Dust brought in by the constant flow of tourists into the Cenacolo, the convent refectory in Milan where Leonardo Da Vinci's Last Supper hangs, threatens to cause permanent damage to the masterpiece.The fresco, which became even more popular as a result of the claim in Dan Brown's novel The Da Vinci Code that the male figure at the right of Christ was Mary Magdalene, was restored in 1999 at a cost of nine billion lira or €4.6m, and is seen by thousands of visitors every week. Numbers are restricted, and it is often necessary to book to see the painting weeks in advance, but, even so, the amount of dust brought in on the clothing and skin of visitors threatens to blacken the canvas, according to experts. The Italian daily Corriere della Sera reported yesterday that the volume of PM10 – particulate matter between 2.5 and 10 micrometers – has been increasing steadily over the past six years, and while it is below 50 micrograms, the level considered dangerous, during the morning it can go as high as 44 micrograms. Technical experts with Chiama Milano, the firm which monitors data collected by the dust-monitoring machine installed in the refectory, said: "There is a phenomenon known as blackening: the dust contains particles of carbon, which have the inevitable effect of darkening the images. Additionally, we don't know if there are acid components in the dust, which could provoke a reaction in the structure of the painting itself." Da Vinci used an innovative fresco techniuqe to paint the work, but the method proved faulty, leading to its dramatic deterioration and a challenging task for restorers. Vittorio Sgarbi, Milan's cultural councillor, denied the painting was in danger. The dust, he said, "will not cause any damage ... the only damage in this painting is due to the fact that Leonardo painted it so badly" . The fresco's real problem, he added, was caused by humidity, "which even the nine billion lira restoration did not address".