Air quality alerts are public notifications issued when outdoor air pollution reaches levels that could harm your health. They’re based on the Air Quality Index (AQI), a scale from 0 to 500 that translates pollution measurements into a single, easy-to-read number. When that number climbs above 100, conditions start posing risks, first for vulnerable populations and then for everyone.
How the Air Quality Index Works
The AQI tracks six common pollutants regulated under the Clean Air Act: ground-level ozone, particle pollution (fine particles called PM2.5 and coarser particles called PM10), carbon monoxide, sulfur dioxide, nitrogen dioxide, and lead. Each pollutant gets its own AQI value based on concentrations measured at monitoring stations, and the highest value among them becomes the reported AQI for your area. So if ozone scores 85 and particle pollution scores 130, the AQI is 130 and particle pollution is listed as the “main pollutant.”
The scale is divided into six color-coded categories:
- Green (0–50), Good: Air quality poses little or no risk. A great day to be active outside.
- Yellow (51–100), Moderate: Acceptable for most people. Unusually sensitive individuals may want to shorten outdoor activities and watch for symptoms like coughing or shortness of breath.
- Orange (101–150), Unhealthy for Sensitive Groups: People with heart disease, lung conditions, asthma, diabetes, older adults, children, and lower-income communities face increased risk. Everyone else can still be active but should pay attention to how they feel.
- Red (151–200), Unhealthy: Everyone may begin to experience health effects. Sensitive groups should avoid long or intense outdoor activities altogether.
- Purple (201–300), Very Unhealthy: Health alert territory. Sensitive groups should stay indoors. Everyone else should cut back significantly on outdoor exertion.
- Maroon (301–500), Hazardous: The entire population is at serious risk. The recommendation is to avoid all outdoor physical activity and remain indoors with low activity levels.
Who Counts as a “Sensitive Group”
You’ll see the phrase “sensitive groups” in nearly every air quality alert, and it covers more people than you might expect. The EPA defines sensitive groups for particle pollution as people with heart or lung disease, people with asthma, people with diabetes, adults 65 and older, and children under 18. Lower socioeconomic status is also recognized as a risk factor, because income, housing quality, and access to healthcare all influence how much pollution a person is exposed to and how well they can recover from it.
If you fall into any of those categories, the orange “Unhealthy for Sensitive Groups” level is your signal to start adjusting plans. At that level, the EPA recommends shorter, less intense outdoor activities, more frequent breaks, and keeping quick-relief medication handy if you have asthma. People with heart disease should treat symptoms like palpitations, unusual fatigue, or shortness of breath as potentially serious.
How Air Quality Gets Measured
The backbone of the system is a network of government-run monitoring stations equipped with Federal Reference Method (FRM) or Federal Equivalent Method (FEM) instruments. These are considered the gold standard for air quality data because they go through a formal EPA evaluation process for accuracy.
In recent years, lower-cost air sensors have expanded the number of monitoring locations dramatically. These smaller, more affordable devices give a general picture of local air quality, but they come with limitations. They can occasionally report questionable data points and may systematically over- or underestimate actual pollution levels. They’re useful as a supplement, not a replacement, for the official monitoring network.
For pollutants that change quickly, like particle pollution during a wildfire, the EPA uses a calculation called NowCast. Standard AQI values are based on averages over several hours, which can lag behind fast-moving conditions. NowCast compensates by weighting recent hours more heavily when pollution levels are shifting rapidly, so the number you see on your phone reflects what’s happening closer to right now rather than what was happening six hours ago.
How You Receive Alerts
The primary hub for air quality information in the U.S. is AirNow.gov, run by the EPA along with state, local, and tribal monitoring agencies. AirNow provides real-time AQI maps, forecasts, and the current NowCast values for your area. You can get alerts pushed to you through several channels: the AirNow mobile app, EnviroFlash email notifications (which let you set a threshold so you’re only notified when air quality drops to a level you care about), RSS feeds, and AirNow’s social media accounts.
Many local news stations and weather apps also pull AQI data into their forecasts, so you may see air quality alerts show up alongside your weather notifications without signing up for anything extra.
What to Do When an Alert Is Active
At the moderate level (yellow), most people don’t need to change their plans. The practical adjustments start at orange. If you’re in a sensitive group, shorten your run, move your workout indoors, or swap a long bike ride for a shorter walk. If you’re not in a sensitive group, orange is mostly informational.
At red (151–200), everyone should start dialing back. The EPA recommends reducing long or intense outdoor activities and taking more frequent breaks. Sensitive groups should consider rescheduling outdoor plans entirely or moving them indoors. At purple (201–300), sensitive groups should avoid all outdoor physical activity, and everyone else should seriously limit time outside. At maroon (301–500), the guidance is the same for everyone: stay inside.
One important caveat the EPA highlights: if you don’t have air conditioning, staying indoors with windows closed during extreme heat can be dangerous on its own. In those situations, seeking out a public cooling center or an air-conditioned building is a better option than sealing yourself in a hot home.
Why Alerts Spike During Wildfire Season
Wildfires are the most common cause of dramatic, sudden jumps in AQI. Smoke from large fires produces massive amounts of fine particle pollution, and depending on wind patterns, that smoke can travel hundreds or even thousands of miles from the fire itself. Cities with normally clean air can see their AQI jump from green to purple in a matter of hours.
This is exactly the scenario NowCast was designed for. During stable conditions, the AQI calculation uses a longer averaging window to smooth out noise in the data. When wildfire smoke rolls in and concentrations spike, NowCast shifts to a shorter window so the reported number keeps up with conditions on the ground. If you’re checking AirNow during a smoke event and the number seems to change every hour, that’s the system working as intended.
During wildfire smoke events, particle pollution is almost always the dominant pollutant. The same protective steps apply: limit outdoor exertion, keep windows and doors closed, run air purifiers with HEPA filters if you have them, and set your car’s ventilation to recirculate rather than pulling in outside air.