Airborne lead pollution may have cost ancient Romans 3 IQ points

Why didn’t the Romans invent the steam engine, electricity, or the airplane? Perhaps because they were 3 IQ points less clever than they could have been. The culprit: lead pollution — but not of the kind we already knew about.

Odd and violent behavior

The Romans drank tap water from lead pipes, prepared and ate their food on lead-containing kitchenware, used lead in their cosmetics, and even sweetened and preserved their wine with lead acetate. Ingesting lead is harmful to the body and brain, and it has been blamed for the odd and violent behavior of some emperors, and even the fall of the Roman Empire itself.

However, these dangers would have only affected a small urban elite. A recent study in the Proceedings of the National Academy of Sciences (PNAS) quantifies a much more widespread form of lead pollution: via the air. This affected everyone, including the Empire’s rural majority.

According to the researchers, this is history’s “first large-scale pollution event from industrial activities,” and it happened a millennium and a half before the Industrial Revolution.

Examining Arctic ice samples up to 11,000 feet (3.3 km) deep from Greenland and Russia for data on atmospheric conditions from 500 BC to 600 AD, researchers found a spike in lead pollution lasting from 15 BC to around 180 AD.

Rome’s (dirty) Golden Age

That period coincides almost perfectly with the so-called Pax Romana: two centuries of unbroken peace and prosperity — Rome’s Golden Age. During that period, the Romans mined massive amounts of lead-silver ore for currency, which released huge quantities of lead into the atmosphere.

To produce one ounce (28 grams) of silver, researchers calculated that Roman metallurgists would need to produce some 10,000 ounces (283 kilos) of lead. Extrapolating from the amounts they found in the ice samples, the researchers estimate that Roman silver mines emitted between 3,300 and 3,600 tons of lead dust into the atmosphere for every year of the Pax Romana.

This resulted in concentrations of atmospheric lead greater than 1 nanogram (ng) per cubic meter (m³) across most of the Empire, and beyond. Of course, pollution was worst closest to the mines, with concentrations of airborne lead as high as 150 ng/m³.

Correlating these findings with modern data on the link between lead levels and mental impairment, the scientists deduced that children (who are more susceptible to lead pollution) would have an extra 2.4 micrograms of lead from air pollution per deciliter of blood, which corresponds to an IQ score up to 3 points lower than normal.

While that may not seem much, such an average over a wide area and for a sustained period equates to “widespread cognitive decline,” notes the article in PNAS.

The cleverest of all Gauls

The maps above show two scenarios for airborne lead pollution in the Roman Empire during the Pax Romana. On the left, the assumption is that virtually all silver mining (and airborne lead pollution) was centered on Rio Tinto, a river region in southwestern Spain that has been mined for millennia. On the right is a model that assumes mining in a variety of locations.

As indicated by the blue lines, which show the presumed extent of lead-induced IQ deficit, there is not much of a difference between both scenarios. The unipolar model spares the northern part of Gaul, suggesting that the Belgae were not just the bravest inhabitants of that country, as per Julius Caesar’s Gallic Wars, but also the cleverest.

The multipolar model sees a much more widespread two-point reduction in IQ — up to what is now northern England, and also large parts of Central and Eastern Europe that were far beyond the Roman borders. In both cases, the worst IQ deficit is located in southern Spain, close to Rio Tinto — by far the largest mine of its kind in that era.

Let’s not feel smug about the remarkably high level of lead pollution in Ancient Rome. As recently as the 1970s, children in the United States had around 15 micrograms of lead per deciliter of blood. This was before leaded gasoline, lead paint, and other common sources of lead pollution were banned. This much more recent and severe level of lead pollution is thought to have caused a 9-point drop in IQ among affected children.

It’s a good thing the steam engine, electricity, and the airplane had all been invented by then.

For more on this study, read Pan-European atmospheric lead pollution, enhanced blood lead levels, and cognitive decline from Roman-era mining and smelting, by Joseph McConnell et al., in PNAS.

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