An air quality alert is a public notification issued when air pollution levels rise high enough to pose health risks. These alerts are tied to the Air Quality Index (AQI), a numerical scale from 0 to 500 that translates pollution measurements into a simple color-coded system. When the AQI is forecast to reach unhealthy levels, typically 101 or above, local or state agencies issue an alert so residents can take steps to protect themselves.
How the Air Quality Index Works
The AQI tracks six pollutants regulated by the EPA: ground-level ozone, particle pollution (fine dust and soot), carbon monoxide, lead, nitrogen oxides, and sulfur oxides. Ground-level ozone and particle pollution are the two most common triggers for alerts. Each pollutant gets its own AQI value based on monitoring station readings, and the highest individual value becomes the overall AQI for that area.
The scale breaks into six color-coded categories:
- Green (0 to 50): Air quality is satisfactory with little or no risk.
- Yellow (51 to 100): Acceptable, though people unusually sensitive to pollution may notice mild effects.
- Orange (101 to 150): Unhealthy for sensitive groups, including people with asthma, heart disease, or lung conditions. The general public is less likely to be affected.
- Red (151 to 200): Unhealthy for everyone. Sensitive groups face more serious effects.
- Purple (201 to 300): Very unhealthy. Health risks increase for the entire population.
- Maroon (301 and higher): Hazardous. Emergency conditions where everyone is likely to be affected.
Most air quality alerts are issued at the orange level or above. You’ll often see them on local news, weather apps, and the federal AirNow website, which provides real-time AQI readings for locations across the country.
What Triggers an Alert
The two pollutants responsible for the vast majority of air quality alerts are ground-level ozone and fine particle pollution, often called PM2.5 (particles smaller than 2.5 micrometers, or about 30 times thinner than a human hair). These tiny particles are especially dangerous because they can travel deep into the lungs and even enter the bloodstream.
Ozone alerts tend to spike on hot, sunny summer days when vehicle exhaust and industrial emissions react with sunlight. PM2.5 alerts are common during wildfire season, in areas with heavy traffic or industrial activity, and on stagnant winter days when cold air traps pollution close to the ground. In 2024, the EPA tightened the annual safety standard for PM2.5 from 12.0 to 9.0 micrograms per cubic meter, reflecting growing evidence that even low levels of particle pollution cause harm over time.
Who Issues Alerts
Air quality alerts come from a patchwork of state environmental agencies, local air quality management districts, and county health departments. These agencies operate networks of ground-level monitors that continuously measure pollutant concentrations. When current readings or forecasts indicate the AQI will cross into unhealthy territory, the responsible agency issues the alert for its jurisdiction. The EPA’s AirNow platform aggregates this data nationally, but the on-the-ground monitoring and alert decisions happen at the state and local level.
Who Needs to Pay the Most Attention
At the orange level (101 to 150), the people most at risk are those with asthma, chronic obstructive pulmonary disease, heart disease, and other respiratory or cardiovascular conditions. Children, older adults, and people who work or exercise outdoors also fall into this sensitive category. At red and above, the risks extend to healthy adults as well.
Short-term exposure to unhealthy air can cause coughing, throat irritation, chest tightness, shortness of breath, and worsening of existing lung or heart conditions. People with asthma may notice increased frequency of attacks. Even healthy individuals exercising outdoors during a red or purple alert can experience reduced lung function and irritation that lasts hours after the exposure ends.
How to Protect Yourself During an Alert
Staying indoors with windows and doors closed is the single most effective step. If you have central air conditioning, use high-efficiency filters rated MERV 13 or higher, and set the system to recirculate mode rather than pulling in outside air. A portable air cleaner in a closed room can create a “clean air” space if your home doesn’t have central air. DIY box fan filter units work in a pinch, though they should never be left running unattended.
Avoid adding pollution sources inside your home during an alert. That means skipping candles, incense, wood fires, gas stoves if possible, and vacuuming, which kicks fine particles back into the air.
If you need to go outside during a red or higher alert, an N95 or P100 respirator filters out fine particles effectively. Cloth masks and surgical masks do not provide meaningful protection against PM2.5. Children ages two and older can wear respirators, but manufacturers don’t make NIOSH-approved sizes for very young children, so keeping toddlers and infants indoors is especially important.
Reduce or reschedule outdoor exercise on alert days. The harder you breathe, the more polluted air you pull deep into your lungs. Moving a run or bike ride indoors, or shifting it to early morning when ozone levels are typically lower, can significantly cut your exposure.
How to Check Your Local Air Quality
The easiest way to stay informed is through AirNow.gov, where you can enter your zip code and see the current AQI, a forecast for the next day, and which pollutant is driving the number. Most smartphone weather apps now display AQI readings as well. You can also sign up for email or text alerts through AirNow’s EnviroFlash notification system, which sends you a message whenever your area’s AQI is forecast to reach a level you choose. Setting your threshold at 101 (the start of the orange zone) gives you advance warning before conditions become a problem.