The white vapor trails left by high-altitude air traffic are composed almost entirely of condensed water vapor, and they have never been shown to contain the toxic chemicals that the conspiracy theory proclaims to exist. The warm air expelled by aircraft engines serves as the catalyst for the condensation of high altitude water vapor.
The idea that aircraft are seeding the air with unknown harmful chemicals first surfaced in 1996, forcing the U.S. Air Force to issue an official denial that the government was deliberately exposing its citizens to harmful chemical or biological agents. Speculation persisted, however, and several governments and scientific agencies have engaged the public in an effort to explain cloud formation.
Aircraft contrails generally form at altitudes between 5 and 10 miles, usually in cold, moist air. Hot jet exhaust, or the somewhat cooler exhaust from piston engines, triggers the condensation of water under these conditions in the same way water condenses against the side of a cold pitcher or window pane. "Chemtrails" are alleged to persist in the sky for longer periods than ordinary contrails, but atmospheric conditions, such as wind speed, determine the duration of contrails. Whether the cloud contains agents of chemical or biological warfare would have no effect on their duration.