When the Black Death swept across Europe in the 14th century, it not only killed millions, it also brought lead smelting, among many other commercial activities, to a halt.

That cessation is reflected in a new analysis of historical and ice core data, which researchers say provides evidence that the natural level of lead in the air is essentially zero, contrary to common assumptions.

“These new data show that human activity has polluted European air almost uninterruptedly for the last [about] 2,000 years,” the study’s authors say. “Only a devastating collapse in population and economic activity caused by pandemic disease reduced atmospheric pollution to what can now more accurately be termed background or natural levels.”