Police in Paris said lead levels were found to be between 32 and 65 times the recommended limit by French health authorities.
The areas closest to the cathedral are currently closed.
Police said the main danger is lead dust that could coat surfaces of nearby homes and businesses.
To avoid lead poisoning, authorities have recommended a good cleaning with a damp cloth, and that pregnant women and children should wash their hands frequently.
Hundreds of tons of lead were used in Notre Dame's frame, as well as the church spire that burned and collapsed.
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