Chemtrails are theorized to be chemicals and heavy metals that are being distributed by airplanes in order to make the population sick. Some evidence from chemtrails has been tested in laboratories and does contain barium and aluminum, among other substances that are toxic to humans. These chemtrails float down from the sky and deposit themselves into plants, water and soil. Another theory is that the sun is too hot and the chemtrails are to shield us from the ill effects of the sun's rays.
According to the chemtrail conspiracy theory, long-lasting trails left in the sky by high-flying aircraft are chemical or biological agents deliberately sprayed for sinister purposes undisclosed to the general public. Believers in the theory argue that normal contrails dissipate relatively quickly, and contrails that do not dissipate must contain additional substances. These arguments have been dismissed by the scientific community: such trails are normal water-based contrails (condensation trails) that are routinely left by high-flying aircraft under certain atmospheric conditions. Although proponents have attempted to prove that the claimed chemical spraying does take place, their analyses have been flawed or based on misconceptions. - Source: Wikipedia