We've all seen the white, hazy streaks in the sky before, usually in the wake of a plane. These marks are called contrails, and they're actually considered lines of clouds that form from the aircraft's engine exhaust.
But this explanation is seemingly too insufficient for conspiracy theorists, who cite contrails as evidence that the government is up to no good. That is, they think chemicals are in sprayed in the air to control the minds of the mass public, or even as an attempt to control climate and weather. This is why they dubbed contrails as "chemitrails."
To offer up scientific evidence to quell these rumors, scientists at the Carnegie Institution for Science embarked on a study of these contrails. The scientists showed chemitrail evidence to 77 atmospheric scientists and geochemists - the people who would know this topic best. Unsurprisingly, the consensus is that what theorists perceive to be chemitrails were no different than normal contrails. In addition, soil and sediment analysis could not definitively prove the chemitrails, and that other phenomena were much more likely.
Of course these explanations could be perceived to be all too convenient for conspiracy theorists. However, perhaps Occam's razor is true for this situation: the simplest explanation is usually the right one.