A moat and fortified walls have protected the Tower of London from vandals for almost 800 years, but against the ravages of pollution, the iconic royal palace doesn’t stand a chance. The entire complex is turning yellow from the exhaust of cars and trucks, according to a new study.

The discoloration is most noticeable at the complex’s White Tower, the original square fortress built by William the Conqueror in 1078. “When we question visitors, they say the color is cream,” said study co-author Peter Brimblecombe, an atmospheric chemist at the University of East Anglia in the United Kingdom. The results are detailed in this month’s issue of the journal Environmental Science and Technology.

In the next 20 years, Brimblecombe said, the famous tower will turn a more brownish yellow. “You might imagine how angry people are going to get about paying a large sum of money to see a yellow tower that’s supposed to be white,” he said.