AQI Information Brochure

Air Quality Index (AQI) Basics 

The AQI is an index for reporting daily air quality. It tells you how clean or polluted your air is, and what associated health effects might be a concern for you. The AQI focuses on health effects you may experience within a few hours or days after breathing polluted air. To calculate the AQI five major air pollutants are included: ground-level ozone, particle pollution, carbon monoxide, sulfur dioxide, and nitrogen dioxide. Ground-level ozone and airborne particles, particularly those known as pm 2.5, are the two pollutants that pose the greatest threat to human health. 

How Does the AQI Work?

The higher the AQI value, the greater the level of air pollution and the greater the health concern. For example, an AQI value of 50 (pm 2.5 around 12) represents good air quality with little potential to affect public health, while an AQI value over 300 (pm 2.5 around 250) represents hazardous air quality.

AQI values below 100 (pm 2.5 around 35) are generally thought of as satisfactory. When AQI values are above 100, air quality is considered to be unhealthy - at first for certain sensitive groups of people, then for everyone as AQI values get higher.

The purpose of the AQI is to help you understand what local air quality means to your health. To make it easier to understand, the AQI is divided into six categories:

Air Quality Index
(AQI) Values

Levels of Health Concern


When the AQI is in this range:

..air quality conditions are:


0 to 50



51 to 100



101 to 150

Unhealthy for Sensitive Groups


151 to 200



201 to 300

Very Unhealthy


301 to 500



Each category corresponds to a different level of health concern. The six levels of health concern and what they mean are:

  • "Good" AQI is 0 to 50 (pm 2.5 is 0 to 12). Air quality is considered satisfactory, and air pollution poses little or no risk.
  • "Moderate" AQI is 51 to 100 (pm 2.5 is 13 to 35). Air quality is acceptable; however, for some pollutants there may be a moderate health concern for a very small number of people. For example, people who are unusually sensitive to ozone may experience respiratory symptoms.
  • "Unhealthy for Sensitive Groups" AQI is 101 to 150 (pm 2.5 is 36 to 60). Although general public is not likely to be affected at this AQI range, people with lung disease, older adults and children are at a greater risk from exposure to ozone, whereas persons with heart and lung disease, older adults and children are at greater risk from the presence of particles in the air.
  • "Unhealthy" AQI is 151 to 200 (pm 2.5 is 61 to 200). Everyone may begin to experience some adverse health effects, and members of the sensitive groups may experience more serious effects.
  • "Very Unhealthy" AQI is 201 to 300 (pm 2.5 is 201 to 250). This would trigger a health alert signifying that everyone may experience more serious health effects.
  • "Hazardous" AQI greater than 300 (pm 2.5 above 250). This would trigger a health warnings of emergency conditions. The entire population is more likely to be affected.

Particle Pollution (PM)

Particle pollution, also called particulate matter or pm, is a mixture of solids and liquid droplets floating in the air. Some particles are released directly from a specific source, while others form in complicated chemical reactions in the atmosphere.

Particles come in a wide range of sizes. Particles less than or equal to 10 micrometers in diameter are so small that they can get into the lungs, potentially causing serious health problems. Ten micrometers is less than the width of a single human hair.

  • Coarse dust particles (pm 10) are 2.5 to 10 micrometers in diameter. Sources include crushing or grinding operations and dust stirred up by vehicles on roads.
  • Fine particles (pm 2.5) are 2.5 micrometers in diameter or smaller, and can only be seen with an electron microscope. Fine particles are produced from all types of combustion, including motor vehicles, power plants, residential wood burning, forest fires, agricultural burning, and some industrial processes

Health Effects

People with heart or lung diseases, older adults and children are most likely to be affected by particle pollution exposure. However, even if you are healthy, you may feel temporary symptoms if you are exposed to high levels of particle pollution. Numerous scientific studies connect particle pollution exposure to a variety of health issues, including:

  • irritation of the eyes, nose and throat
  • coughing, chest tightness and shortness of breath
  • reduced lung function
  • irregular heartbeat
  • asthma attacks
  • heart attacks
  • premature death in people with heart or lung disease

 (adapted from