The Vatican says the Sistine Chapel’s precious frescoes are starting to turn white from the air pollution caused by so many visitors passing through each day.

Officials first noticed the whitening patina in 2010 and immediately launched an investigation. While the exact origin still unknown, officials told an academic conference Thursday that the powdery patina — which covers some frescoes like cracked sugar icing — was believed to be calcium carbonate and calcium bicarbonate deposits. It was easily removed.

Vatican officials have said the Sistine Chapel’s new air conditioning system inaugurated this week would prevent potential damage from rising levels of carbon dioxide and humidity brought on by crowds. But they never revealed that damage was already underway and that the new system was aimed at preventing further problems.