An AQI of 87 means the air is moderately polluted. An AQI of 187 means it is unhealthy for everyone. The number is not a measurement of pollution itself — it is a translation of pollutant concentrations onto a single 0-to-500 scale, so you can judge air quality at a glance.
The scale runs from 0 to 500 and is divided into six color-coded categories. Each category describes who is at risk and what precautions make sense. Higher numbers mean more pollution and greater health concern. The categories are the same nationwide because the scale was designed to be consistent across different pollutants and different cities.
What Is The AQI And Who Creates It?
The Air Quality Index is a standardized tool that converts measurements of major air pollutants into one easy-to-read number.
It was developed by the U.S. Environmental Protection Agency under the Clean Air Act. The EPA sets the breakpoints — the pollution concentrations that correspond to each AQI value — and state and local agencies use those breakpoints to report daily air quality in their areas.
The index covers five pollutants regulated by federal law:
- Ground-level ozone
- Particle pollution (PM2.5 and PM10)
- Carbon monoxide
- Sulfur dioxide
- Nitrogen dioxide
Each day, monitoring stations measure the concentration of each pollutant. The EPA’s formula converts each concentration into a separate AQI value. The highest of those values becomes the reported AQI for that location. This is why the news sometimes says “ozone is driving today’s air quality” — that pollutant had the highest sub-index.
The AQI does not measure every airborne substance. It does not capture pollen, mold spores, or many industrial chemicals. If you have allergies, the AQI tells you little about what is triggering your symptoms.
What Do The Six AQI Colors And Levels Mean?
Each color band represents a different level of health concern, and the bands are consistent across the country.
| AQI Range | Color | Category | Who Is Affected |
|---|---|---|---|
| 0–50 | Green | Good | No one |
| 51–100 | Yellow | Moderate | People unusually sensitive to air pollution |
| 101–150 | Orange | Unhealthy for Sensitive Groups | People with heart or lung disease, older adults, children, pregnant people |
| 151–200 | Red | Unhealthy | Everyone may begin to feel effects |
| 201–300 | Purple | Very Unhealthy | Everyone; health alert |
| 301–500 | Maroon | Hazardous | Everyone; emergency conditions |
An AQI of 100 generally corresponds to the level of a pollutant that the EPA considers the upper limit of acceptable air quality for the general public, based on the national ambient air quality standards.
One detail people often miss: the AQI is not linear in the way you might assume. The jump from 150 to 200 represents a larger increase in actual pollution than the jump from 50 to 100. The scale is designed to reflect health risk, not just raw concentration.
What Does Each Level Actually Feel Like?
The AQI categories describe risk, not guaranteed symptoms. A person can be in the “unhealthy” range and feel fine — or be in the “moderate” range and notice irritation.
At green levels, outdoor air is considered clean. No precautions are needed for any group.
At yellow levels, air quality is acceptable for most people. However, a small number of people who are unusually sensitive to air pollution may experience mild symptoms. This group is not precisely defined by the EPA, but generally includes people with asthma or other respiratory conditions who notice symptoms at lower pollution levels than most.
At orange levels, sensitive groups may experience health effects. This includes people with heart or lung disease, older adults, children, and pregnant people. The general public is less likely to be affected. Symptoms can include irritation of the eyes, nose, and throat, coughing, chest tightness, and shortness of breath.
At red levels, some members of the general public may begin to experience health effects. Sensitive groups may experience more serious effects.
At purple levels, the risk of health effects is increased for everyone.
At maroon levels, emergency conditions exist. This level is rarely reached in the U.S., but it has occurred during severe wildfires.
Why AQI Numbers Mean Different Things To Different People
Two people can breathe the same air and have very different responses. The AQI describes population-level risk, not individual risk.
Several factors affect how any one person reacts to air pollution:
- Age. Children breathe more air per pound of body weight than adults. Their lungs are still developing. Older adults may have reduced lung function and more existing health conditions.
- Existing health conditions. Asthma, COPD, heart disease, and diabetes can make a person more vulnerable to pollution.
- Duration of exposure. A short walk at AQI 130 is different from working outdoors for eight hours at the same level.
- Activity level. Breathing harder pulls more pollution deeper into the lungs.
- Genetics and overall health. Individual responses vary in ways researchers do not fully understand.
The AQI does not account for these individual factors. It is a general guide, not a personalized risk assessment.
Research has consistently linked both short-term and long-term exposure to elevated levels of particle pollution and ozone with increased respiratory and cardiovascular health problems. The evidence for this association is strong. What is harder to predict is exactly how any one person will respond on any given day.
How Should You Adjust Your Day Based On AQI?
The AQI is designed to help you make practical decisions. The general principle is simple: as the number rises, reduce strenuous outdoor activity and limit time outdoors.
The EPA’s guidance follows the categories:
- Green: No restrictions. Enjoy normal outdoor activities.
- Yellow: Most people can go about their day normally. Unusually sensitive people might consider reducing prolonged or heavy outdoor exertion.
- Orange: Sensitive groups should reduce prolonged or heavy outdoor exertion. Everyone else can continue normal activity but may want to take it easier.
- Red: Sensitive groups should avoid prolonged or heavy outdoor exertion. Everyone else should reduce it.
- Purple: Sensitive groups should avoid all outdoor exertion. Everyone else should reduce outdoor exertion significantly.
- Maroon: Everyone should avoid all outdoor exertion and stay indoors if possible.
These are general recommendations. Your own health history and your doctor’s advice matter more than any general guideline.
What The AQI Does Not Tell You
The AQI is a useful tool with real limits. Understanding those limits helps you use it well.
It does not tell you the source of the pollution. Smoke from a wildfire, traffic exhaust, and industrial emissions can all produce the same AQI number, but their health effects may differ in ways that are not fully captured by a single index.
It does not measure indoor air quality. Indoor air can be significantly better or worse than outdoor air depending on ventilation, cooking, smoking, and other factors.
It does not capture all pollutants. As mentioned earlier, allergens and many chemical compounds are not included.
It does not predict your personal response. The AQI is a population-level indicator. It cannot tell you whether you will have symptoms at a given level.
Some research suggests that even at lower AQI levels — in the “moderate” range — there may be health effects for certain vulnerable people. The evidence here is not uniform, and the thresholds at which effects begin are still being studied.
Where To Find Your Local AQI
The AQI is reported daily by the EPA through its AirNow system, which provides current conditions and forecasts by zip code or city. Many local health departments and weather apps also display AQI data.
If you live in an area prone to wildfires, ozone exceedances, or industrial pollution, checking the AQI before planning outdoor activities can be a reasonable habit. It takes seconds and can inform decisions about exercise, children’s outdoor play, and time spent outside.
People with asthma or heart conditions may find it especially useful. The AQI is not a substitute for medical advice, but it can be one more piece of information to help you manage your exposure.
Frequently Asked Questions
What is a good AQI number?
An AQI of 0 to 50 is considered good, meaning air quality poses little or no risk. Any number in the green category is generally safe for all groups.
Is AQI 150 bad?
An AQI of 150 falls at the top of the orange category, which means air quality is unhealthy for sensitive groups. People with heart or lung conditions, older adults, children, and pregnant people should reduce outdoor exertion.
What does AQI 200 mean?
An AQI of 200 is in the red category, meaning the air is unhealthy for everyone. Sensitive groups should avoid prolonged outdoor exertion, and others should reduce it.
Can I exercise outside when the AQI is high?
The higher the AQI, the more you should limit or avoid strenuous outdoor activity, especially if you are in a sensitive group. At red levels and above, outdoor exercise is generally not recommended.
