High Quality Content by WIKIPEDIA articles! Air pollution is the introduction of chemicals, particulate matter, or biological materials that cause harm or discomfort to humans or other living organisms, or damages the natural environment into the atmosphere. In the 1960s, 70s, and 90s, the United States Congress enacted a series of Clean Air Acts which significantly strengthened regulation of air pollution. Individual U.S. states, some European nations and eventually the European Union followed these initiatives. The Clean Air Act sets numerical limits on the concentrations of a basic group of air pollutants and provide reporting and enforcement mechanisms. In 1999, the United States EPA replaced the Pollution Standards Index with the Air Quality Index to incorporate new PM2.5 and Ozone standards. In an October 2006 letter to EPA, the agency's independent scientific advisors warned that the ozone smog standard - needs to be substantially reduced - and that there is scientific justification for retaining the current, weaker standard. The scientists unanimously recommended a smog threshold of 60 to 70 ppb after they conducted an extensive review of the evidence.

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