Have you ever looked up and seen straight lines of clouds across the sky in the wake of a jet airliner? These are "contrails," which is short for "condensation trails." Contrails are made of ice particles and tiny specs of jet fuel exhaust. Depending on the temperature and humidity at the flight level, the condensation may quickly dissipate or they may linger. Under very humid conditions, contrails may expand and last for hours.
Contrails have a measurable effect on the amount of sunlight that reaches Earth and they do contribute to air pollution. Plus, jet fuel is carbon-based and so it contributes to climate change. Because of all of this, jet engines are held to emission standards. Some people believe the federal government is lacing jet exhaust with chemicals intended to fight climate change or to get people hooked on drugs or even to control our minds. These government conspiracy theories about so-called "chem-trails" are nonsense.