Remember when smokestacks emitted black smoke? Visible emissions from plants of all kinds were once fixtures of industrialization, but today have all but disappeared. Modern exhaust stacks emit water vapor with little to no opacity, and air pollutants aren't generally visible to the human eye. Major regulatory activity continues, however, building from an especially notable expansion in 1987, when the U.S. Environmental Protection Agency formally acknowledged the health risks associated with smaller-particle emissions by creating the particulate-matter-smaller-than-10-microns, or PM10, standard, which sets the mark for "inhalable coarse particulate matter." In 2004, the agency cracked down further and began regulating an even smaller particle, PM2.5, which includes fine inhalabLe particles that can remain in the lungs and cause health issues. The EPA last revised its National Ambient Air Quality Standards, or NAAQS, in 2012, according to a calendar by which the agency, every five years or so, formally differentiated between what it considered clean and unacceptably dirty air. In fact, NAAQS standards in one form or another go as far back as the 1960s and are perhaps best known for the momentum their enforcement gained with creation of the EPA in 1970 during the Nixon administration.
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