Unhealthy air quality days are triggered by a handful of common causes: wildfire smoke drifting into your area, high ozone levels building up on hot sunny days, traffic and industrial emissions getting trapped near the ground by stagnant weather, or some combination of all three. The EPA’s Air Quality Index (AQI) measures pollution on a scale from 0 to 500, and anything above 100 is considered unhealthy for at least some people. Above 150, it’s unhealthy for everyone.
How the AQI Scale Works
The AQI tracks five major pollutants regulated under the Clean Air Act: ground-level ozone, fine particulate matter (both PM2.5 and PM10), carbon monoxide, sulfur dioxide, and nitrogen dioxide. Each pollutant gets its own score, and the highest one becomes the overall AQI for the day.
The color-coded breakdown looks like this:
- Green (0 to 50): Good air quality with little to no risk.
- Yellow (51 to 100): Acceptable, though a small number of unusually sensitive people may notice effects.
- Orange (101 to 150): Unhealthy for sensitive groups, including children, older adults, and people with asthma or other lung conditions.
- Red (151 to 200): Unhealthy for everyone. Most people will start to experience some effects.
- Purple (201 to 300): Very unhealthy. Serious health effects are more likely.
- Maroon (301+): Hazardous. Emergency-level air quality.
You can check your local AQI in real time at AirNow.gov or through most weather apps. The number updates throughout the day as conditions shift.
Wildfire Smoke: The Most Common Culprit
If your AQI spiked suddenly, wildfire smoke is the most likely explanation. Wildfires produce enormous quantities of fine particulate matter, the tiny particles (PM2.5) that pose the greatest health risk. These particles are small enough to travel extraordinary distances. During the 2023 Canadian wildfires, smoke crossed the entire North American continent, triggered air quality alerts across much of the United States, and even reached Europe and Asia.
This means your air can be unhealthy even if no fire is burning anywhere near you. Smoke from fires hundreds or thousands of miles away can settle over cities for days, pushing the AQI into the orange, red, or purple range. The particles are so fine that they’re often invisible at lower concentrations, so the air can look relatively clear while still being harmful.
Ozone Buildup on Hot Days
Ground-level ozone is the other major driver of unhealthy air days, especially in summer. Unlike wildfire smoke, ozone isn’t emitted directly. It forms when nitrogen oxides from car exhaust, power plants, and industrial facilities react with volatile organic compounds in the presence of sunlight. The hotter and sunnier the day, the more ozone builds up.
This is why ozone-driven air quality warnings tend to peak in the afternoon on hot summer days, particularly in urban areas with heavy traffic. Ozone can still reach unhealthy levels during cooler months, but it’s far more common when temperatures climb. If your AQI is high on a sweltering day with no visible haze, ozone is likely the pollutant responsible.
Weather Patterns That Trap Pollution
Sometimes the pollution isn’t worse than usual. The weather is just refusing to clear it out. Under normal conditions, warm air near the ground rises and carries pollutants upward, where winds disperse them. During a temperature inversion, that process reverses. The ground cools rapidly overnight (especially under clear skies in high-pressure systems), creating a layer of cold air at the surface with warmer air sitting on top of it like a lid.
That warm lid prevents pollutants from rising and dispersing. Traffic exhaust, industrial emissions, and any other pollution sources keep adding to the trapped layer, and concentrations climb hour after hour. This continues until the weather pattern breaks. Inversions are especially common during winter high-pressure systems, but they happen year-round and often explain why air quality degrades over several consecutive days before suddenly improving.
Why PM2.5 Is the Biggest Health Concern
Fine particulate matter, classified as PM2.5 (particles 2.5 micrometers or smaller), is the pollutant most closely linked to serious health effects. These particles are about 30 times thinner than a human hair, small enough to pass through your lungs and into your bloodstream. Ultrafine particles, a subset even smaller than 1 micrometer, can accumulate in organs throughout the body.
Once in the bloodstream, these particles trigger an inflammatory response. Your body releases inflammatory compounds that circulate systemically, stressing your cardiovascular system and aggravating existing conditions. This is why air pollution doesn’t just cause coughing. It’s linked to heart attacks, strokes, and worsening of chronic diseases. Short-term exposure on a single bad air day can affect you, though the risk compounds with repeated or prolonged exposure.
The World Health Organization tightened its recommended annual PM2.5 exposure limit in 2021 from 10 to just 5 micrograms per cubic meter, citing growing evidence that even low-level exposure causes harm. Over 90% of the world’s population breathes air that exceeds this guideline. In the U.S., the EPA strengthened its own annual PM2.5 standard in February 2024, lowering it from 12 to 9 micrograms per cubic meter.
Who Is Most at Risk
When the AQI is in the orange range (101 to 150), the general population is largely unaffected, but sensitive groups can experience noticeable symptoms. The EPA defines these sensitive groups as children, older adults, people with lung diseases like asthma, chronic bronchitis, or emphysema, and people who work or exercise outdoors. The common thread is either underdeveloped or compromised lungs, or higher breathing rates that increase the volume of polluted air inhaled.
There’s also a category of people who react at even lower levels. Controlled studies have identified individuals who experience symptoms during moderate exertion at AQI levels in the yellow range (51 to 100), well below what affects most people. There’s no easy way to predict who falls into this group, but if you consistently feel chest tightness, shortness of breath, or unusual fatigue on moderate AQI days, you’re likely one of them.
How to Protect Yourself
Staying indoors with windows closed is the simplest and most effective step. If you have a portable air purifier with a HEPA filter, run it in the room where you spend the most time. Look for a unit with a clean air delivery rate (CADR) matched to your room size. The CADR rating for tobacco smoke is the best proxy for fine particulate filtration, since it covers the smallest particle sizes. A general rule: the CADR number (in cubic feet per minute) should be at least two-thirds of your room’s square footage.
If you need to go outside on a high-AQI day, respirator-style masks (N95, KN95, or FFP2) offer the best protection. Against wildfire smoke particles, respirators filter roughly 90% of inhaled particulates. Surgical masks capture about 68%, and cloth masks only about 33%. The key is fit. A respirator with gaps around the nose or chin performs closer to a surgical mask. Flat cloth masks or bandanas provide minimal meaningful protection against fine particulate matter.
Reduce your exertion level when air quality is poor. Breathing harder and faster during exercise dramatically increases the volume of polluted air reaching your lungs. On red or purple AQI days, moving outdoor workouts indoors or skipping them entirely makes a measurable difference in your exposure.