"So this was maybe nine months after the fire in Notre Dame, and I had been struck visually by the fire, the yellow smoke, which is a telltale indicator of lead. The fact that 400 tons of lead constituted the covering of the roof of the cathedral. And a lot of that had volatilized, presumably, but no one really knew how much. So that got me thinking, and I happened to be in Paris at the time, so I thought if it's so much lead, could it be that it affected the population living within say a kilometer of the cathedral? I thought there wasn't really a lot of clear information about what had happened, and what had been measured. I thought some more openness and transparency was needed."

Geochemist Lex van Geen is a research professor at Lamont-Doherty Earth Observatory and is member of the Earth Institute at Columbia University. His research fo ...