"In this day and age, there are so many possible techniques and materials that we could obviously have found a solution to preserve the colour of lead without using it," said Rainhorn, who teaches history at the University of Paris I - Panthéon-Sorbonne.

She called it "a missed opportunity, given the symbolism that Notre-Dame represents for the whole world."

"A restoration project that took into account knowledge of lead and toxic products would have demonstrated genuine concern for the public environment, particularly in large cities. In my view, this is a political error with significant health consequences. And it's a choice I find hard to explain," Rainhorn said.

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