Ancient lead pollution may have lowered IQs across the Roman Empire
A general view of (L-R) the church of Santi Luca e Martina, the Arch of Septimius Severus, the ruins of the Temple of Saturn and - in the background - the Amphitheatrum Flavium, commonly known as Colosseum (in Italian: Colosseo), on March 31, 2024 in Rome, Italy.Credit: Emanuele Cremaschi/Getty Images ShareThe Industrial Revolution was undeniably a major turning point in history. Beginning in the late 1700s, human environmental impacts reached previously impossible new heights. Yet pollution wasnt invented in the 18th century. Humans have been mucking up Earth for far longer, to our own detriment, as exemplified by new research linking lead air pollution to cognitive losses during the Pax Romana. People living during the golden age of the Roman Empire experienced an average 2.5 to 3 point reduction in IQ due to atmospheric lead, according to a study published January 6 in the journal Proceedings of the National Academy of Sciences.The new research adds context to a long-standing debate about the role lead pollution and poisoning may have played in the collapse of the Roman Empire. Some historians have argued that Roman elites and emperors who purportedly displayed odd, often violent behavior like Caligula and Nero were actually suffering from lead poisoning, and thus that lead and the erratic actions it caused critically undermined societal stability. The study doesnt prove, one way or the other, if or how the fall of Rome was linked to lead. However, it does demonstrate that environmental health and the effects of pollution on people has roots stretching back millenia.[ Related: Childhood leaded gasoline exposure damaged Americans mental health ]Using Arctic ice cores, atmospheric modeling, epidemiological data, and previously published health and cognition studies, the scientists estimate fluctuating levels of lead air pollution over centuries, how that likely translated to blood lead levels in people, and how those blood lead levels might have impacted cognitive capacity in denizens of the Roman Empire.The research is not the first to find a notable peak of lead pollution and human lead exposure during Roman times, lots of previous work has established the prevalence of lead contamination in antiquity through analysis of ice and peat cores, skeletal remains, and ancient infrastructure. But the study is unique for quantifying the effects of that Roman-era pollution on blood lead levels and IQ losses. The authors estimate that children living during the 200-year Pax Romana (between about 27 BCE and 180 CE) would have had average blood lead levels of about 3.4 micrograms per deciliter (2.4 mcg/dl above Neolithic background levels), from air pollution alone, and that those levels would have translated to a 2.5-3 point drop in IQ levels, population-wide.Though IQ is a flawed metric, its one of the best scientific shorthands available for tracking the population-level consequences of something like lead. The metal is a well-established neurotoxin, known to be particularly harmful to infants and children. Even low and moderate levels of lead exposure can lead to lifelong health consequences, including developmental delays, learning disabilities, behavioral changes, immunosuppression, heart disease, organ damage, pregnancy complications, and more. There is no level of lead exposure considered safe, according to the World Health Organization and the Centers for Disease Control and Prevention. But when lead permeates the environment, avoiding it is impossible.There were many sources of potential lead exposure in the Roman Empirefrom utensils and cookware to water pipes and wine. All likely contributed to the lead burden borne by the people of the day. Though none would have been as far reaching as air pollution, which would have exposed even those in isolated rural areas to the toxin. Mining and smelting metal ores, particularly the galena ore used as a source for the silver in Roman coins, produced lead emissions that spread far and wide across the Roman Empire.To my knowledge, its the first large-scale pollution event from industrial activities, says Joe McConnell, lead study author and a research professor and hydrologist at the Desert Research Institute in Nevada. Our objective here was to try to understand the potential health impacts resulting from [that], he adds.To do so, he applied his expertise in ice core analysis to assess samples from three different Arctic sites. Ice cores serve as a frozen record of atmospheric conditions throughout history because particles that circulate in the air eventually fallsome onto glaciers and ice sheets, where theyre preserved in a literal timeline. Using these measurements of lead deposited in the Arctic throughout the Roman era, McConnell and his colleagues then applied atmospheric models (the same kind used by climate scientists) to reverse engineer estimates of how much lead must have been circulating in the air over the Roman Empire, thousands of kilometers away from Greenland and Russia where the samples were collected.They ran two different model scenarios: one assuming most of the lead pollution originated from a known mining region in present-day southern Spain, and the second assuming more dispersed sources of lead emissions from across the empire. Both scenarios resulted in similar estimates of atmospheric lead.From there, the interdisciplinary research team turned to contemporary environmental health analyses that establish the relationship between levels of lead in the air and in peoples blood. Finally, they estimated how those levels may have impacted cognitive ability, using data on IQ loss from public health research.The findings are that this lead pollution resulted in clear effects, not only for the air, but also for blood lead levels and cognitive deficits, says McConnell. The levels of atmospheric lead pollution documented in the study are less than the peak of global lead pollution reached in the 20th century, when leaded gasoline use was widespread. But its still a notable and measurable effect, he says.Our data suggests that lead pollution during the 180 years of the peak of the Roman Empire had about one-third as much of an impact on cognitive decline as during the height of 20th century exposure, McConnell explains. The idea that 2,000 years ago, humans were polluting the continent of Europe at a third the level of modern industry is pretty surprising. An awful lot of environmental research assumes that pre-industrial was a pristine world. It was not.Lead exposure from air pollution, as calculated in the study, represents a lower limit of what people were realistically encountering, McConnell adds. In locations nearby mining or smelting operations, air pollution would have been much more intense. And through water, food, and household items, many people living in the Roman empire likely had higher blood lead levels and thus incurred even more harm.[ Related: Without humans, what would happen to Earth? ]Its interesting work, I agree with what theyre trying to do, says Sean Scott, a chemist at Pacific Northwest National Laboratory who has previously studied lead levels in the Roman Era through skeletal remains. Though Scott points out that, by combining estimates and models the researchers have magnified the unreliability inherent in their methods. Im sure that these measurements are very good, but once you make a leap from ice core to human blood, and then to intelligence quotient, thats going to have uncertainty, he says. Granted, thats the best they can do, he adds.McConnell acknowledges this limitation. It would be great if going forward the linkages between background air pollution, childhood blood lead levels, and health were better quantified, he says. It would also be ideal to have ways of quantifying the other health impacts of lead and industrial pollutants, he notes.Still, the new research stands as a sketch of unprecedented environmental change during an endlessly fascinating time in human history, says Scott. It may be impossible to know exactly what precipitated the fall of the Roman Empire (most probably it was not any single thing, say both Scott and McConnell). But perhaps studying the pollution of the era could get people considering the parallels between history and our present-day. When you study the Roman population and the historic environmental science of that, and then you look around at modern times, it changes the way you see the world, Scott says. Likely, Romans didnt fully grasp the consequences of their silver smelting. It makes you wonder what we are doing currently that we dont understand.
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