In April 2019 Dominique Weis was at the Vancouver airport, in a plane waiting on the tarmac about to take off to Paris, when she found out that the city’s famous Notre-Dame cathedral was on fire.

For Weis, who holds the Canada Research Chair in Geochemistry of the Earth’s Mantle, her first thought was for the ancient building’s roof, which had been covered with some 400 tonnes of lead. The fire would allow some particles of this metal to escape into the atmosphere as Pb oxide aerosols and settle on nearby surfaces, a public health hazard that authorities in Paris rushed to confirm and address by cordoning off parts of the city around the site.

Such urban pollution assessments are familiar to Weis, who has recently been working with PhD candidate Kate Smith honing a method of measuring lead isotopes in honey from building-top beehives to identify the source of lead emissions. Earlier in 2019 the two co-authored a paper in Nature Sustainability describing how this technique linked specific industrial activities in downtown Vancouver with lead isotopes found in honey produced nearby.

Although the Notre-Dame fire was a major cultural loss that drew condolences from around the world, Weis also immediately recognized a scientific opportunity to take stock of the public health threat posed by the disaster.

“I just said ‘we can’t miss this’,” she recalls.

French scientists and technicians initially resisted her requests to obtain samples to analyze at UBC’s Pacific Centre for Isotopic and Geochemical Research (PCIGR), that Weis directs and where she works. Later that summer Smith attended a conference in Montreal where she met a colleague who connected her and Weis with Sibyle Moulin of the major Parisian apiary firm Beeopic.

“[Sibyle] is an urban beekeeper,” explains Smith. “She and her colleagues manage hundreds of hives. They’ve got over 350 hives in Paris at any given time and then another several hundred in suburbs and surrounding countryside.”

With Moulin’s cooperation, Smith and Weis were able to obtain 36 honey samples collected from Parisian hives in July 2019, just three months after the fire. Subsequent analysis at PCIGR showed that these samples collected downwind of the fire did in fact contain concentrations of lead that were up to four times higher than levels found in samples from other parts of the city that were unaffected by the fallout of the fire. Those amounts were still well within the European limits for safe human consumption, but they unquestionably reveal the footprint of Pb fallout from the cathedral fire.

More specifically, the highest concentration of lead measured in Parisian honey was 0.077 µg/g Pb, which Weis compares with 77 drops of water in an Olympic-size swimming pool. As concerning as lead might be, she adds, this is no higher than what can be found in honey collected from downtown Vancouver hives.

On the other hand, Paris is a much older city, with a much longer history of lead from various activities making its way into the local environment, which makes it harder to pin down the sources for different isotopic signatures. In this respect, this study offered a useful introduction to the challenges of using honey samples in this kind of urban setting.

“While Pb isotopic compositions did not readily offer source apportionment leverage in this case (because of the overlap in Pb isotopic compositions among sources), the results are supported by the history of Pb use in Paris,” concludes a recently published paper in Environmental Science & Technology Letters, co-authored by Smith, Weis, Catherine Chauvel and Moulin. Dr. Chauvel is a collaborator scientist of France’s Centre national de la recherche scientifique that also provided samples from the French countryside in the Rhône-Alpes area.

“Honey is, of course, no substitute for thorough public health assessments (e.g., indoor dust and blood Pb testing), whether performed routinely or immediately after an event like the Notre-Dame fire. Honey offers an accessible (hives are plentiful, sampling is low cost) and unique tool for visualizing urban metal distribution, and this work serves as an interesting and timely case study for the application of honey as a biomonitor immediately following an acute Pb pollution event.”