Air quality is considered “bad” starting at an Air Quality Index (AQI) of 101, the point where pollution begins affecting people with asthma, lung disease, or other vulnerabilities. For the general public, air becomes unhealthy at an AQI of 151 or higher. The AQI scale runs from 0 to 500, and understanding where you fall on it determines whether you should change your plans for the day.
The AQI Scale, Level by Level
The U.S. EPA uses a color-coded AQI system that translates raw pollution measurements into a single number. Here’s what each range means in practical terms:
- 0 to 50 (Green, Good): Air quality poses little or no risk. You can exercise and spend time outdoors without concern.
- 51 to 100 (Yellow, Moderate): Acceptable for most people, but unusually sensitive individuals may notice mild effects. An AQI of 100 is the upper boundary before the EPA starts issuing cautions.
- 101 to 150 (Orange, Unhealthy for Sensitive Groups): Children, older adults, and people with asthma, COPD, or heart disease should reduce prolonged outdoor exertion. The general population is unlikely to feel symptoms.
- 151 to 200 (Red, Unhealthy): Everyone may begin experiencing respiratory irritation. People in sensitive groups should avoid extended time outdoors.
- 201 to 300 (Purple, Very Unhealthy): Health warnings are triggered for the entire population. Outdoor activity should be limited for everyone.
- 301 and higher (Maroon, Hazardous): Emergency-level conditions. Serious health effects are possible for all people with any outdoor exposure.
What’s Actually in Bad Air
The AQI tracks five major pollutants, but the two most commonly responsible for high readings are fine particulate matter (PM2.5) and ground-level ozone.
PM2.5 refers to tiny particles, roughly 30 times smaller than the width of a human hair, that penetrate deep into the lungs and can enter the bloodstream. Air is considered “good” when PM2.5 stays at or below 12 micrograms per cubic meter over a 24-hour period. Once it crosses 35.5 µg/m³, the AQI enters the orange “Unhealthy for Sensitive Groups” range. At 55.5 µg/m³ and above, air quality is unhealthy for everyone. During severe wildfire smoke events, PM2.5 can soar past 250 µg/m³, pushing into hazardous territory.
Ground-level ozone is created when sunlight reacts with vehicle exhaust and industrial emissions. It tends to peak on hot, sunny afternoons. The effects vary dramatically depending on how hard you’re breathing: an average young adult playing an active outdoor sport for two hours can experience measurable lung function decline and airway inflammation at just 120 parts per billion. That same person sitting still outdoors wouldn’t feel effects until concentrations hit 300 to 400 ppb. This is why AQI warnings specifically mention “heavy exertion” and “prolonged outdoor activity” rather than simply being outside.
Why Activity Level Matters
Your breathing rate is the single biggest factor in how much pollution reaches your lungs. Sitting on a patio and running a 5K on the same day with the same AQI are two very different exposures. When you exercise, you breathe faster and more deeply, pulling air past the nose (which filters some particles) and straight into the lower lungs. An outdoor worker doing intermittent physical labor for eight hours can experience small to moderate lung function effects at ozone levels as low as 60 to 70 ppb, a concentration that someone resting indoors would never notice.
This is why most AQI guidance focuses on reducing “prolonged or heavy exertion” rather than telling you to stay inside entirely. At orange-level readings, moving your jog indoors or shortening your time outside is usually enough. At red or purple levels, minimizing all outdoor activity becomes the safer choice.
Who Needs to Pay Attention Sooner
The “Unhealthy for Sensitive Groups” category exists because some people react to pollution levels that most adults wouldn’t notice. The EPA defines sensitive groups differently depending on the pollutant. For ozone, the list includes children, older adults, people with asthma or chronic lung disease like emphysema, and anyone who works or exercises outdoors regularly. For particle pollution, people with heart disease are also included, because fine particles can trigger cardiovascular events, not just breathing problems.
Children are particularly vulnerable because their lungs are still developing and they tend to spend more time in active outdoor play. Older adults are at higher risk partly because they’re more likely to have undiagnosed heart or lung conditions that pollution can aggravate. If you fall into any of these groups, treat an AQI of 101 the way the general population would treat 151: start limiting your time and effort outdoors.
Indoor Air Has Its Own Thresholds
When outdoor air quality drops, most people retreat inside, but indoor air isn’t automatically clean. Buildings with poor ventilation can accumulate carbon dioxide and other pollutants that cause headaches, fatigue, and difficulty concentrating. Office buildings typically see CO2 levels between 350 and 2,500 ppm. The recommended ventilation standard for offices corresponds to keeping indoor CO2 around 870 ppm, and buildings that routinely exceed 1,000 ppm tend to see more complaints of sick building symptoms.
During high-pollution days, keeping windows closed and running an air purifier with a HEPA filter can significantly reduce indoor particle levels. If your home has no mechanical filtration, even a box fan with a furnace filter taped to the back can help. The goal is to create at least one clean-air room where PM2.5 stays low while outdoor conditions are poor.
How to Check Air Quality Where You Are
The EPA’s AirNow website and app provide real-time AQI readings for locations across the United States, broken down by pollutant. Most weather apps also display AQI in their daily forecasts. The number you see reflects conditions at the nearest monitoring station, so keep in mind that air quality can vary within a few miles, especially near highways, industrial areas, or active fires.
When checking the AQI, look at which pollutant is driving the number. A reading of 120 from ozone means afternoon outdoor exercise is the main concern, while a reading of 120 from PM2.5 (often during wildfire season) means the particles are present all day and night, making sustained exposure more of a risk. The same AQI number can call for different responses depending on what’s in the air.