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AQI forecast and maps courtesy of AIRNow
Mid Atlantic AQI map by AIRNow
AQI = Good Quality AQI = Moderate Quality AQI = Unhealthy for Sensitive Groups AQI = Unhealthy AQI = Very Unhealthy AQI = Hazardous
National AQI map by AIRNow

Air Quality Index (AQI)

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

Particulate matter (PM), is a complex mixture of extremely small particles and liquid droplets. Particle pollution is made up of a number of components, including acids, organic chemicals, metals, and soil or dust particles. The EPA is concerned about particles that are 10 micrometers in diameter or smaller. Once inhaled, these particles can affect the heart and lungs and cause serious health effects.
Particle pollution is divided into two categories by their size:
"Inhalable coarse particles", such as those found near roadways and dusty industries, are larger than 2.5 micrometers and smaller than 10 micrometers in diameter.
"Fine particles"(PM2.5), such as those found in smoke and haze, are 2.5 micrometers in diameter and smaller. These particles can be directly emitted from sources such as forest fires, or they can form when gases emitted from power plants, industries, and automobiles react in the air.

Ozone (O3) is a colorless gas composed of three oxygen atoms. "Good" ozone occurs naturally in the stratosphere approximately 10 to 30 miles above the earth's surface. This Ozone forms a layer that protects life on earth from the sun's harmful rays. In the earth's lower atmosphere, ground-level ozone is considered "bad." Motor vehicle exhaust and industrial emissions, gasoline vapors, and chemical solvents as well as natural sources emit oxides of nitrogen (NOx) and volatile organic compounds (VOC) that help form ozone. Ground-level ozone is the primary constituent of smog. Sunlight and hot weather can cause ground-level ozone to form in harmful concentrations in the air.

For more information on air quality and air pollution visit the Environmental Protection Agency (EPA) website.