An air quality alert means pollution levels in your area have risen high enough to pose a health risk, particularly for children, older adults, and people with asthma or heart disease. These alerts are issued by state and local air quality agencies when the Air Quality Index (AQI) is forecast to exceed 100, the threshold where pollution shifts from “moderate” to “unhealthy for sensitive groups.” The two most common culprits behind an alert are ground-level ozone (smog) and fine particulate matter (PM2.5), tiny particles small enough to lodge deep in your lungs.
What the AQI Numbers Mean
The AQI is a scale from 0 to 500 that translates raw pollution measurements into a single number you can act on. It’s color-coded so you can quickly gauge risk:
- Green (0–50): Good. Air quality poses little or no risk.
- Yellow (51–100): Moderate. Acceptable for most people, though unusually sensitive individuals may notice mild effects.
- Orange (101–150): Unhealthy for sensitive groups. This is the level that triggers most air quality alerts. Children, older adults, and people with lung or heart conditions should limit prolonged outdoor exertion.
- Red (151–200): Unhealthy. Everyone may begin to experience respiratory effects. About 5% of otherwise healthy adults doing moderate outdoor activity will develop chest pain on deep breaths or aggravated coughing.
- Purple (201–300): Very unhealthy. Health warnings for the entire population.
- Maroon (301–500): Hazardous. Emergency conditions.
For PM2.5 specifically, a 24-hour average concentration above 35.5 micrograms per cubic meter of air pushes the AQI into the orange zone. For context, the EPA tightened its annual PM2.5 health standard in February 2024 to 9.0 micrograms per cubic meter, down from the previous 12. The World Health Organization goes further, recommending that daily PM2.5 not exceed 15 micrograms per cubic meter more than three or four days a year.
The Most Common Triggers
The EPA tracks six “criteria” pollutants: ozone, particulate matter, carbon monoxide, sulfur dioxide, nitrogen dioxide, and lead. In practice, nearly every air quality alert you see on your phone is driven by one of the first two.
Ground-Level Ozone
Ozone forms when emissions from cars, power plants, and industrial facilities react with sunlight. That’s why ozone alerts peak on hot, sunny, windless summer afternoons. You won’t see a visible haze, but your lungs will feel it. Exposure to ozone concentrations between 60 and 80 parts per billion for four to eight hours of moderate outdoor activity (think jogging or yard work) can trigger coughing, throat irritation, chest tightness, and reduced lung function. At levels above 120 parts per billion, symptoms can appear after just one to three hours of heavy exertion.
Fine Particulate Matter (PM2.5)
PM2.5 refers to particles less than 2.5 micrometers across, about 30 times smaller than the width of a human hair. They come from vehicle exhaust, industrial smokestacks, construction dust, and burning wood or fossil fuels. Because they’re so small, they bypass your nose and throat and settle deep into lung tissue, where they can enter the bloodstream. PM2.5 alerts are common year-round but spike dramatically during wildfire season.
Why Wildfires Cause Severe Alerts
Wildfire smoke is one of the fastest ways to push an entire region’s AQI into the red or purple zone, sometimes hundreds of miles from the fire itself. Smoke particles are rich in carbonaceous matter produced by the incomplete burning of wood, leaves, and other vegetation. This sets wildfire PM2.5 apart from typical urban pollution, which contains a different chemical mix with greater oxidative potential. Both are harmful, but wildfire smoke can blanket a wide area for days or even weeks, keeping AQI levels elevated far longer than a typical smog event.
If you live in the western U.S. or Canada, wildfire smoke is the most likely explanation for a sudden, severe air quality alert during summer and fall. But smoke can travel thousands of miles. In recent years, Canadian wildfires have pushed AQI readings above 300 in cities like New York and Chicago.
How Weather Traps Pollution Near the Ground
Pollution levels don’t depend only on emissions. Weather conditions determine whether pollutants rise and disperse or sit in a concentrated layer at ground level. The most important factor is a temperature inversion, which happens when a layer of warm air settles on top of cooler air near the surface. Normally, warm air rises and carries pollutants upward. During an inversion, the warm lid traps cold, heavy air below it, and pollutants accumulate instead of dispersing.
In summer, inversions typically break apart by mid-morning as the sun heats the ground. In winter, the sun may not generate enough warmth to dissolve the inversion, and pollution episodes can persist for several days. Low wind speeds and stagnant high-pressure systems make the problem worse by removing the horizontal movement that would normally dilute pollutants across a wider area. This is why mountain valleys and basins, like Salt Lake City and Los Angeles, are especially prone to multiday air quality alerts during winter.
How Your Body Responds
Respiratory symptoms are usually the first sign. Coughing, a scratchy throat, tightness in the chest, and shortness of breath can all appear within hours of exposure to elevated ozone or PM2.5. People with asthma may notice their symptoms flare well before a healthy person feels anything.
The effects aren’t limited to your lungs. Fine particulate matter triggers inflammation throughout the body, and short-term spikes in PM2.5 are linked to increased rates of heart attacks and strokes, especially in people with existing cardiovascular disease. Even a single day of elevated PM2.5 raises emergency room visits for chest pain and breathing problems in the surrounding area.
Healthy adults who exercise outdoors are also at higher risk, not because they’re more vulnerable, but because heavy breathing pulls more polluted air deeper into the lungs. That’s why alerts specifically recommend reducing outdoor exertion rather than simply staying outside less.
What to Do During an Alert
The simplest and most effective step is to reduce the amount of polluted air you breathe. Move vigorous exercise indoors or shift it to early morning, when ozone levels are typically lowest. Keep windows closed and run air conditioning on recirculate if you have it. A portable air purifier with a HEPA filter can significantly lower indoor PM2.5 concentrations in a single room.
If you need to be outside during a PM2.5 alert, a well-fitting N95 or KN95 respirator filters out fine particles effectively. Standard cloth and surgical masks do very little for PM2.5. For ozone, no consumer mask helps, because ozone is a gas, not a particle. Limiting time outdoors and exertion level is the only practical protection.
Pay attention to how you feel. Coughing, chest discomfort on deep breaths, or unusual shortness of breath during an alert are signs your body is reacting to the pollution, and a signal to get indoors.
Checking Air Quality Yourself
The most reliable real-time readings come from AirNow.gov, which aggregates data from government-operated regulatory monitors across the U.S. You can also check the AQI through most weather apps on your phone.
Consumer-grade sensors, like those in the popular PurpleAir network, provide hyperlocal readings that can be useful for seeing conditions on your specific block. However, these laser-based sensors tend to read higher than regulatory monitors, especially during wildfire smoke events. They’re sensitive to humidity, temperature, and the type of particle in the air, and there’s no standardized calibration or maintenance. Think of them as a useful supplement to official readings, not a replacement. If your PurpleAir sensor shows alarming numbers but AirNow shows moderate levels, the truth is likely somewhere in between.