barely any wind — a perfect day to be outside. Then you notice another number sitting quietly next to the forecast: AQI 162, glowing orange or red. Suddenly the "perfect" day comes with a warning. That number is the Air Quality Index, and on days when wildfire smoke, smog, or dust is in the air, it matters far more than the temperature.
The AQI condenses something invisible — how much pollution you're actually breathing — into a single, easy-to-read figure. Once you know how to read it, you can decide at a glance whether to go for that run, open the windows, or keep the kids inside. Here's exactly what the number means.
The Air Quality Index is a scale that runs from 0 to 500, where lower is cleaner and higher is more polluted. It isn't a measure of one single thing — it rolls up several major pollutants and reports whichever one is currently the worst. The pollutants it tracks are:
Fine particles (PM2.5 and PM10) — tiny specks of soot, smoke, and dust small enough to travel deep into your lungs and even into your bloodstream. This is the pollutant that spikes during wildfire smoke and dust storms, and it's the one that drives most high-AQI days.
Ground-level ozone — the main ingredient in summer smog. It forms when sunlight cooks pollutants from traffic and industry, which is why ozone tends to peak on hot, sunny, stagnant afternoons.
Carbon monoxide, sulfur dioxide, and nitrogen dioxide — gases mostly tied to traffic, power plants, and industry. These usually matter most near busy roads and industrial areas.
Whichever pollutant is highest at the moment sets the AQI. So an AQI of 155 on a smoky day is telling you about fine particles, while the same number on a hot, hazy city afternoon is more likely about ozone.
The index is split into six color-coded categories. The colors are deliberately intuitive — green is good, red is bad, and the deep purples and maroons are reserved for genuinely hazardous air. This is the table worth memorizing:
| AQI | Category | What it means for you |
|---|---|---|
| 0–50 | 🟢 Good | Air is clean. Enjoy outdoor activity freely. |
| 51–100 | 🟡 Moderate | Acceptable. Unusually sensitive people may notice symptoms. |
| 101–150 | 🟠 Unhealthy for Sensitive Groups | Kids, older adults, and people with asthma or heart conditions should ease up on hard outdoor exertion. |
| 151–200 | 🔴 Unhealthy | Everyone may feel effects. Sensitive groups should limit time outdoors. |
| 201–300 | 🟣 Very Unhealthy | Health alert. Avoid prolonged outdoor exertion; stay inside if you can. |
| 301–500 | 🟤 Hazardous | Emergency conditions. Everyone should stay indoors with windows closed. |
The "Unhealthy for Sensitive Groups" category exists because not everyone reacts to polluted air at the same point. Some people feel it long before the average person does. You're in a sensitive group if you:
• have asthma, COPD, or another lung condition
• have heart disease
• are pregnant
• are a young child or an older adult
• work or exercise hard outdoors
If that's you, treat the orange band (101–150) as your personal red line. For everyone else, the real caution starts in the red band (151+). Children are worth extra attention even when healthy: they breathe faster than adults relative to their body size, so they take in more polluted air per minute.
Air quality is tightly linked to weather, which is why it belongs right next to your forecast. A few patterns push the number up:
Wildfire smoke is the big one. Smoke can travel hundreds or even thousands of miles, turning the sky hazy and sending PM2.5 soaring in cities nowhere near the actual fire. A region can go from green to purple in a matter of hours when the wind shifts.
Heat and stagnant air brew ozone. Hot, sunny, windless days — often under the same high-pressure ridge that causes a heat dome — are prime conditions for smog to build up because there's no wind to flush it away.
Temperature inversions trap pollution near the ground. Normally air cools with height and pollutants rise and disperse. During an inversion, a layer of warm air acts like a lid, sealing dirty air in the valley below — a common cause of winter smog in cities ringed by mountains.
You don't need to barricade yourself indoors every time the number ticks up. Match your response to the level:
Orange (101–150): Sensitive groups should move intense activity earlier or later in the day, or take it indoors. Everyone else can carry on but might skip a hard workout outside.
Red (151–200): Everyone should cut back on strenuous outdoor exertion and shorten time outside. Keep windows closed and run air conditioning on recirculate.
Purple and maroon (201+): Stay indoors as much as possible. A well-fitted N95 mask genuinely helps against fine particles if you must go out — cloth and surgical masks do little against PM2.5. A portable air purifier with a HEPA filter can keep one room noticeably cleaner.
The AQI turns invisible pollution into a number you can act on in seconds. Green and yellow mean breathe easy. Orange is the moment for sensitive groups to take care. Red and above mean everyone should pay attention and scale back time outdoors. It's a habit worth building — especially in wildfire and heat season, when the air can change far faster than the temperature on the forecast.
→ Check the current conditions for your city on ClearCast before you head outside