Definitely one from the conspiracy-theory end of the word-coining spectrum, this term seems to have appeared first about three years ago and is still going strong. Chemtrails are supposedly contrail-like formations produced by military aircraft over the US, Europe, and Australia, among other places. It’s suggested that substances are mixed with the engine exhausts for some purpose — among those often mentioned are climate control, immunisation of populations against future biowarfare, or a systematic attempt to kill off the old and sick as part of the New World Order (some people say they have suffered pneumonia-like symptoms, or mystery rashes). The Illuminati (which is supposedly a secret group that controls major world governments) are often mentioned as being behind chemtrails. Those who believe claim that the trails have characteristic shapes that can be distinguished from ordinary contrails — caused by engine exhausts freezing to ice particles in the upper atmosphere. The whole idea seems to be based on the idea of seeding clouds with silver iodide to make rain fall.
“They call that white thing the chemtrail,” he said. “They can mix all kinds of chemicals in there, mix them in with the jet fuel. Barium salt. Aluminum oxide. All different polymers. The chemicals come out with the exhaust. They linger in the clouds. They can easily change the weather.”
Newsday, Apr. 2002
Believers call these tracks “chemtrails.” They say they don’t know why the chemicals are being dropped, but that doesn’t stop them from speculating. Many guess that the federal government is trying to slow global warming with compounds that reflect sunlight into the sky. Some propose more ominous theories, such as a government campaign to weed out the old and sick.
USA Today, Mar. 2001