Even small amounts of lead pollution in the air can affect our health. How heavily the atmosphere was contaminated with lead in the past can be seen from Arctic ice cores. In this way, researchers have now discovered that the air during the rule of the Roman Empire was heavily polluted with lead – apparently a result of the silver mining at the time. This probably affected the mental abilities of the European population, causing their IQ to drop.
Lead is a known environmental toxin. If it is present in the air, we inevitably breathe it in. It then enters the bloodstream via the lungs – with sometimes devastating consequences. “Lead is known to have a variety of effects on human health,” says co-author Nathan Chellman of the Desert Research Institute (DRI) in Nevada. In adults, high lead exposure is associated with, among other things, a reduced immune response, infertility and anemia. Cardiovascular diseases, cancer and memory loss can also be a consequence. In children, even a small amount of lead can cause concentration problems and lower IQ, resulting in poor academic performance. Therefore, strict guidelines now apply, including for leaded gasoline. But what consequences did lead have before its disease-causing effects were known?
Ice cores show lead pollution during the Roman Empire
To answer this question, a team led by DRI’s Joseph McConnell examined how polluted the Earth’s atmosphere was with lead in the past, particularly during the Roman Empire between 500 BC and 600 AD. To do this, the hydrologists, archaeologists and historians took three ice cores from the Arctic, where thick ice sheets have formed over thousands of years. The climate of that time is archived centimeter by centimeter in the ice pillars. The team melted the ice in the laboratory and determined the concentration of lead and other air pollutants it contained. Using computer models of aerosol movement in the atmosphere, the researchers then created maps of past lead pollution across Europe.
The investigations showed that lead pollution in the air began during the Iron Age and initially increased until the late 2nd century BC. At this point the Roman Republic was at its peak. During the subsequent crisis of the Roman Empire in the 1st century BC, lead pollution temporarily fell sharply before rising again around 15 BC with the renewed rise of the Roman Empire. Lead contamination subsequently remained high until the Antonine Plague, which raged from 165 to 180 AD and killed around ten percent of people in Europe.
According to the research, more than 500 kilotons of lead were released into the atmosphere during the almost 175-year heyday of the Roman Empire, the Pax Romana. Although these values were subsequently significantly exceeded on several occasions, they still represent long-term historical highs to date. “This research changes our understanding of the era by showing precise connections between the lead pollution records and historical events such as population declines associated with periodic plagues and pandemics “says senior author Andrew Wilson from the University of Oxford.
Lead from silver mines caused IQ to drop
The ice cores suggest that lead pollution during the Roman Empire was linked to its economic and social boom and bust. But where did the lead come from? It is known from historical texts that lead was found in water pipes, dishes, cosmetics and paints at that time. However, analysis of lead isotopes revealed that most of the lead probably came from mining and smelting operations across Europe during this period. Lead was released into the atmosphere, especially during silver mining, where the lead-rich mineral galena was melted down to produce silver. For every ounce of silver mined, thousands of ounces of lead were released, the team explains. At that time, silver formed the basis of the Roman economy. The study shows that “humans have been influencing their health through industrial activities for thousands of years,” says McConnell.
But what health consequences did the lead-containing air have in Roman times? To find out, McConnell and his colleagues also compared their lead analyzes with recent epidemiological studies linking lead exposure to cognitive decline. As a result, lead pollution in the air during the Pax Romana resulted in an average lead level in the blood of children under five of about 3.4 micrograms per deciliter. Blood values also increased in children under eleven years of age. As a result, the average IQ level of the European population fell by at least 2.5 to three points during Roman rule. “A two to three point IQ reduction doesn’t sound like much, but when you apply that to the entire European population, it’s a big deal,” says Chellman. Accordingly, lead exposure significantly reduced the mental abilities of people living at the time.
Source: Joseph McConnell (Desert Research Institute) et al.; Proceedings of the National Academy of Sciences (PNAS), doi: 10.1073/pnas.2419630121