evidence: If you’ve never heard of chemtrails, don't fret. Before spending time in Southern California I hadn’t heard of them either. But whether it’s being in the land of obsession-with-healthy-living, or perhaps just being near a few major airports, I now hear about chemtrails quite frequently in SoCal. So what are chemtrails? Should we be worried about their effects on our health? [...] Chemtrails, short for chemical trails, are what some call the white trails you see left behind as a plane passes overhead. Believers in the chemical aspect of chemtrails say those trails are actually clouds of chemicals used by the government or someother large scale entity for a variety of nefarious purposes from weather modification, to human population control via sterilization, to even mind control. The theories usually claim that normal jet plane output should dissipate quickly so any clouds that do not disappear immediately must be full of additional, undisclosed substances. Chemtrail theories are not only a U.S.-based phenomena—almost 17% of respondents in an international survey noted a belief in the existence of a "secret large scale atmospheric program" to be at least partly true.