When it burned last year, the cathedral of Notre Dame in Paris released hazardous lead dust that landed in parks and buildings across the city, raising health alarms for its residents. Now, researchers have found that the lead found its way into honey produced by urban beehives.

Researchers analyzed concentrations of metals, including lead, in 36 honey samples collected from Parisian hives in July 2019. All the honey was within tolerable limits for consumption, but the honey from hives near Notre Dame had lead concentrations four times higher than the samples from Parisian suburbs.

“Because of the way the wind was blowing the night the fire burned, the direction that the smoke plume traveled is well-defined. The elevated lead concentrations were measured in honey that was collected from beehives within that plume footprint,” said Kate Smith, lead author of the study, in a press release.

Smith and her team compared the honey obtained after the fire with a Parisian blend from 2018 and with samples from 2017 from the region of Auvergne-Rhone-Alpes. A sample from a hive five kilometers west of Notre Dame had the highest concentration of lead, 0.08 micrograms, while the pre-fire honey had 0.009 micrograms.

The European Union allows a maximum lead content of 0.10 micrograms per gram for syrups, sweeteners, and juices. Lead is a potent neurotoxin and a high exposure to it can kill, while lower levels can lead to health problems, such as cognitive and physical damage and shortened attention spans.

Lead was a highly used building material in Paris during the time of the construction of Notre Dame, which dates back to the 12th century. The roof and the spire of the cathedral had several hundred tons of lead and while most melted during the fire, some flames reached temperatures high enough to aerosolize lead oxides.

“We were able to show that honey is also a helpful tracer for environmental pollution during an acute pollution event like the Notre-Dame fire. It is no surprise, since increased amounts of lead in dust or topsoil, both of which were observed in neighborhoods downwind of the Notre Dame fire, are a strong indicator of increased amounts of lead in honey,” said co-author Dominique Weis.

The fact that honey bees usually forage within a two- to three-kilometer radius of their hive allowed the researchers to use honey as a localized snapshot of the environment. Bees collect dust and airborne particles, which then end up in the honey. The researchers worked with an apiary company, which manages 350 hives in Paris and provided them with the samples for the study.

It’s the first time a heavy-metal analysis through honey has been done in a megacity. The method came out of previous work by the same researchers, in which they measured trace amounts of metals in honey from urban beehives in six Metro Vancouver neighborhoods, demonstrating the use of bees as an effective biomonitor.

The study was published in the journal Environmental Letters.