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Despite much progress in recent years in cleaning the nation's air, air pollution remains a serious threat to public health and the environment. The American Lung Association estimates that over half of the nation's population lives in counties receiving a grade of "F" on their rating system for ozone pollution. Outdoor air pollution in the U.S. due to particulate pollution alone was estimated by the Environmental Protection Agency (EPA) in 1997 to cause at least 20,000 premature deaths each year. Other estimates place this number at 50,000 to 100,000 deaths per year.(1) Globally, about 800,000 people per year die prematurely due to outdoor air pollution, according to a 2005 study published in the Journal of Toxicology and Environmental Health. This represents about 1.2 percent of total annual global deaths. The death toll due to air pollution only begins to touch the vast magnitude of human suffering caused by breathing our dirty air--for every 75 deaths per year due to air pollution in the U.S., health scientists have estimated that there are 505 hospital admissions for asthma and other respiratory diseases, 3,500 respiratory emergency doctor visits, 180,000 asthma attacks, 930,000 restricted activity days, and 2,000,000 acute respiratory symptom days. (2)

The Clean Air Act requires the U.S. Environmental Protection Agency (EPA) to set National Ambient Air Quality Standards for six pollutants, called "criteria pollutants", considered harmful to public health and the environment. The EPA maintains a database of air pollution forecasts generated for over 150 locations for these six pollutants by the local or state agencies responsible. These forecasts are sent to The Weather Underground as they are updated, and we post the forecasts for the affected areas. Many smaller cities and rural areas do not have air pollution forecasts available. A comprehensive web page indexing all the available air pollution forecasts and real-time air pollution observations is available at the EPA's AIRNOW site.

The National Ambient Air Quality Standards for the six criteria pollutants are listed below. Units of measurement are parts per million (ppm) by volume, milligrams per cubic meter of air (mg/m³), and micrograms per cubic meter of air (µg/m³). There are two types of national air quality standards. Primary standards set limits to protect public health, including the health of "sensitive" populations such as asthmatics, children, and the elderly. Secondary standards set limits to protect public welfare, including protection against decreased visibility, damage to animals, crops, vegetation, and buildings.