Air Quality Index (AQI) Legend and Additional Information
New York State Air Quality Hotline: (800) 535-1345
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The U.S. Air Quality Index – daily index
The U.S. Air Quality Index (AQI) is the tool used by DEC and EPA for communicating daily air quality. It uses color-coded categories and provides statements for each category that tell you about air quality in your area, which groups of people may be affected, and steps you can take to reduce your exposure to air pollution. It’s also used as the basis for air quality forecasts and current air quality reporting.
EPA has issued a national index for air quality since 1976 to provide an easy-to-understand daily report on air quality in a format that’s the same from state to state. The AQI as we know it today was issued in 1999; it’s been updated several times since to reflect the latest health-based air quality standards.
There is an AQI for five major pollutants that are regulated by the Clean Air Act: ozone, particle pollution (also called particulate matter), carbon monoxide, nitrogen dioxide and sulfur dioxide. The AQI for each pollutant is generally based on the health-based national ambient air quality standard for that pollutant and the scientific information that supports that standard. The ozone AQI is an 8-hour index; for particle pollution, it’s 24 hours.
AQI forecasts are a prediction of the day’s AQI. Forecasts are issued in the afternoon for the next day, then the current day’s forecast is updated in the morning. The forecasts are prepared by air pollution meteorologists at DEC. They use a number of tools – including weather forecast models, satellite images, air monitoring data, and computer models that estimate how pollution travels on the air. They also use their own knowledge of how pollution behaves in certain communities to issue the air quality forecast for the next day. DEC forecasters issue forecasts for ozone and particle pollution, which are two of the most widespread pollutants. Forecasts are available on this website, at airnow.gov, and can be received by email by signing up at Enviroflash.info.
AQI forecasts issued in the afternoon focus on the next day. For ozone, an AQI forecast focuses on the period during the day when average 8-hour ozone concentrations are expected to be the highest. For fine particle pollution, the forecast predicts the average 24-hour concentration for the next day.
AQI forecasts tell you what the next day’s AQI is expected to be, which groups of people may be affected, and steps individuals can take to reduce their exposure to air pollution.
Use AQI forecasts to help you plan your outdoor activities for the day. Much like a weather forecast lets you know whether to pack an umbrella, an air quality forecast lets you know when you may want to change your outdoor activities to reduce the amount of air pollution you breathe in. Many forecasters also provide a “forecast discussion,” which lets you know when pollution is expected to be highest during the day – and if there are times when air quality is expected to be better.
The NowCast AQI
EPA issues the NowCast AQI, which shows your current air quality using the AQI colors and scale. They use two algorithms, called “NowCasts,” to relate hourly readings from air quality monitors to the AQI for ozone and the AQI for particle pollution.
The NowCast shows you air quality for the most current hour available by using a calculation that involves multiple hours of past data. The NowCast uses longer averages during periods of stable air quality and shorter averages when air quality is changing rapidly, such as during a wildfire. Current NowCast values are available on airnow.gov and on phone apps.
Fine Particle pollution (also known as “particulate matter” or “PM2.5”) in the air includes a mixture of solids and liquid droplets. Some particles are emitted directly; others are formed in the atmosphere when other pollutants react. Particles come in a wide range of sizes. Those less than 2.5 micrometers in diameter are so small that they can get into the lungs, potentially causing serious health problems. 2.5 micrometers is 1/7th the width of a single human hair.
Ozone is a gas composed of three atoms of oxygen. Ozone occurs both in the Earth's upper atmosphere and at ground level. Ozone can be good or bad, depending on where it's found.
Ozone occurs naturally in the Earth's upper atmosphere 6 to 30 miles above the Earth's surface where it forms a protective layer that shields us from the sun's harmful ultraviolet rays. This beneficial ozone is gradually being destroyed by man-made chemicals. An area where the protective ozone layer has been significantly depleted for example, over the North or South pole's sometimes called the ozone hole.
In the Earth's lower atmosphere, near ground level, ozone is formed when pollutants emitted by cars, power plants, industrial boilers, refineries, chemical plants, and other sources react chemically in the presence of sunlight. Ozone at ground level is a harmful air pollutant.