Work began Thursday to clean the area around Notre-Dame cathedral in Paris, contaminated by lead particles in the wake of the fire that ravaged the historic building in April. But with access to the monument sealed off, some unsuspecting tourists were left disappointed.

Tests last month revealed dangerously high levels of lead at the cathedral and in the surrounding streets, believed to have been caused when hundreds of tonnes of the toxic metal in Notre-Dame’s roof and steeple melted during the fire.

Workers will use high-pressure hoses and a special gel that will trap the particles as it dries to decontaminate the area.

It means that the cathedral’s parvis and adjacent streets have been closed to public access since Tuesday, catching some visitors hoping to admire the exterior of the famous monument unawares.

“I thought it was a bit odd that there was so much security, police, barriers preventing us from getting really near,” said Maxime, a tourist from Canada. “I found that a bit concerning.”