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In 1976, Herbert Needleman, a psychiatrist at Harvard Medical School, suspected even trace amounts of lead ravaged children’s brains. His opinion was not popular at the time—the prevailing wisdom was only extremely high amounts could inflict such damage.

So Needleman asked teachers in two Massachusetts towns to collect baby teeth from first- and second-grade students. He knew even if blood tests were negative, the history of the children’s lead exposure was stored in their enamel. His results, published in the New England Journal of Medicine in 1979, transformed scientists’ view of lead as one of humanity’s most potent neurotoxins and helped win him a nomination for a Nobel prize.

Needleman’s study revealed that lead profoundly transformed children’s brains, even at levels far below those considered dangerous. As exposure to lead rose, so did deficits: lower IQs, slower reaction times, hyperactivity, and impaired attention (today known as ADHD). Even at levels deemed “safe” at the time by health authorities, the cognitive damage was evident.

Over the next five decades, study after study began to converge on the same disturbing conclusion: “No safe blood lead level has been identified,” as the US Centers for Disease Control (CDC) eventually put it.

Since 1990, more than 6,000 studies have all shown that “exposures to low levels of lead early in life have been linked to effects on IQ, learning, memory, and behavior.” The damage is permanent and untreatable given the brain’s limited ability to repair itself. Society has yet to fully act on that fact.

After a flurry of activity that began 50 years ago to remove lead from the lives of Americans—the phaseout of leaded automobile gasoline began in the 1970s and residential lead paint was banned in 1978—progress has stalled out. Lead pipes and paint (the largest source of exposure for children) remain ubiquitous. In the air, aviation fuel, the last major fuel to contain lead,  is the largest source of airborne lead pollution in the US, exposing 3 million children living near general aviation airports.

Meanwhile, scientists have steadily pushed down the acceptable lead levels in children’s blood. The CDC, for instance, has lowered this threshold for “elevated” blood lead levels from 30 micrograms per deciliter in 1986 to 3.5 micrograms today. Even that amount is not considered safe, but rather is a reference level warranting immediate action.

“In children, we now know there is no safe level of lead in the human body,” says Philip Landrigan, a pediatrician and epidemiologist at Boston College who directs its Global Pollution Observatory, which tracks pollution-related diseases. “The appropriate blood lead level in the child is zero. Even very low levels damage the child’s brain.”

The US is now entering a new era of lead poisoning. The 20th century saw extremely high doses of lead across the entire population: the average American had blood lead levels five times higher than what’s considered elevated today. In the 21st century, blood lead levels have come down significantly. But millions of children are still chronically exposed to low levels of lead from multiple sources, including old lead paint and leaded aviation gasoline. Scientists are rediscovering there is no safe level of lead.

In the 20th century, most humans carried concentrations of lead 100  times higher than their prehistoric ancestors, based on analyses of ancient skeletons. The element, naturally leached from rocks over millennia, began to permeate our environment when early civilizations added the metal to weapons, tools, cups, paint, face powder, pipes, and medicine. Thanks to lead’s sweet taste, it was even sprinkled on food or wine as a preservative and a deadly condiment.

Yet lead’s toxicity was evident even then. Accounts of acute lead poisoning made their way into the written record 2,000 years ago. “Gleaming, deadly white lead,” the Greek poet Nicander noted, induced symptoms such as frothing mouth, parched throat, dry retching, chills, delusions, and crushing fatigue. Less fortunate sufferers endured dementia, hallucinations, convulsions, and even death. The Greek physician Discorides said “lead makes the mind give way.”

Lead pollution rose throughout the Industrial Revolution, but exposure skyrocketed after the first gallon of gasoline with tetraethyl lead was sold in 1923. The additive, designed to prevent engine knocking (a rattling characteristic of uneven combustion in engines), was so toxic, even a splash on the skin could be lethal.

By 1973, more than 200,000 tons of lead were entering the atmosphere each year and settling over the planet. Tiny airborne lead particles penetrated deep in the lungs and nasal passages, crossing into the brain and bloodstream. The liver, kidneys, lungs, brain, spleen, muscles, and heart sponged it up. Soon, almost every human on Earth was suffering from lead exposure affecting every organ, especially the brain.

It’s tempting to think of lead as a bulldozer in the brain, razing delicate neural machinery. At high levels, that’s true. At low doses, it’s more like an assassin killing neurons, severing branching interconnections that form new memories and thoughts, and attacking myelin sheaths connecting nerve cells to one another. As thousands and then millions of these neural connections are severed, the mind begins to flicker less brightly. Gray matter, particularly in brain regions controlling executive judgment, impulsivity and mood regulation, is notably smaller in adults with childhood lead exposure, according to brain imaging studies.

The effect appears even at the lowest doses. And researchers have confirmed the first doses of lead are more damaging than the next. That is, the early exposure, microgram for microgram, does the most harm, says Sammy Zahran, an economist and public health researcher at Colorado State University who studies lead pollution from airports.

What makes lead so pernicious is how it replaces the minerals the body uses to build itself. In almost every tissue, cells build their cellular machinery and keep it running by recruiting molecules of calcium, magnesium, and zinc. Lead, with its heavy atomic weight, interferes with all of this as it gets swapped in for blood cells, neurons, and other crucial bits of biology. “Lead’s much much heavier than the basic building elements of our body,” says Michael Santa Maria, a neuropsychologist who has studied lead extensively. “It’s quite literally dead weight.”

As lead atoms displace the crucial molecules the body uses to construct itself, cellular mechanisms begin to go haywire. Cells repave themselves with lead. Cellular protein factories falter. Genetic instructions get lost. Nerves’ electrical impulses, usually gliding along beds of calcium, are derailed. Every organ system is attacked, according to a 2021 report by the National Academies of Science. The body’s own defenses against lead, a volley of enzymes meant to avoid tissue damage, falter and eventually turn against the body itself. “Lead exhausts the system,” say researchers.

Only in the last few decades have scientists and health officials been able to see what low blood levels look like in the general population. Once leaded gasoline consumption started to fall in the 1970s, blood lead levels fell in lockstep. Blood lead levels of the US population dropped 94% between 1976 and 2016, from 12.8 micrograms to less than one, according to the CDC.

Yet lead never fully leaves the body. After circulating in the blood for about a month, it stays in the brain for years, especially the cerebral cortex, home to complex mental functioning. Much of the rest is entombed in teeth or bone. These traces get liberated again during pregnancy and breastfeeding, when mothers transfer calcium to their newborns, as well as when bones weaken in old age. This unleashes a new cycle of destruction. Scientists estimate lead from bones may account for as much as 70% of the lead burden in older adults. For baby boomers, the most lead burdened generation in history, scientists worry this will trigger a rash of cardiovascular and neurological diseases from Alzheimer’s to Parkinson’s.

“Research shows that lead has unambiguous and long-lasting effects on intelligence, behavior, and health,” the Amherst College economics professor Jessica Wolpaw Reyes has noted (pdf). “The research establishes causality: Lead causes these bad outcomes.”

Bruce Lanphear, a physician and health researcher at Simon Fraser University, argues lead is already behind as many as 400,000 deaths in the US annually, with more than half of these from heart disease. “The steepest increase in risk occurs at the lower concentrations of lead in blood,” he wrote in his 2018 study in Lancet Public Health.

A century of lead exposure is responsible for enormous intellectual deficits. For every two microgram increase in blood lead levels, a child loses approximately one IQ point. Much of this damage occurs during the earliest exposure, since a non-linear effect means even the smallest lead doses can have an outsize impact on children’s futures. For Americans born between 1960 and 1980, a period when childhood lead exposure was nearly universal in the US, the average person lost a stunning five IQ points, writes Michael McFarland, a sociologist at Florida State University, who co-authored one of the most comprehensive studies on lead in the US population, in the Proceedings of the National Academy of Sciences in 2022.

At least half the people in the US today have “meaningful cognitive loss because of lead exposure,” according to the study. These exposures show up decades later in a child’s income, health, and socioeconomic status. “Lead influences the trajectory of a person’s life,” McFarland warns. “Even relatively small deficits in achieved IQ…meaningfully predict a person’s educational and occupational attainment, health, wealth, and happiness.”

One long-term study that followed more than 500 children in New Zealand born between 1972 and 1973 found that cognitive impairment from lead was associated with nearly half of their downward social mobility as adults 38 years later.

Even modest exposure to lead can push someone below the range of average intelligence. The late John Rosen, a pediatrician who advocated for lead paint removal in the 1990s, said early lead exposure for a child with an IQ of 85 “could mean the difference between a menial job in a fast-food restaurant or a meaningful career.”

Children don’t need to be exposed to lead to suffer from it. Claudia Persico, a policy researcher at American University,  found (pdf) that even the presence of lead-exposed peers in classrooms is associated with lower test scores, lower graduation rates, and more suspensions for all students.“Even kids who are not exposed to lead are being influenced by the behavior of lead-poisoned kids,” Persico tells Quartz. “Peer effects can have really long-run consequences. Lead poisoning is much more costly than we ever thought.”

Researchers have, in fact, put a number on it. One study concluded (pdf) that the cost per child for each microgram of lead in the blood was $50,000. For society, the cost is billions of dollars every year. Aaron Reuben, a PhD candidate in clinical psychology at Duke University and a co-author of the study led by McFarland, says the past 80 years represent trillions of dollars in lost earning potential alone—that’s before factoring in the heart disease, bone disease, dementia, ADHD, teen pregnancy, crime rates, and other ills associated with lead exposure.

Economists and epidemiologists do their best to calculate the social cost of lead by measuring what’s most measurable, like IQ, earnings, and special education programs. But that fails to capture the true cost in people’s lives: parents whose children can’t curb their aggression; students who never go on to college; the fallout of an unplanned teen pregnancy; adults whose careers falter when they fail to master complex skills.

“The legacy of lead continues to shape the health and wellbeing of the country in ways we do not yet fully understand,” McFarland concluded in his study noting millions of people are still exposed to lead around the world. “Our estimates provide a springboard to understand the global extent that populations were harmed, and continued to be harmed, by legacy lead exposures.”

Quartz has built maps, searchable by city and state, of neighborhoods surrounding 95 of the top lead-emitting general aviation airports in the US.

More stories in this series:

Leaded airplane fuel is poisoning a new generation of American children

Living with the risk of childhood lead exposure: A day in the life

This series was reported with grant support from:

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