Silver fuelled the rise of the Roman Empire as the coin-based currency accelerated trade, filled tax coffers and funded military conquests.
But the ancient process of mining and extracting silver was also making the air so thick with lead pollution that it was probably making the population slightly stupider, one study has found.
“To get the silver out of the ore, you have to crush it,” Andreas Stohl, a professor of atmospheric sciences at the University of Vienna and a co-author of the study, said Tuesday. “It’s a dusty business – and this dust contained a lot of lead.”
The peer-reviewed study, published Monday in the Proceedings of the National Academy of Sciences, found the mining and smelting activities released enough of the neurotoxin into the atmosphere that it would have caused “widespread cognitive decline” – which could have reduced the typical person’s intelligence quotient (IQ) score by up to three points.
“The concentrations were high enough to cause cognitive decline, especially in children,” Stohl said in a phone interview Tuesday.
The findings would make the roughly 200-year span of Rome’s golden age known as Pax Romana one of the earliest examples of industrial pollution harming human health at scale. The study also could add fuel to a fraught and long-standing debate whether mass lead poisoning could have contributed to the fall of the Roman Empire.
“Ancient texts and archaeological evidence indicate substantial lead exposure during antiquity that potentially impacted human health,” researchers wrote. “Although lead exposure routes were many and included the use of glazed tablewares, paints, cosmetics, and even intentional ingestion, the most significant for the nonelite, rural majority of the population may have been through background air pollution from mining and smelting of silver and lead ores that underpinned the Roman economy.”
To conduct their research on lead pollution, scientists analyzed the presence of lead in Arctic ice cores – preserved there from the time of the Pax Romana – the epoch of relative peace and prosperity that began in 27 B.C.
They found that at the same time Roman smelters were releasing lead into the atmosphere, the amount of the neurotoxin deposited by air currents in the Arctic spiked. The tiny particles of lead found in the ice had drifted from Europe on air currents, they said.
By reconstructing the flow of air currents using atmospheric models, the scientists were able to estimate how much lead pollution was being released across Europe at the time: between 3 million and 4.3 million kilograms per year.
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Relying on modern epidemiological studies, the researchers estimated blood lead levels among child inhabitants probably increased by 2 to 5 micrograms per deciliter. (This aligns with another analysis of tooth enamel from Roman skeletal remains in 2021, which also pointed to pervasive childhood exposure to lead at the time.)
The authors estimated the exposure was enough to reduce the typical IQ score by 2.5 to three points across the empire, based on data from modern studies that show a link between childhood lead exposure and cognitive decline. The impact was likely to be even greater for those living close to silver mines, they found.
“To my knowledge, it is probably the earliest example in history” of industrial-scale pollution causing harm to human health, said Jorgen Peder Steffensen, a professor of physics at the University of Copenhagen who helped analyze the ice cores as part of the study, in an email Tuesday.
According to the Centers for Disease Control and Prevention, no level of lead exposure is safe for children. Even low levels can cause lifelong harmful effects, including cognitive and nervous system damage and lower IQ.
Airborne lead pollution levels have fluctuated throughout human history. To put the study’s findings in perspective: It most recently soared after the Industrial Revolution and its widespread use in gasoline in the 20th century.
During the peak era of leaded gasoline in the United States, around the 1970s, lead pollution was so heavy that the average level for a child was around 15 micrograms of lead per deciliter of blood. Such levels corresponded to a decline in IQ of nine points, the study noted.
The level of lead pollution ingested by children in the United States has since dropped markedly, as a result of stringent restrictions introduced in the 1970s and ’80s on its use in things like gasoline and paint. According to the Environmental Protection Agency, the level of airborne lead in the United States declined by 98 percent between 1980 and 2014.