The Black Death tore through Europe in the years 1346–1353, killing as many as 200 million people as the deadliest plague known to humans carved its path through history.

Now, an analysis of ancient ice dating back through those dark days reveals an unexpected quirk of the plague – and researchers say the discovery provides evidence that the 'natural' level of lead in the atmosphere should be effectively zero.

When the sickness came, it caused massive social upheaval in the populations it infected, shutting down entire human industries as ravaged communities went into damage control.

One of these affected industries, according to historian Alexander More from Harvard University, was lead mining and smelting by medieval workers – and thanks to his team's new study, we've got more than historical records to show that.

After analysing an ancient ice core extracted from a glacier in the Swiss-Italian Alps, the team found only one instance in the last 2,000 years when atmospheric lead readings dropped to negligible levels.

That blip occurred between 1349 and 1353 – the timeframe in which the Black Death effectively brought the medieval lead industry to a standstill.

"When we saw the extent of the decline in lead levels, and only saw it once, during the years of the pandemic, we were intrigued," says More.

"In terms of the labour force, the mining of lead essentially stopped in major areas of production. You see this reflected in the ice core in a large drop in atmospheric lead levels, and you see it in historical records for an extended period of time."

But the fact that lead levels appear to rise sharply on either side of the Black Death era shows that environmental lead pollution isn't something that just started with the Industrial Revolution.

Instead, it shows that measurable levels of lead pollution can be seen as far as two millennia back, and would have been present ever since humans first started significantly toiling with the metal.

Why that matters, the team says, is because it shows that there's no safe 'natural' or 'background' level of lead in the atmosphere, not as we currently understand those terms – as readings taken before the Industrial Revolution (the 'natural' threshold) were still affected by human activity.

In other words, while lead may be a naturally occurring metal, it shouldn't actually be present in the atmosphere in detectable levels – unless humans are around, polluting the environment with it, that is.

The closest we've come to an actual 'background' reading is the negligible levels seen during the Black Death.

The researchers also picked up on other lesser drops in lead levels in 1460 (likely epidemic-related) and in the 1970s, when new restrictions on lead in petrol and air pollution came into place.

According to the team, the results give us "for the first time a lead reading of what it would look like without humans on this part of the planet", as one of the researchers, archaeologist Chris Loveluck from the University of Nottingham in the UK, put it to The Guardian.

Given everything we know about lead and how bad it is for us, that new baseline is a pretty valuable measurement to have – even if it signifies a rather sombre reality.

"We have basically been poisoning ourselves for about 2,000 years," More told The Guardian.

The findings are due to be published this week in GeoHealth.