About US Air Quality

US Air Quality provides real-time air quality information for cities across the United States. Our data comes from the EPA's AirNow network, which monitors air pollution levels at thousands of stations nationwide.

What is the Air Quality Index (AQI)?

The Air Quality Index is the EPA's standardized way of measuring and communicating air quality. Think of it like a yardstick that runs from 0 to 500. The higher the AQI value, the greater the level of air pollution and the greater the health concern.

0-50
Good
Air quality is satisfactory, and air pollution poses little or no risk.
51-100
Moderate
Air quality is acceptable. However, there may be a risk for some people, particularly those who are unusually sensitive to air pollution.
101-150
Unhealthy for Sensitive Groups
Members of sensitive groups may experience health effects. The general public is less likely to be affected.
151-200
Unhealthy
Some members of the general public may experience health effects; members of sensitive groups may experience more serious health effects.
201-300
Very Unhealthy
Health alert: The risk of health effects is increased for everyone.
301-500
Hazardous
Health warning of emergency conditions: everyone is more likely to be affected.

Major Air Pollutants

The AQI is calculated based on five major pollutants regulated by the Clean Air Act:

PM2.5 (Fine Particulate Matter)
Tiny particles less than 2.5 micrometers that can penetrate deep into lungs and bloodstream.
Measured in: µg/m³
PM10 (Coarse Particulate Matter)
Particles between 2.5 and 10 micrometers, including dust, pollen, and mold.
Measured in: µg/m³
Ozone (Ground-level Ozone)
A harmful gas created by chemical reactions between pollutants in sunlight.
Measured in: ppb
NO2 (Nitrogen Dioxide)
A reddish-brown gas from burning fuel, can irritate airways.
Measured in: ppb
CO (Carbon Monoxide)
A colorless, odorless gas from incomplete combustion.
Measured in: ppm
SO2 (Sulfur Dioxide)
A gas from burning fossil fuels, can affect breathing.
Measured in: ppb

Who is Most at Risk?

While air pollution can affect everyone, certain groups are more sensitive to its effects:

  • People with asthma may experience more frequent or severe attacks
  • People with heart disease are at increased risk for heart attacks
  • Children breathe more air relative to their body size and their respiratory systems are still developing
  • Older adults may have undiagnosed respiratory or heart conditions
  • People who exercise outdoors breathe faster and more deeply, pulling more pollution into their lungs

How to Protect Yourself

When air quality is poor, there are several steps you can take to reduce your exposure:

  1. Check the AQI before outdoor activities - Plan strenuous activities for when air quality is better, typically early morning
  2. Limit outdoor exercise - When AQI is in the unhealthy range, consider moving your workout indoors
  3. Keep windows closed - On poor air quality days, keep windows and doors closed to prevent outdoor air from coming in
  4. Use air purifiers - HEPA air purifiers can remove fine particles from indoor air
  5. Create a clean air room - Designate one room with a portable air cleaner where you can spend time when air quality is poor
  6. Wear a mask - N95 or P100 respirators can filter out fine particles when you must be outdoors during poor air quality

Our Data Sources

The air quality data on this site comes from the EPA's AirNow program, which collects real-time air quality data from monitoring stations operated by federal, state, local, and tribal agencies. AirNow receives data from over 2,000 monitoring stations across the United States.

For more information about air quality monitoring and the AQI, visit:

Contact Us

Have questions or feedback? We'd love to hear from you. While we strive to provide accurate and timely air quality information, please note that this site is for informational purposes only and should not replace official EPA guidance or medical advice.