The chemtrails conspiracy theory suggests that the condensation trails that appear as thin clouds behind airplanes are, in fact, vaporized chemicals being emitted into the atmosphere as part of a plot, generally to either control global weather or harm people below. Theorists often cite the real practice of "cloud seeding," or the release of silver iodide in clouds to encourage precipitation, as evidence that even greater geoengineering may be occurring.
That idea that the clouds that follow aircraft are chemtrails, rather than mere condensation trails, dates back to at least the 1990s. Art Bell, the radio host of "Coast to Coast AM," frequently discussed chemtrails on his radio show.
Proponents of the chemtrails conspiracy theory argue that prior to the 1990s, the trails behind airplanes did not last as long as they do today, suggesting that this perceived difference is the result of chemical additives.
Believers in chemtrails also look to the Space Preservation Act of 2001, which was introduced in the U.S. Congress but died in committee. The legislation specifically mentioned chemtrails as a form of "exotic weapons" that would be banned under the Act. Chemtrail conspiracy theorists took the bill's introduction, and its ultimate failure to pass committee, as evidence that the United States was already releasing chemicals into the atmosphere, that some members of Congress learned of the chemtrails program and tried to stop it, and that forces within the Department of Defense ensured the bill did not pass.