People have been getting poisoned by lead in the air for at least two millennia, according to a new study analyzing an ice core extracted from a glacier in the Swiss Alps.
Only the Black Death, the most devastating plague in human history, briefly returned atmospheric lead to its natural level of undetectability in the 1300’s. The ice core also documents the sharp spike in atmospheric lead in the 20th century that peaked in 1975, when clean air laws began mandating the removal of lead from gasoline.
“In reality, there is no safe blood lead level,” lead study author Alexander More of Harvard’s Initiative for the Science of the Human Past told BuzzFeed News. “This study shows we almost never lived in some idyllic pre-industrial nature where there wasn’t lead pollution.”
This historical finding matters because public health agencies have been steadily marching the “safe” blood lead levels downward over recent decades, epidemiologist Philip Landrigan of Mount Sinai Hospital, who was not part of the study, told BuzzFeed News. Improved tests have shown lead has negative effects on IQ and health, he noted, even at levels once considered low. What’s more, incidents of lead poisoning in water, such as the catastrophe in Flint, Michigan, have raised pressure for examining the effects of lead when many might be unwittingly exposed.
Findings that point to the real, natural level of lead in the atmosphere being zero, he argued, add weight for cutting allowable levels — halved to 5 micrograms per deciliter in children’s blood by the US Centers for Disease Control and Prevention in 2012 — even further. Since there is no natural level of lead, and we are apparently the ones responsible for what lead there is in the air, that means we should be getting lead levels as close to zero as possible, Landrigan argued.
“I think it is a very cool study, they are seeing lead levels on an almost daily basis stretching back for centuries,” he said.
People have been mining and smelting lead for 6,500 years. To help take an atmospheric look at this history, glaciologists on the study team drilled 236 feet into the glacier on the Swiss-Italian border and retrieved the ice core, a compressed record of snowfall and lead deposition on the glacier for the last 2,000 years.
In the lab, a laser burned off microscopic slices of a sample of the ice core. Each slice was chemically analysed, sometimes delivering up to 300 lead measurements a year from the glacier, which dated at its base — the oldest segment of ice — back to about 1 AD.
“It’s an an amazing effort,” toxicologist Steven Gilbert of the Institute of Neurotoxicology and Neurological Disorders told BuzzFeed News. “That is a super refinement and a big amount of data.”
While past studies of Peruvian mummies have pointed to similar contrasts between a natural level of lead — a 1979 study found blood lead levels from 1600 years ago that were 700 times lower than modern ones — an ice core provides a much better survey of the historical atmosphere, said Gilbert.
The most surprising finding for the researchers was the sharp drop to undetectable lead levels from 1349 to 1353, which historians on the team knew was the height of the Black Death in Europe, when the plague killed 30% to 50% of the population.
“We looked at the drop and said, ‘look at that, why does it drop so sharply?' Then we identified the date.” More said. “Well, that was the largest pandemic that ever hit Eurasia.”
Lead mining — along with practically everything else — stopped when the plague reached its peak in Italy, Germany, and England, countries that had previously been centers of the industry. (There was no need for lead seals or in silver coins with society collapsing.) A labor shortage made lead mining, famously dangerous work, unpalatable as well.
By 1400, with lead mining resumed, the lead levels in the air recorded by the ice core resumed their old levels. The data also shows declines in atmospheric lead tied to market crashes in 1460 and 1885.
“The history and the science all came together very nicely,” More said.
One lead expert contacted by BuzzFeed News, Kim Dietrich of the University of Cincinnati, suggested that findings of continuous lead in the atmosphere for 2000 years would be unsurprising, given the history of mining. While lead pollution is terrible in Africa, China, and India, as well as in disasters like Flint, he didn’t see its long history as an argument for further lowering lead safety standards for children.
“I just don’t see likely long term effects in children from levels lower than the current CDC limits,” he said.
Gilbert, however, has called for lowering the level to 2 micrograms per deciliter in children’s blood, arguing that studies find deleterious effects from lead at every level. “There’s no good biological use for lead. The (ice core) study underlines that we really haven’t evolved defenses to it for most of our history.”