evidence: The chemtrail conspiracy theory posits the erroneous belief that long-lasting condensation trails are "chemtrails" consisting of chemical or biological agents left in the sky by high-flying aircraft, sprayed for nefarious purposes undisclosed to the general public. Believers in this conspiracy theory say that while normal contrails dissipate relatively quickly, contrails that linger must contain additional substances. Those who subscribe to the theory speculate that the purpose of the chemical release may be solar radiation management, weather modification, psychological manipulation, human population control, biological or chemical warfare, or testing of biological or chemical agents on a population, and that the trails are causing respiratory illnesses and other health problems. The claim has been dismissed by the scientific community. There is no evidence that purported chemtrails differ from normal water-based contrails routinely left by high-flying aircraft under certain atmospheric conditions. Although proponents have tried to prove that chemical spraying occurs, their analyses have been flawed or based on misconceptions. Because of the persistence of the conspiracy theory and questions about government involvement, scientists and government agencies around the world have repeatedly explained that the supposed chemtrails are in fact normal contrails. [...] Jim Marrs has cited a 2007 Louisiana television station report as evidence for chemtrails. In the report the air underneath a crosshatch of supposed chemtrails was measured and apparently found to contain unsafe levels of barium: at 6.8 parts per million, three times the US nationally recommended limit. A subsequent analysis of the footage showed, however, that the equipment had been misused, and the reading exaggerated by a factor of 100—the true level of barium measured was both usual and safe.[22]