The Chemtrails conspiracy theory claims that some contrails, visible in the Earth's atmosphere, are biological or chemical agents, sprayed at high altitudes for reasons unknown to the public. There are many versions of the theory that propose different explanations, similar to how urban legends are spread.
The conspiracy theory, however, has never found any credit in the scientific community, as it is devoid of objective evidence. What advocates of the theory identify as "chemtrails" are normal condensation trails (contrails), which under certain weather conditions take an unusual appearance.
The spread of this conspiracy theory throughout the world, via mass media and especially via the Internet, has led to government agencies receiving thousands of complaints from people who demand explanations for these alleged activities.
Spraying chemicals from a high altitude for people to inhale would also be extremely inefficient. This is one of the reasons why airplanes spraying pesticides on fields fly so low.
The same government agencies and scientists have repeatedly denied the existence of chemical trails.
The US Air Force has argued that the theory is a hoax, a view which is recognized by many universities, scientific organizations, and the mainstream media. The Department for Environment, Food and Rural Affairs (a British regulatory agency) stressed that the "chemical trails" theory is unscientific. A Canadian parliamentary leader said that "chemical trails" is just a popular expression and that there is no evidence supporting their existence. Similar responses were given by various Italian governmental figures as well as by pilots and weather experts. Magazines and programs have established this scientific theory as a hoax.
The supposed release of chemical trails should not be confused with the technique known as cloud seeding (cloud insemination), which consists of spreading silver iodide in clouds to encourage rainfall.