The Roman Empire's pervasive influence once reached across land, sea, and even sky.
During the golden age of Roman imperialism, air pollution grew so inescapable that researchers now suspect it caused widespread neurological damage over much of Europe, including most of what is now Great Britain.
Based on ice-core records from the Arctic, atmospheric levels of toxic lead spiked between 100 BCE and 200 CE as the Roman Empire began mining and smelting metals like never before.
A child born within the Roman Empire at this time would have roughly three times as much lead in their blood on average as kids in the US today, researchers estimate.
Using modern evidence of lead pollution and its health effects, the international team calculates that lead exposure in the Roman golden age could have caused a population-wide drop in IQ of about 2.5 to 3 points per person. And that goes for most of the Roman Empire, including its provinces of Gaul, northwestern Africa, Iberia, and Britannia.
"An IQ reduction of 2 to 3 points doesn't sound like much, but when you apply that to essentially the entire European population, it's kind of a big deal," says snow and ice hydrologist Nathan Chellman from the Desert Research Institute in the US.
And these are just estimates of lead pollution in the air. Lead pipes and lead vessels also delivered toxic particles straight into the mouths of elite Romans and the urban population.
"All Europeans, their livestock, and agricultural fields were exposed for centuries to background atmospheric lead pollution resulting from the large-scale mining and processing of lead/silver ores that underpinned the Greek and Roman economies," writes the international team of researchers, including climatologists and epidemiologists who hail from institutions in Denmark, the UK, the US, Canada, Austria, and Switzerland.
"This background lead pollution in air and soil may have been the most significant exposure route in rural, non-elite populations."
No amount of lead in the blood or body is safe, but the higher the concentration, the more disastrous the results.
Today, epidemiological studies suggest that blood lead levels as low as 3.5 µg/dl in children are linked to decreased intelligence and learning impairments later in life, commonly measured using IQ tests.
In the US, lead pollution is sometimes referred to as the "longest-running epidemic" in the nation. Since the 1940s, scientists estimate lead exposure has lowered the IQ of half the population, mostly from the burning of fossil fuels.
In the last forty years, lead exposure has thankfully plummeted. Now that leaded gasoline is restricted, along with numerous lead-based products, children in the US have a blood lead level of around 0.6 to 0.8 µg/dl.
In Roman times, children probably had on average a blood lead level of 3.4 µg/dl, according to the current study's models. Given this is a mean calculation, it's likely many children had concentrations of lead that put them at great risk of neurological impairment.
The results align with previous ice-core studies, which have also noted a spike in atmospheric lead at the peak of the Roman Empire, and archaeological finds, which have revealed high levels of lead in the teeth of many Roman children.
"This is the first study to take a pollution record from an ice core and invert it to get atmospheric concentrations of pollution and then assess human impacts," says Joe McConnell, hydrologist and lead author of the study.
"The idea that we can [calculate] this for 2,000 years ago is pretty novel and exciting."
The study was published in PNAS.
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