An air quality alert is an official warning that pollution levels in your area have risen 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 climbs above 100, the air is considered unhealthy for at least some portion of the population, and local or state agencies issue an alert.
How the AQI Scale Works
The AQI breaks air quality into six categories, each tied to a color and a range of numbers:
- Green (0 to 50): Good. Air quality poses little or no risk.
- Yellow (51 to 100): Moderate. Acceptable for most people, though unusually sensitive individuals may notice mild effects.
- Orange (101 to 150): Unhealthy for sensitive groups. People with asthma, heart disease, lung conditions, older adults, children, and people who are active outdoors may experience symptoms.
- Red (151 to 200): Unhealthy. Everyone may begin to feel effects, and sensitive groups face more serious risks.
- Purple (201 to 300): Very unhealthy. Health warnings for the entire population.
- Maroon (301 and higher): Hazardous. Emergency conditions where everyone is at risk.
Most air quality alerts are issued when the AQI is forecast to reach the orange level or above. The specific threshold varies by region, but the core message is the same: pollution is high enough that some people need to change their behavior.
What Pollutants Trigger Alerts
The AQI tracks five major pollutants: ground-level ozone, fine particulate matter (PM2.5), coarse particulate matter (PM10), carbon monoxide, and sulfur dioxide. In practice, the two most common culprits behind air quality alerts are ozone and PM2.5.
Ground-level ozone forms when emissions from cars, power plants, and industrial sources react with sunlight. It’s not the same as the protective ozone layer high in the atmosphere. This is a lung irritant that builds up on hot, sunny days, which is why ozone alerts peak in summer.
PM2.5 refers to tiny particles less than 2.5 micrometers across, small enough to bypass your body’s natural filters and lodge deep in lung tissue or even enter your bloodstream. Wildfire smoke is a major source of PM2.5, but so are vehicle exhaust, industrial emissions, and wood-burning stoves. In February 2024, the EPA tightened the annual PM2.5 standard to 9.0 micrograms per cubic meter, down from the previous 12.0, reflecting growing evidence that even low levels of particle pollution contribute to heart attacks, respiratory illness, and premature death.
Weather Patterns That Make It Worse
Poor air quality isn’t just about how much pollution is being produced. Weather plays an equally important role in whether that pollution disperses or concentrates over your area.
High-pressure systems are one of the biggest factors. They create stagnant air that acts like a dome over a region, trapping vehicle and factory exhaust near the ground. Temperature inversions have a similar effect: normally, warm air near the surface rises and carries pollution upward, but during an inversion a layer of warm air sits above cooler surface air, acting like a lid. These inversions are especially common in winter and in cities surrounded by mountains or valleys, like Los Angeles, Denver, and Salt Lake City.
Heat waves compound the problem from multiple directions. The extreme heat accelerates the chemical reactions that form ground-level ozone, while stagnant conditions during the heat wave prevent both ozone and particulate pollution from dispersing. Drought often accompanies heat waves, drying out vegetation and increasing wildfire risk. Wildfires dump enormous quantities of particulate matter and carbon monoxide into the atmosphere, sometimes pushing AQI readings into the purple or maroon range across hundreds of miles.
Who Is Most at Risk
Alerts often reference “sensitive groups,” which sounds vague but refers to specific populations. People with asthma, chronic obstructive pulmonary disease (COPD), or other lung conditions are among the first to feel the effects. So are people with heart disease, because fine particles can trigger cardiovascular events. Children breathe more air relative to their body size and their lungs are still developing, making them more vulnerable. Older adults face higher risk as well, and anyone who works or exercises outdoors gets a larger dose of polluted air simply because physical activity increases breathing rate and depth.
At orange levels, these groups may experience coughing, shortness of breath, chest tightness, or worsening of existing conditions. At red and above, even healthy adults without preexisting conditions can develop throat irritation, reduced lung function, and difficulty breathing during exertion.
What to Do During an Alert
The single most effective action is reducing your time outdoors, especially during physical activity. Running, cycling, or doing yard work on an alert day dramatically increases the amount of pollution your lungs take in compared to sitting indoors.
If you’re staying inside, keep windows and doors closed. Run your air conditioning on recirculate mode rather than pulling in outside air. If your HVAC system uses replaceable filters, upgrading to MERV 13 or higher can significantly reduce indoor particle levels. A portable air cleaner with a HEPA filter in a single room creates a clean-air refuge, which is especially useful during wildfire smoke events that can last days or weeks. Avoid adding to indoor pollution by burning candles, incense, or wood, and skip vacuuming, which can kick particles back into the air.
If you need to go outside when particle levels are high, an N95 or P100 respirator filters out fine particles effectively. Standard cloth or surgical masks do very little against PM2.5. The respirator needs to fit snugly against your face to work. Children aged 2 and older can wear respirators, but manufacturers don’t make sizes small enough for infants and toddlers.
How to Check Your Local Air Quality
The most reliable source is AirNow.gov, which aggregates data from monitoring stations run by the EPA, state agencies, and tribal governments across the United States. You can search by zip code to see current and forecast AQI levels for your area, broken down by pollutant.
AirNow also offers a mobile app for real-time readings and an email notification service called EnviroFlash that sends you alerts when air quality in your area is forecast to reach unhealthy levels. Most smartphone weather apps now display AQI data as well, though AirNow remains the primary government source. During wildfire events, the AirNow Fire and Smoke Map overlays satellite data with ground-level sensor readings to give a more detailed picture of smoke impacts.
These monitoring networks use high-precision instruments known as Federal Reference Method and Federal Equivalent Method monitors. Low-cost consumer sensors (like PurpleAir) can give a rough neighborhood-level reading, but they’re less accurate and are best used as a supplement to official data rather than a replacement.