Notre-Dame-de-Paris analysed by Andrew [not a book review]

As reported in Le Monde, Alexander van Geen, Yuling Yao, Tyler Ellis, and Andrew Gelman wrote a paper analysing the impact of the destruction of Notre-Dame last year in terms of lead concentration in the ground. As 460 tons of lead from the roof melted overnight. Based on  100 samples of surface soil collected by one author (not Andrew!) from tree pits, parks, and other sites in all directions within 1 km of the cathedral. Here is a plain language summary of the findings.

“This study attempts to estimate the extent to which the population of Paris was exposed to lead as a result of the Notre‐Dame cathedral fire of April 15, 2019. The concern stems from the large quantity of lead that covered the cathedral, some of which was injected into the air by the fire for several hours. In order to evaluate how much lead rising from the fire was redeposited nearby, surface soil samples were collected in all directions within a 1 km radius of the cathedral. Elevated levels of lead observed downwind of the cathedral indicate that surface soil preserved the mark of lead fallout from the fire. Although the estimated amount of lead redeposited within 1 km corresponds to only a small fraction of the total covering the cathedral, it could have posed a health hazard to children located downwind for a limited amount of time. Environmental testing on a larger scale immediately after the fire could have provided a more timely assessment of the scale of the problem and resulted in more pointed advice to the surrounding population on how to limit exposure to the fallout of lead.”

The statistical modelling is one of a spatial pattern of the lead distribution, using a mean-zero Gaussian process prior. And of a discretisation of the neighbourhood of the cathedral into uniform 30×30 locations. Without any further input, the model identifies properly the direction of the wind on that fateful evening. And logically concludes to a higher exposure than measured weeks after the fire. (Minor quibbles: a bias in self-declared test toward “a more educated, wealthier segment of the population” is unlikely in the immediate neighbourhood of Notre-Dame where the average flat sells at 16,000 euros per m², and the LCPP (Laboratoire Central de la Préfecture de Police) is not affiliated with the City of Paris but the Ministry of the Interior.)

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