Chemtrails are the basis of a theory that states some contrails left behind jets are not merely condensation, but include chemicals or biological agents deliberately sprayed on the unknowing public for an unidentified purpose. The term chemtrail comes from the phrase "chemical trail," much like contrail is derived from "condensation trail." Chemtrails do not include crop-dusting or chemicals released during aerial attempts to fight fires. The word refers only to aerial trails that are said to be created by the high-altitude release of substances not found in ordinary contrails, resulting in the appearance what proponents consider to be uncharacteristic sky tracks.
The chemtrail conspiracy theory, as it's known, started in 1998, after the Air Force Air University created a strategy paper filled with hypothetical scenarios for maintaining future military dominance. Conspiracy theorists accused the Air Force of using aircraft to spray unwitting Americans with unknown substances. Those aircraft created unusual contrail patterns, called "mystery contrails." Investigative journalist and author William Thomas is credited with coining the phrase "chemtrails" when his reports about the issue received national attention.
Proponents of this theory say that chemtrails can be differentiated from contrails in a variety of ways. Not only do they last longer, but they're also thicker and form patterns that resemble "X"s, grids, and cross-hatched or parallel lines. They often become wispy "clouds" that join together to form a veil. Chemtrails are also purported to have color spectra in the streams. Unmarked or military aircraft flying at unusual altitudes or excessive concentrations of sky tracks in a single area also indicate their presence.
Chemtrails are generally believed to be toxic, although supporters disagree on their purpose. Ideas vary from benign measures like trying to vaccinate citizens against disease or bioterrorism and global warming mitigation measures, to more sinister scenarios like ID tracking, military weapons testing, chemical population control, and biowarfare. Symptoms are said to range from flu-like conditions, including neck pain and stomach cramps, to severe respiratory distress similar to pneumonia.
Scientists, universities, publications, and governments around the world have all denied the existence of chemtrails. The United States received so many complaints that NASA, the EPA, the FAA, and NOAA issued a jointly published fact sheet in 2000 that explains how contrails are formed and outlines their known and potential impacts on temperature and climate. Other countries — including Britain and Canada, where chemtrails are purported to be prevalent — have also vehemently refuted the chemtrail conspiracy theory.