Milan is one of Europe's most polluted cities. And that puts Leonardo da Vinci's The Last Supper at risk. The painting has been on the wall of a dining hall in Milan's Santa Maria Delle Grazie monastery for more than 500 years. Particulates in the air from motor vehicles can accumulate and damage works of art. In response, Italian authorities installed a high-tech system of heating, ventilation and air conditioning. To test its efficacy, researchers installed air monitors throughout the room housing the masterpiece, and outdoors. Over a year, they found that fine particulates were reduced indoors by 88 percent, and coarse particulates by 94 percent, compared to the levels outside. The research was in the journal Environmental Science and Technology. The scientists say that's a huge success. The painting has been threatened before — by Napoleon's army, by the poor attempts of previous conservators, by bombs during World War II. Even airborne lipids from visitors' skin can pose a danger, mitigated by a strict regulation on the number of viewers. But if conservation measures are successful, The Last Supper should feed art lovers for centuries to come.