Courtesy of an analysis of 72-meter ice cores extracted from the 4450 meter high Colle Gnifetti in the Swiss-Italian Alps, the above graph reveals changing levels of lead(Pb) in the Earth’s atmosphere. The highest concentration occurred in the mid-1970s. This was about 50 years after refiners started to add tetraethyl lead to gasoline to make engines run more smoothly. Then from 1950 onwards car sales mushroomed, and so did traces of the lead they released. Luckily, by the 1980s, thanks to environmental legislation in both Europe and North America, unleaded gasoline sales rose while those of the toxic metal plummeted. By the 1990s, lead was no longer used in house paint, which caused its concentration in dust to drop. Consequently, as the graph reveals, by the late 1990s lead levels in the atmosphere had started to drop significantly.
But what was the source of lead in previous years? Lead has been used for a long time to make pipes, coins, roofs, gutters, and cisterns. In the Middle Ages, the compound, lead acetate, was even used as an artificial sweetener. The amount of lead in the environment also tracks silver production because galena, lead’s ore, often lies side-by-side with ores of silver. So whenever lead or silver production peaked, the industrial boost would leave its signature in the atmosphere. In spring, especially, wind carried deposits of the heavy metal hundreds of miles from the mines of England to the Alps.
For example, the data reveals that lead-levels dropped around the year 1170, only to peak a few years later. What happened? For years King Henry II had been in conflict with the archbishop Thomas Becket. Eventually the latter was brutally murdered by the king’s knights. Henry II was excommunicated, people dodged taxes, and mining came to a halt. But when the king atoned for the murder by building many churches, since their roofs and gutters were made of lead, mines were busy and they spewed out more of the pollutant. Corroborating evidence comes from records of the spike in taxes on mines in the Peak District and at Carlisle in England.
What caused the most dramatic drop, around the year 1350? The lowest level in the last two thousand years coincided with the shutting down of lead and silver mines at the peak of the Black Death. Another drop in lead concentrations occurred in the mid 1400s. This time, production of the toxic metal plummeted during the The Great Slump, an economic depression in England lasting from the 1430s to the 1480s. The third sharpest drop in the amount of lead occurred in the late 1800s. This was related to the fact that in 1878, a fall in the price of lead and a general world economic depression caused smelting to slow down at some mines and come to a complete halt at others.
In the past, only economic disasters, murders or plagues could lower emissions of lead, and only temporarily. Now we could do it rationally and with a more lasting effect.
Next-generation ice core technology reveals true minimum natural levels of lead (Pb) in the atmosphere: Insights from the Black Death. GeoHealthVolume 1, Issue 4 p. 211-219 https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2017GH000064
Lead pollution in ancient ice cores may track the rise and fall of medieval kings. Science. 30 March 2020 Ann Gibbons. https://www.science.org/content/article/lead-pollution-ancient-ice-cores-may-track-rise-and-fall-medieval-kings