Extensive silver mining may have exposed ancient Romans to high levels of lead pollution in the air, probably leading to a general drop in their IQ and their resistance to disease.
Analyses of Arctic ice layers point to concentrations of atmospheric lead at the Roman Empire’s height that were about threefold what they are today in the US. The effect on the intelligence of ancient people across Europe – especially in major coin-making regions in Iberia – would have been significant and might have even made them more susceptible to conflict and to plagues, says at the Desert Research Institute in Reno, Nevada.
“The idea that you had this human-caused heavy metal pandemic 2000 years ago is kind of mind-boggling,” he says.
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Lead poisoning is known to cause cognitive deficits, health problems like heart disease and reduced immunity, and behavioural issues like a heightened propensity for violence.
Roman water pipes and pottery exposed citizens to lead contamination, which is reflected in previous analyses of the and of mostly well-off, urban Romans. However, McConnell and his colleagues suspected that atmospheric lead pollution might have had a wider effect, including on the rural non-elite, whose remains are rare and less studied.
According to , Romans smelted a lead-rich mineral called galena to obtain silver for coining. That would have released about 10,000 grams of lead for every gram of harvested silver, says McConnell.
While most of that lead hovered within the Roman Empire – and especially over metal-working areas – smaller quantities would have spread across Europe and ended up in the glaciers of Greenland within a matter of days.
Glacier ice tends to pack into easily distinguishable layers about 10 centimetres thick every year, so scientists can accurately date them and the chemicals they harbour. McConnell and his team revisited archived lead records from three glacier ice cores in Greenland and the Russian Arctic representing the years 500 BC to AD 600.
Adding in atmospheric modelling, the researchers determined that in areas closest to where smelting took place, lead concentrations probably exceeded 150 nanograms per cubic metre of air at the height of the Roman Empire, from 27 BC to AD 180. That translates into more than 500,000 tonnes of lead released into the atmosphere during that time.
Based on studies of modern lead exposure in children under age 5, the team estimates that young Roman children would have had an average blood concentration of about 3.5 micrograms per decilitre.
Between 1976 and 1980, at the end of the leaded petrol phase, young US children had blood lead levels higher than this, averaging 15.2 micrograms per decilitre. But since then, environmental measures to cut lead pollution have brought these levels down to .
Based on epidemiological studies, the ancient levels of lead air pollution would have caused an average drop in IQ of 2.5 to 3 points across the entire Roman Empire – with much bigger effects in the smelting zones.
Beyond the empire, all of Europe – and probably even North America – would have been affected at some level by the Romans’ lead pollution, says McConnell.
There was a major drop in lead concentrations in about AD 165, corresponding with the Antonine Plague from AD 165 to 180. Silver mining also dropped dramatically at this time – perhaps because enslaved people were dying off.
In their paper, McConnell and his colleagues say it is “intriguing” that the Antonine Plague ripped through the Roman Empire following nearly two centuries of highly elevated atmospheric lead emissions.
Still, that doesn’t mean lead pollution killed off the Romans, says at The Ohio State University. “While the magnitude of exposure and the correlated blood lead levels were enough to negatively affect the cognitive function of that population, this is still a far cry from causing the downfall of the Roman Empire,” she says.
On the contrary, the findings provide hope for the future, says Pyle-Eilola. “I am encouraged by the fact that some of the most successful societies in human history were plagued by some degree of IQ impairment from lead exposure. If the last few generations put a man on the moon, developed modern computers and developed innumerable medical advancements, I can only imagine what the coming generations will do for humanity.”
PNAS
