What is the Air Quality Index (AQI)?

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. EPA calculates the AQI for five major air pollutants regulated by the Clean Air Act: ground-level ozone, particle pollution (also known as particulate matter), carbon monoxide, sulfur dioxide, and nitrogen dioxide. For each of these pollutants, EPA has established national air quality standards to protect public health.

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. The six levels of health concern and what they mean are:

• "Good" The AQI value for your community is between 0 and 50. Air quality is considered satisfactory, and air pollution poses little or no risk.
• "Moderate" The AQI for your community is between 51 and 100. 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" When AQI values are between 101 and 150, members of sensitive groups may experience health effects. This means they are likely to be affected at lower levels than the general public. For example, people with lung disease are at greater risk from exposure to ozone, while people with either lung disease or heart disease are at greater risk from exposure to particle pollution. The general public is not likely to be affected when the AQI is in this range.
• "Unhealthy" Everyone may begin to experience health effects when AQI values are between 151 and 200. Members of sensitive groups may experience more serious health effects.
• "Very Unhealthy" AQI values between 201 and 300 trigger a health alert, meaning everyone may experience more serious health effects.
• "Hazardous" AQI values over 300 trigger health warnings of emergency conditions. The entire population is more likely to be affected.

AQI colors

EPA has assigned a specific color to each AQI category to make it easier for people to understand quickly whether air pollution is reaching unhealthy levels in their communities. For example, the color orange means that conditions are "unhealthy for sensitive groups," while red means that conditions may be "unhealthy for everyone," and so on.

How is AQI Displayed on the New Mexico Environment Department Website?

There are some differences in the official EPA AQI calculation and the method for calculation and display on the NMED website. The biggest difference is in the averaging period used. In order to make the AQI for particulate matter be more responsive during dust storm events, the AQI is calculated for particulate matter based upon the readings from the last two hours.

The AQI is displayed on the NMED website in a number of ways. Browsing the map of the state (Maps > New Mexico Map) will show each region as a colored dot. Those dots are dynamically generated and colored based upon the worst air quality at any station in that region. Clicking on a region will show dots for each of the region's stations. In the same manner, the colors of these dots are displayed based upon the AQI calculated for the most recent data collected at the site. When the mouse hovers over a dot, the pop up information has an indicator in AQI colors and the text AQI level ("Good" or "Moderate"), for example. When one of the regional dots is clicked, the Station Summary Screen is displayed. On that page, a graphic display in the form of a dial is displayed based upon the AQI as in this example - 

Note that the date, time, index value and pollutant used to calculate the AQI is displayed on the graphic. If a station measures more than one pollutant that can be used to calculate AQI, the one that measures the worst air quality is used as the "responsible" pollutant and is displayed.

Where to get more information

EPA has a number of publications about the AQI. They are available at the AIRNow Publications page.