Lead is a well known neurotoxin. It’s also a common pollutant. New research estimates the toll that those two truths, combined, have had on Americans’ mental health. Between 1940 and 2015, childhood lead exposure (specifically from the use of leaded gasoline) resulted in about 151 million additional instances of psychiatric illness that wouldn’t have otherwise occurred, according to the study published December 4 in the Journal of Child Psychology and Psychiatry. The authors came to that conclusion by applying findings from previously published work to a model of the entire U.S..
“Writ large, across the population, we’ve shifted the curve away from normal, healthy functioning and towards greater rates of mental illness,” says Aaron Reuben, a study co-author and clinical neuropsychologist at the University of Virginia, where he’s an incoming assistant professor of psychology.
Currently, lead is found in industrial emissions, water lines, paint, and contaminated foods and consumer products. But for decades it was also a gasoline additive. Between the early 1920s and 1980s, much of the gasoline used in the U.S. was leaded. (Though usage peaked in the 1970s, it wasn’t banned as a passenger car fuel additive in the U.S. until 1996, and it’s still used in some aircraft and off road vehicle fuels).
All that fuel, burned up by all those miles driven, made lead pervasive in the air and soil and constituted the single largest exposure source of lead for more than a generation.
The most up-to-date science states that no level of lead exposure is safe–especially during childhood when the toxic metal causes developmental mayhem. “We know from about 100 years of evidence from human and animal studies that lead is harmful for almost every organ system that we’ve studied, and we know it’s particularly harmful to the developing brain,” says Reuben.
Yet some level of exposure in the present day is unavoidable. Accordingly, the CDC has set a blood reference level for identifying children with high exposure relative to the rest of the population at 3.5 micrograms per deciliter, lowered from 5 mcg/dL in 2021.
However, more than 170 million people in the U.S.–over half the country’s current population–experienced childhood exposure to lead at dangerous levels, higher than that old, lax 5 mcg/dL reference value, per a 2022 analysis from Reuben and colleagues. The researchers derived this estimate from stats on leaded gasoline consumption and nationally representative blood-lead level survey data. In that same 2022 study, they determined lead pollution had cost Americans approximately a collective 824 million IQ points.
Admittedly, IQ is a deeply flawed metric. But it is one of the best measures we have to understand population-level responses to something like environmental contamination. In this case, the researchers weren’t comparing individuals to one another, but rather people to themselves under a theoretical, different environmental condition. And, regardless, the science is clear: lead damages the brain and diminishes cognitive function.
The authors based their new analysis off of this and other previously published research. They combined past work quantifying the levels of lead exposure incurred by different U.S. age cohorts as environmental policy shifted, with associative studies of smaller groups that tracked childhood lead levels and the subsequent development of certain maladaptive personality traits, mental illness symptoms, and diagnoses later in life.
They did not conduct any new data collection or monitoring. Nor did they look at mental health or personality trait trends through time across the whole population. Instead, they used past, small-scale findings to model approximate, nationwide effects, assuming those prior studies were representative of the wider population.
Using a metric of mental health similar to IQ points, called “General Psychopathology factor points,” they found that leaded gasoline and the corresponding childhood lead exposure caused a gain of 602 million GP points over the 75-year study period, among the U.S. population. Contrary to the IQ scale, a higher GP score is considered worse, as it translates to an increase in mental illness symptoms and their severity. Population-wide symptoms of depression, anxiety, and ADHD symptoms also increased, due to lead exposure, according to Reuben and his co-authors.
[ Related: Leaded gas may have lowered the IQ of 170 million US adults ]
Beyond these markers of clinical mental disorders, the new analysis further determined that early life lead exposure has altered the average U.S. personality. Again, they used previously published studies which tracked adult personality traits in relation to childhood lead exposure levels in a cohort, and modeled that data across the U.S. population. Due to childhood lead poisoning, the research suggests that Americans, overall, are less conscientious and more neurotic.
All of the above trends in mental health and personality vary in intensity depending on birth cohort, per the study, as leaded gasoline use, and thus lead exposure, has fluctuated with changing environmental regulations. Because leaded gasoline consumption ramped up rapidly in the 1960s and reached its highest level in the 1970s, the effects noted by the new research are most pronounced for those born between 1966 and 1986 (Generation X) who experienced the highest estimated levels of lead exposure between ages 0 and 5 years.
For most Gen X-er’s across the U.S., the lead exposure they incurred as children probably didn’t lead to obvious, individual changes. But the new research demonstrates that, big picture, lead pollution has had serious national consequences. “Our takeaway is that, even though these exposures went unnoticed for most people, they undoubtedly influenced their mental health. They made life just a little bit harder for some and a lot harder for others.”
Though the exact mechanisms aren’t yet entirely understood, it’s well-established that lead disrupts multiple physiological processes, says Bruce Lanphear a professor of health sciences at Simon Fraser University in British Columbia who wasn’t involved in the new research. Lead, he explains, can replace calcium–a critical neurotransmitter and necessary mineral–in biological reactions. Prior research has also found that lead interferes with the brain’s dopamine system, he notes. Lanphear has spent decades studying lead poisoning as a population health scientist, and as a result he says he was utterly unsurprised by the findings. “There’s no question in my mind that lead has had a huge impact on mental health over the past two or three generations,” Lanphear says.
The new study has some big limitations. For instance it relies on very strong correlational evidence as a proxy for causational studies and it formally evaluates just a handful of datasets. Plus, by focusing on the role of leaded gasoline alone, it ignores other potential sources of childhood lead exposure like paint dust and drinking water, which could mean the research is actually an underestimate of the problem. But the authors acknowledge these weaknesses and “overall, I think they presented the data fairly,” says Lanphear.
By combining existing, narrow analysis into a broad population assessment, the study researchers have gotten us closer to understanding the global burden of disease and total societal damage wrought by lead pollution. “That’s really the novel part of this: helping people appreciate the impact over time,” he adds. Lanphear hopes that studies like this will bring more attention to the outsized and understudied role that environmental exposures have on health and well-being.
Despite ever-tightening restrictions on lead, about one-third of children worldwide still suffer from high blood-lead levels. Even in the U.S., most people continue to be exposed to lead levels hundreds of times higher than natural background levels, says Reuben. “It’s making life harder for all of us, and that goes unnoticed and untreated,” he says.
But turning the tides and reducing damage to future generations is possible. “We need to replace our lead service lines. We need to stop putting lead into paint. We need to finally finish removing lead from fuel sources,” Reuben says. And with more awareness, perhaps childhood lead tests will become more common, he adds, allowing families to identify and eliminate harmful exposure sources earlier. “There’s lots you can do when you start to actually look the problem in the face.”
