Childhood lead exposure in the United States is ubiquitous and much more concerning than previous estimates have suggested, according to a new study.

When researchers analyzed leaded gas use from 1940 and combined it with data on blood-lead levels from the mid 1970s, they found more than 54 percent of Americans alive in 2015 had been exposed to dangerous levels of lead as children.

That's more than 170 million adults who are now at greater risk of neurodegenerative disease, mental illness and cardiovascular issues, because of the lead they breathed in, ingested or absorbed as kids.

No level of lead exposure is safe at any point in a person's life, but this highly toxic metal can be especially detrimental for children as it can impede brain development, leading to permanent learning difficulties and behavioral issues.

Altogether, researchers estimate leaded gas has reduced the nation's cumulative IQ score by 824 million points, which is nearly three points per person.

And that's just the average. Those born in the 1960s and 1970s, when leaded gas use was reaching a peak, could have lost an average of six to seven IQ points. The cohort's lead exposure was eight times over today's health limits.

For most people, these effects are not easily noticed, but for some who have lower than average cognitive ability, it can lead to a diagnosis of intellectual disability.

"I frankly was shocked," says sociologist Michael McFarland from Florida State University (FSU). "And when I look at the numbers, I'm still shocked even though I'm prepared for it."

Ever since the US government banned leaded gasoline for cars in 1996, childhood lead exposure has gradually fallen. Yet there are still many Americans alive today who are dealing with the fallout of their upbringing.

Children born after 1996 generally have lower blood-lead levels than their parents and grandparents, but compared to generations before the preindustrial era, their lead exposure is still much higher.

What's more, there are thousands of communities in the US, like Flint Michigan, that continue to suffer from the nation's legacy of unlimited lead use, and the racial disparities are stark.

Black adults over the age of 45, for instance, were found to have considerably higher levels of blood-lead than their White counterparts, and that was true even for those born after 1996.

The authors of the study are now examining the long-term consequences of that exposure, and whether it can account for racial disparities in health outcomes, like kidney disease, coronary heart disease, and dementia.

"Millions of us are walking around with a history of lead exposure," says clinical psychologist Aaron Reuben from FSU.

"It's not like you got into a car accident and had a rotator cuff tear that heals and then you're fine. It appears to be an insult carried in the body in different ways that we're still trying to understand but that can have implications for life."

Lead poisoning is insidious by nature. The invisible and odorless pollutant has historically been used in paints, pipes, and gasoline, and even though restrictions are better than they once were, at least in the US, enormous quantities of lead have already seeped into our drinking water, our airways, and our homes.

Leaded gas from car exhaust may no longer be the threat it once was, but other sources of lead pollution, like hunting ammunition, plumbing and industrial waste, still pose a threat to humans and the wider environment.

In 2021, for instance, a study of over a million American children found detectable levels of lead in the blood of half the cohort. Those children who live in zip codes with predominantly Black populations were more likely to fall in this group.

Some researchers consider lead pollution the "longest-running epidemic" in the nation, and calculating IQ points lost from lead exposure is a commonly used proxy for its detrimental health effects.

Last year, researchers found lead exposure was linked to "astonishingly high" and "alarming losses" in IQ from 1999 to 2010.

The new estimates have looked further back in time, only to find even higher blood-lead levels among older adults.

"By providing more complete estimates of the number of people exposed to lead in early life, this study makes a considerable step toward understanding the full extent of the damage done to the US population in one specific domain: cognitive ability," the authors conclude.

The study was published in PNAS.