An air quality alert is a public notification issued when outdoor pollution levels rise high enough to pose health risks. These alerts are tied to the Air Quality Index (AQI), a numerical scale that runs from 0 to 500. Any reading above 100 signals that the air has become unhealthy for certain groups of people, and alerts typically go out at that threshold or higher to warn residents in affected areas.
How the Air Quality Index Works
The AQI translates complex pollution measurements into a single number with a color-coded system that makes it easy to gauge risk at a glance:
- Green (0 to 50): Good. Air quality poses little or no risk.
- Yellow (51 to 100): Moderate. Acceptable for most people, though some unusually sensitive individuals may notice effects.
- Orange (101 to 150): Unhealthy for sensitive groups. This is the level where alerts begin.
- Red (151 to 200): Unhealthy. Everyone may start to experience effects.
- Purple (201 to 300): Very unhealthy. Health warnings for the entire population.
- Maroon (301 and higher): Hazardous. Emergency conditions affecting everyone.
State and local agencies are required to report AQI readings daily in U.S. cities with populations over 350,000. Many smaller cities report voluntarily. The data comes from a network of ground-level monitors run by state, local, and tribal agencies, then compiled through AirNow, a partnership between the EPA, NOAA, the National Park Service, NASA, the CDC, and those local monitoring agencies.
What Pollutants Trigger Alerts
The EPA tracks seven major pollutants, but two are responsible for the vast majority of air quality alerts: ground-level ozone and fine particulate matter, known as PM2.5.
Ozone forms when emissions from cars, power plants, and industrial sources react with sunlight. It tends to spike on hot, sunny afternoons in summer, which is why air quality alerts are especially common during heat waves. Ozone irritates the airways and can make breathing painful even for healthy people at high concentrations.
PM2.5 refers to tiny particles less than 2.5 micrometers in diameter, small enough to pass deep into the lungs and even enter the bloodstream. Wildfire smoke is one of the most common sources, but vehicle exhaust, industrial emissions, and wood-burning stoves also contribute. In 2024, the EPA tightened the annual standard for PM2.5 from 12.0 to 9.0 micrograms per cubic meter of air, reflecting growing evidence that even low levels of fine particle exposure carry health risks. The AQI breakpoints for PM2.5 were updated to match: a 24-hour average reading of 9.0 micrograms per cubic meter now marks the upper boundary of “Good” air quality.
The remaining tracked pollutants, including carbon monoxide, sulfur dioxide, nitrogen dioxide, lead, and coarser particulate matter (PM10), trigger alerts less frequently but still have established health-based standards.
Who Is Most at Risk
When an alert reaches the orange “Unhealthy for Sensitive Groups” level, the people who need to pay closest attention fall into several overlapping categories. For ozone, that includes people with asthma, chronic bronchitis, or emphysema, as well as children and teenagers (whose lungs are still developing), older adults, and anyone who exercises or works vigorously outdoors. Even some otherwise healthy people have an above-average sensitivity to ozone that they may not discover until exposure happens.
For particle pollution, the sensitive group expands to include people with heart disease, including coronary artery disease and heart failure. Older adults are at particular risk because many have undiagnosed heart or lung conditions. For carbon monoxide, people with cardiovascular disease face the greatest danger, and pregnant women should also take precautions because elevated exposure can affect fetal development.
If you have asthma, the combination of ozone and sulfur dioxide is especially problematic. People with asthma who are physically active outdoors are the most susceptible to sulfur dioxide’s effects, which can trigger sudden airway constriction.
What to Do During an Alert
The core advice is straightforward: stay inside as much as possible, especially if you fall into a sensitive group. If you need to go out, limit your time outdoors to essential activities and avoid strenuous exercise. Running, cycling, or doing yard work forces you to breathe harder and pull more polluted air deep into your lungs.
You can also help keep pollution from getting worse by minimizing car trips and skipping gas-powered lawn equipment. Burning yard debris or using a fire pit during an alert adds directly to the problem.
Keeping Indoor Air Clean
Staying indoors only helps if the air inside your home is actually cleaner than outside. During a wildfire smoke event or a high-ozone day, outdoor pollution seeps in through open windows, door gaps, and your HVAC system’s fresh air intake.
The single most effective step is running a portable air cleaner with a HEPA filter in the room where you spend the most time. If you have central heating and cooling, switch to a filter rated MERV 13 or higher, which captures fine particles far more effectively than a standard filter. Check whether your system can handle that rating first, since a filter that’s too dense for your unit can strain the blower motor. Set your HVAC system to recirculate mode rather than pulling in outside air. This keeps the system filtering the same indoor air repeatedly instead of bringing in contaminated air from outdoors.
Once outdoor conditions improve, even temporarily, open windows to flush out any pollutants that accumulated inside. Cooking, cleaning products, and off-gassing from furniture all contribute to indoor pollution, so periodic ventilation when the outdoor AQI drops back into the green or yellow range helps reset your indoor air.
How to Get Alerts
The EPA’s free AirNow app (available on both iOS and Android) shows current and forecast AQI readings for your location and lets you save multiple areas for quick reference. It also includes a Fire and Smoke Map that overlays real-time PM2.5 readings with active fire locations and smoke plumes, which is particularly useful during wildfire season.
For email notifications, the EPA’s EnviroFlash service sends daily air quality forecasts and alert-day notifications for your zip code. NOAA Weather Radio also broadcasts air quality warnings alongside severe weather alerts, so if you already monitor weather radio, you’ll get air quality information automatically. Local news stations and weather apps increasingly surface AQI data as well, though the AirNow app pulls directly from the monitoring network and tends to update fastest.