Unhealthy air quality means the air contains enough pollution to cause health symptoms in the general population, not just people with pre-existing conditions. In the United States, the EPA measures this using the Air Quality Index (AQI), a scale from 0 to 500. An AQI reading of 151 to 200 falls in the “Unhealthy” category, but air quality problems start well before that threshold, and the scale extends into more dangerous territory above it.
How the AQI Scale Works
The AQI translates complex pollution measurements into a single number you can act on. It tracks five major pollutants: fine particulate matter, ground-level ozone (smog), carbon monoxide, sulfur dioxide, and nitrogen dioxide. The highest reading among those pollutants determines the overall AQI for your area. Each range corresponds to a color-coded category:
- 0 to 50 (Green, Good): Air quality poses little or no risk.
- 51 to 100 (Yellow, Moderate): Acceptable for most people, though unusually sensitive individuals may notice symptoms.
- 101 to 150 (Orange, Unhealthy for Sensitive Groups): People with asthma, heart disease, or lung conditions, along with children and older adults, are likely to be affected.
- 151 to 200 (Red, Unhealthy): Everyone may begin to experience health effects. Sensitive groups face more serious effects.
- 201 to 300 (Purple, Very Unhealthy): Health alert for the entire population. Significant symptoms are likely.
- 301 to 500 (Maroon, Hazardous): Emergency conditions. Everyone is at risk of serious health effects.
When local news or weather apps warn of “unhealthy air quality,” they typically mean the AQI has crossed 150 or is forecast to do so. But in practice, the “Unhealthy for Sensitive Groups” range (101 to 150) already affects a large portion of the population, since roughly one in three Americans has at least one qualifying risk factor.
What Pollutants Are Actually in the Air
The two pollutants most often responsible for pushing air quality into unhealthy territory are fine particulate matter (PM2.5) and ground-level ozone.
PM2.5 refers to tiny particles less than 2.5 micrometers across, about 30 times smaller than a human hair. At that size, they bypass your nose and throat’s natural filtering and penetrate deep into the lungs, reaching the tiny air sacs where oxygen enters your bloodstream. From there, these particles can trigger inflammation throughout the body. Their surfaces carry metals like iron, copper, and zinc, along with organic compounds that generate cell-damaging free radicals in lung tissue. This is why particle pollution affects not just breathing but also heart health.
Ground-level ozone forms when emissions from vehicles, power plants, and industrial facilities react with sunlight. It’s not emitted directly. Instead, nitrogen oxides and volatile organic compounds cook together in warm, sunny conditions to produce ozone. That’s why unhealthy ozone days peak in summer and in urban areas with heavy traffic, though cold-weather ozone spikes do occur.
What Unhealthy Air Feels Like
Even healthy people can experience noticeable symptoms when air quality enters the unhealthy range. Particle pollution commonly causes eye irritation, coughing, phlegm, chest tightness, and shortness of breath. These symptoms are usually temporary and resolve once you’re breathing cleaner air.
Ozone irritates the respiratory system differently. It can create a burning sensation in your airways, reduce how deeply you can breathe, and cause wheezing. If you have asthma, ozone exposure can trigger full attacks. People who exercise outdoors are especially vulnerable because they breathe more deeply and pull more polluted air into their lungs.
For people with heart or lung disease and for older adults, the stakes are higher. Unhealthy air days are associated with increased emergency room visits and hospitalizations for heart attacks, strokes, and severe asthma episodes.
Who Counts as a “Sensitive Group”
You’ll see this phrase constantly in air quality alerts, and it covers more people than you might expect. The EPA defines sensitive populations as people with asthma or other lung diseases, people with heart disease, children, and older adults. Pregnant people and anyone who works or exercises outdoors for extended periods also face elevated risk. Children are particularly vulnerable because their lungs are still developing and they breathe at a faster rate relative to their body size.
Long-Term Risks of Repeated Exposure
A single unhealthy air day is unlikely to cause lasting harm for most people. The real concern is cumulative exposure. PM2.5 particles corrode the delicate walls of the lung’s air sacs over time, gradually impairing lung function. The free radicals generated by these particles damage cells in ways that accumulate, contributing to chronic respiratory disease, cardiovascular disease, and lung cancer with prolonged exposure.
This growing body of evidence is why regulators keep tightening standards. In 2024, the EPA lowered the annual PM2.5 standard from 12.0 to 9.0 micrograms per cubic meter of air. The World Health Organization is even stricter, recommending an annual limit of just 5 micrograms per cubic meter as of 2021. Many cities worldwide remain well above both thresholds.
How to Protect Yourself
On unhealthy air days, staying indoors with windows closed is the simplest and most effective step. A portable air purifier with a HEPA filter can reduce indoor particle levels significantly, though how well it works depends on how airtight your home is. In a well-sealed home, a purifier can remove up to 90% of infiltrating particles. In a drafty older home, much of that cleaned air gets replaced by leaking outdoor air, so you’d need a purifier with a higher output to keep up. Sealing obvious gaps around doors and windows makes any purifier work harder for you.
If you need to go outside, mask choice matters. An N95 respirator filters about 90% of wildfire smoke particles when fitted properly. A surgical mask captures around 68%, which is a meaningful step down. Cloth masks perform worst, catching roughly 30% of the smallest smoke particles. Fit is critical with any mask: gaps around the nose or cheeks let unfiltered air straight through.
Reducing your activity level outdoors also helps. Heavy exercise can increase the volume of air you inhale by 10 to 20 times compared to rest, so moving a run indoors or switching to a lower-intensity walk meaningfully cuts your exposure. Checking your local AQI before heading out, through apps like AirNow or most weather apps, takes seconds and tells you whether it’s a day to modify your plans.