The chemtrail conspiracy, which was first promulgated in 1996 as an accusation against the U.S. Air Force, revolves around the idea that high-flying aircraft disperse harmful chemicals into the atmosphere in the form of unusually long-lasting contrails. The Environmental Protection Agency has since studied and disproven the theory.
The term "chemtrail" is a portmanteau of the words "chemical" and "trail," and shares its etymology with the contraction of the words condensation and trail into contrail, the white line of ice crystals sometimes left behind by jet aircraft.
William Thomas, an environmental journalist, first popularized the idea that contrails might contain dangerous chemicals in 1999. In an article entitled "Contrails: Poison From the Sky," he alleged that a large number of American residents had fallen ill after witnessing jet aircraft flying in strange formations and leaving oddly resilient contrails. Thomas suggested that the U.S. Air Force might be using these chemical contrails to modify the weather.
In 2000, the Environmental Protection Agency, in conjunction with the Federal Aviation Administration, the National Aeronautics and Space Administration and the National Oceanic and Atmospheric Administration, published a report explaining the nature and formation of contrails. The report, entitled Aircraft Contrails Fact Sheet and posted on the agencys website, states that contrails are in no way directly harmful to the public. It acknowledged, however, that it is possible that the cloudiness of contrails may contribute to climate change. As of 2015, no reputable scientific evidence supports the existence of chemtrails, but proponents of the theory still exist.