What Is the Air Quality Index?
The Air Quality Index, or AQI, is a simple color-coded scale that tells you how clean or polluted the air is and what that means for your health. It is used by the U.S. Environmental Protection Agency (EPA) through AirNow and by many agencies around the world.[1],[2]
AQI is based on the concentrations of five major air pollutants regulated by the Clean Air Act:[2],[4]
- Ground-level ozone (O₃) — formed when pollutants from cars, industry, and other sources react in sunlight.[5]
- Particulate matter (PM2.5 and PM10) — tiny solid or liquid particles from smoke, dust, wildfires, and vehicle exhaust.[4]
- Carbon monoxide (CO) — a colorless, odorless gas from combustion.
- Sulfur dioxide (SO₂) — primarily from burning fossil fuels at power plants and industrial facilities.
- Nitrogen dioxide (NO₂) — from vehicle emissions and industrial sources.
The overall AQI reflects the highest risk from any single pollutant at a given time and location. It is not a single sensor reading — it is a calculated summary.[1],[2]
For each pollutant, scientists compare its measured concentration to a health-based standard. The result is converted to a number on a 0–500 scale, then assigned to one of six color categories. The highest of these individual values becomes the reported AQI for that location.[1],[2]
AQI is designed to be easy to understand: the higher the number, the greater the health concern. The scale is uniform across the United States, so an AQI of 100 in Denver means the same thing as an AQI of 100 in Atlanta.[1]
Each category has a color, a numeric range, and a simple health message. These are the official EPA categories used by AirNow:[1],[2]
| Category | AQI range | Color | Hex | Meaning |
|---|---|---|---|---|
| Good | 0–50 | Green | #00E400 | Air quality is satisfactory; little or no risk. |
| Moderate | 51–100 | Yellow | #FFFF00 | Acceptable; unusually sensitive people may notice symptoms. |
| Unhealthy for Sensitive Groups | 101–150 | Orange | #FF7E00 | Sensitive groups may experience health effects. |
| Unhealthy | 151–200 | Red | #FF0000 | Everyone may begin to feel effects; sensitive groups more so. |
| Very Unhealthy | 201–300 | Purple | #8F3F97 | Health alert: risk of effects is increased for everyone. |
| Hazardous | 301–500 | Maroon | #7E0023 | Emergency conditions; entire population is more likely to be affected. |
Source: U.S. EPA AirNow — official AQI color codes and numeric breakpoints.[1],[2]
Air quality is satisfactory and poses little or no risk.
Air quality is acceptable; unusually sensitive people may experience minor symptoms.
Members of sensitive groups may experience health effects; the general public is less likely to be affected.
Some members of the general public may experience health effects; sensitive groups may experience more serious effects.
Health alert: the risk of health effects is increased for everyone.
Health warning of emergency conditions: everyone is more likely to be affected.
Note: the descriptions above match EPA terminology. Some apps, including Air Diary, may use a simpler 5-tier scale in everyday language while preserving the same color logic.
- Good (0–50)
- Air quality is satisfactory and poses little or no risk.
- Moderate (51–100)
- Air quality is acceptable; unusually sensitive people may experience minor symptoms.
- Unhealthy for Sensitive Groups (101–150)
- Members of sensitive groups may experience health effects; the general public is less likely to be affected.
- Unhealthy (151–200)
- Some members of the general public may experience health effects; sensitive groups may experience more serious effects.
- Very Unhealthy (201–300)
- Health alert: the risk of health effects is increased for everyone.
- Hazardous (301–500)
- Health warning of emergency conditions: everyone is more likely to be affected.
Some people feel the effects of air pollution earlier than others. Groups that are often more sensitive include:[1],[3]
- Children and teenagers
- Older adults
- People with asthma, COPD, or other lung conditions
- People with heart disease
- Pregnant people
- People who work or exercise outdoors
When the AQI reaches the orange “Unhealthy for Sensitive Groups” level or higher, these groups are encouraged to reduce prolonged outdoor exertion.[1]
- Green/Good (0–50): Enjoy outdoor activities as usual.
- Yellow/Moderate (51–100): Air is acceptable for most. Unusually sensitive people may consider limiting long outdoor workouts.
- Orange/USG (101–150): Sensitive groups should reduce prolonged outdoor exertion. Everyone else can usually continue normal activity.
- Red/Unhealthy (151–200): Everyone may begin to feel effects. Avoid long or intense outdoor activity.
- Purple/Very Unhealthy (201–300): Health alert for everyone. Avoid outdoor exertion and keep windows closed if possible.
- Maroon/Hazardous (301–500): Emergency conditions. Everyone should avoid all outdoor activity and follow local health warnings.
These are general guidelines, not medical advice. Always listen to your body and consult a healthcare provider for personal health concerns.
Air Diary can auto-fill outdoor air quality from public sources like the EPA AirNow program and OpenWeather. The app then helps you notice how the air quality on a given day relates to how you felt — building your own gentle picture over time.
You remain in control. You can choose the data source, enter a reading manually, or skip the AQI section entirely. Air Diary is a wellness tool, not a medical device.
- What is the difference between AQI and PM2.5?
- PM2.5 is one pollutant; AQI is a summary index that can include PM2.5, ozone, and other pollutants. PM2.5 is often the main driver of AQI during wildfire smoke.[4]
- Why does AQI change during the day?
- Pollution levels vary with traffic, sunlight, weather, wind, and wildfire activity. Morning rush hour and sunny afternoons can raise ozone, while smoke can spike PM2.5 suddenly.[5]
- Is AQI the same everywhere?
- AQI values are computed for specific locations. A reading from a city monitor may differ from your exact neighborhood, especially near busy roads or fires.[1],[3]
- Can I use Air Diary without AQI data?
- Yes. AQI is optional. You can log how the air feels and how you feel without any numeric reading.
This guide is based on public guidance from the U.S. Environmental Protection Agency, AirNow, and the American Lung Association. Air Diary is not affiliated with these organizations.