evidence: The chemtrail conspiracy theory /ˈkɛmtreɪl/ is the erroneous[1] belief that long-lasting condensation trails left in the sky by high-flying aircraft are actually "chemtrails" consisting of chemical or biological agents, sprayed for nefarious purposes undisclosed to the general public.[2] Believers in this conspiracy theory say that while normal contrails dissipate relatively quickly, contrails that linger must contain additional substances.[3][4] Those who subscribe to the theory speculate that the purpose of the chemical release may be solar radiation management,[3] 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.[2][5] [...] The term 'chemtrail' is a portmanteau of the words 'chemical' and 'trail', just as 'contrail' blends 'condensation' and 'trail'.[12] [...] 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 nationally recommended limit. But a subsequent analysis of the footage showed 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.[8]