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Health Department

Air Quality Index Explained

The Air Quality Index (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.

The AQI for Nashville and Davidson County, Tennessee is reported by the Metro Public Health Department, Air Pollution Control Division. The reported AQI is the maximum value for the previous day from midnight to midnight. It incorporates the measured concentrations of five pollutants: carbon monoxide, ozone, sulfur dioxide, PM2.5 and PM10. For each of these pollutants, EPA has established national air quality standards to protect public health. Ground-level ozone and airborne particles are the two pollutants that pose the greatest threat to human health in this country.

The AQI is updated daily, Monday through Friday, at approximately 9:00 A.M. In addition to this website, a daily recorded update of the AQI can be obtained by calling 615-340-0488.

How Does the AQI Work?

Think of the AQI as 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. For example, an AQI value of 50 represents good air quality with little potential to affect public health, while an AQI value over 300 represents hazardous air quality.

An AQI value of 100 generally corresponds to the national air quality standard for the pollutant, which is the level EPA has set to protect public health. AQI values below 100 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.

Understanding the AQI

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. Each category corresponds to a different level of health concern.

AQI Colors and Definitions

EPA has assigned a specific color to each AQI category so the public can quickly determine if air pollution is reaching unhealthy levels in their community.

Air Qualitiy Index Levels of Health Concern Numerical Value Meaning
Good 0-50 Air quality is considered satisfactory, and air pollution poses little or no risk.
Moderate 51-100 Air quality is acceptable; however, for some pollutants there may be a moderate health concern for a very small number of people who are unusually sensitive to air pollution.
Unhealthy for Sensitive Groups 101-150 Members of sensitive groups may experience health effects. The general public is not likely to be affected.
Unhealthy 151-200 Everyone may begin to experience health effects; members of sensitive groups may experience more serious health effects.
Very Unhealthy 201-300 Health alert: everyone may experience more serious health effects.
Hazardous > 300 Health warnings of emergency conditions. The entire population is more likely to be affected.

View the "Air Quality Index Levels" chart as an Adobe Acrobat document.