An air quality alert is a public health notification issued when pollution levels in outdoor air rise high enough to cause health problems. These alerts are triggered when concentrations of specific pollutants, most commonly ground-level ozone and fine particulate matter (tiny airborne particles known as PM2.5), reach levels that can harm people who spend time outdoors. Alerts are created by state and local air quality agencies, then broadcast through the National Weather Service, local news, and apps like AirNow to reach as many people as possible.
How the Air Quality Index Works
Every air quality alert is tied to the Air Quality Index, or AQI, a standardized scale that runs from 0 to 500. The EPA calculates the AQI for five major pollutants: ground-level ozone, particulate matter (including PM2.5 and PM10), carbon monoxide, sulfur dioxide, and nitrogen dioxide. An AQI value of 100 corresponds to the concentration limit set by the national air quality standard for that pollutant, so anything above 100 means air quality has crossed the threshold the EPA considers safe for short-term exposure.
The scale is color-coded to make it easy to interpret at a glance:
- Green (0 to 50): Good. Air pollution poses little or no risk.
- Yellow (51 to 100): Moderate. Acceptable for most people, though unusually sensitive individuals may notice effects.
- Orange (101 to 150): Unhealthy for sensitive groups. This is typically when alerts start being issued.
- Red (151 to 200): Unhealthy. Some members of the general public may experience symptoms, not just sensitive groups.
- Purple (201 to 300): Very unhealthy. Health risks increase for everyone.
- Maroon (301 and higher): Hazardous. Emergency conditions where the entire population is at risk.
Most air quality alerts are issued when the AQI is forecast to reach the orange level (101 or above), though the exact trigger varies by region. Some areas issue alerts at the yellow level during events like wildfire smoke that are expected to worsen.
Who Issues the Alert
Air quality alerts originate from state and local environmental agencies, not the National Weather Service. This is a common misconception. State agencies monitor pollution levels, run air quality forecast models, and decide when conditions warrant an alert. The National Weather Service’s role is strictly to broadcast the message through its alert system once a state or local agency requests it. The NWS does not write, edit, or initiate the content. Any corrections or updates to an alert go back through the originating agency.
You’ll typically see these alerts appear on your phone through weather apps, on highway message signs, and in local news broadcasts. The AirNow website (run by the EPA) also provides real-time AQI maps and forecasts by ZIP code.
What Causes Air Quality to Deteriorate
The two most common triggers for air quality alerts in the U.S. are wildfire smoke and ground-level ozone. Wildfire smoke sends massive amounts of PM2.5 into the atmosphere, and prevailing winds can carry it hundreds of miles from the fire itself. Ground-level ozone forms when pollutants from cars, power plants, and industrial sources react with sunlight, which is why ozone alerts are most common on hot, sunny summer afternoons.
Weather patterns play a major role. Temperature inversions are one of the most important mechanisms. Normally, warm air near the ground rises and disperses pollutants upward. During an inversion, a layer of warmer air sits above cooler air near the surface, acting like a lid that traps pollution close to the ground. Pollutants accumulate hour after hour until the weather pattern shifts. Extended high-pressure systems in winter are particularly prone to this effect: the ground cools rapidly overnight, cold air settles near the surface, and the warm layer above prevents any mixing. Traffic emissions, industrial output, and wood smoke all build up in that trapped layer.
Stagnant wind conditions make things worse. Without wind to disperse pollutants, even modest emission sources can push the AQI into unhealthy territory over the course of a day or two.
Who Is Most at Risk
Air quality alerts frequently reference “sensitive groups,” a term that covers a broader population than many people realize. The EPA identifies these groups as:
- People with heart or lung disease, including asthma, COPD, and coronary artery disease
- People with diabetes
- Children under 18, whose lungs are still developing and who tend to spend more time active outdoors
- Older adults, due to higher rates of underlying cardiovascular and respiratory conditions, plus the gradual decline in the body’s defenses that comes with aging
- People who work or exercise outdoors, because heavy breathing pulls more pollutants deep into the lungs
- Lower-income communities, which often face higher baseline exposure from proximity to highways and industrial facilities
There is no fixed age at which someone becomes an “older adult” for air quality purposes. The EPA notes that people age at different rates, and susceptibility increases gradually as pre-existing conditions become more common and the body’s natural defenses weaken. For sensitive groups, health effects can appear even during moderate physical activity outdoors when the AQI is in the orange range (101 to 150). At the red level, members of the general public with no pre-existing conditions start experiencing symptoms too.
What Poor Air Quality Does to Your Body
Fine particulate matter (PM2.5) is the pollutant that drives most serious health effects during air quality alerts. These particles are less than 2.5 micrometers across, small enough to pass through your nose and throat and lodge deep in your lungs. From there, the smallest particles can enter your bloodstream. Short-term exposure can trigger asthma attacks, worsen COPD symptoms, and cause chest tightness, coughing, and shortness of breath even in healthy people when levels are high enough.
The cardiovascular effects are less intuitive but well documented. Fine particulate pollution causes inflammation in blood vessels, which can raise blood pressure and increase the risk of heart attacks and irregular heart rhythms. This is why people with heart disease, not just lung disease, are listed as a sensitive group. In February 2024, the EPA tightened the annual PM2.5 standard from 12 to 9 micrograms per cubic meter, citing the strength of evidence linking fine particle exposure to heart attacks and premature death.
Ground-level ozone irritates the airways in a different way. It triggers inflammation in the lining of the lungs, which can feel like a sunburn inside your chest. Symptoms include coughing, throat irritation, and a sensation of not being able to take a full breath.
How to Protect Yourself During an Alert
The most effective step is reducing your time outdoors, especially avoiding strenuous activity like running, cycling, or yard work. Exercise forces you to breathe harder and faster, pulling pollutants deeper into your lungs and increasing the total dose you inhale. If the AQI is in the orange range and you’re in a sensitive group, move workouts indoors. At the red level or above, everyone should limit prolonged outdoor exertion.
Keeping your indoor air clean matters just as much. Close all windows and doors. If you have a central HVAC system with a fresh air intake, switch it to recirculate mode so it’s filtering indoor air rather than pulling in outdoor air. Consider upgrading your HVAC filter to MERV 13 or higher if your system can handle it, and run the fan continuously by setting it to “on” rather than “auto.” Window air conditioners should have their outdoor air dampers closed. If you have a single-hose portable air conditioner, use it sparingly during smoky conditions because it pulls outdoor air inside as it operates.
Evaporative coolers (swamp coolers) are particularly problematic during air quality alerts because they work by drawing large volumes of outdoor air through the system. If you can safely access the unit, covering the outside air intakes with 4-inch-thick MERV 13 filters helps, though these filters may need frequent replacement.
If you need to go outside during an alert, an N95 respirator filters 60 to 80 percent of fine particles when worn with a proper seal against your face. Surgical masks and cloth masks are significantly less effective, filtering only 25 to 60 percent of particles, largely because of gaps around the edges. The fit matters as much as the filter material: air follows the path of least resistance, so any gap between the mask and your skin lets unfiltered air in.
How Long Alerts Typically Last
Most air quality alerts last one to three days, tied to the weather pattern driving them. Ozone alerts often follow a daily cycle, peaking in the late afternoon when heat and sunlight are strongest, then improving overnight. Wildfire smoke events can persist for weeks if fires remain active and winds continue pushing smoke into your area. Temperature inversions usually break when a weather front moves through, bringing wind and mixing that disperses the trapped pollution.
You can track real-time and forecast AQI values by ZIP code on AirNow.gov. Many weather apps also display AQI data and will send push notifications when alerts are issued for your area. Checking the forecast before planning outdoor activities, especially in summer or during wildfire season, takes seconds and can help you adjust your schedule on high-pollution days.