Leonardo da Vinci\'s painting The Last Supper is threatened by tourists who come to Italy to view it and leave behind compounds from their skin, scientists say. Having survived bombings in World War II and exposure to some of the worst air pollution in Europe, the mural in the Maria delle Grazie Church in Milan now faces a chemical threat carried in by visitors, ScienceNews.org reported Thursday. Although a recently installed ventilation system in the church protects the painting from soot and other emission-related debris, carbon compounds naturally emitted from human skin and cosmetics and lotions may be damaging it, experts said. The compounds in question, knows as squalanes, can adhere to dust and then stick to and darken the artwork, they said. Keeping dust under control may contain the problem but, if it doesn\'t tourists may have to wash their hands before seeing the masterpiece da Vince created in the 1490s, scientists said.