Leaded Gas Exposure Tied to Millions of Mental Illness Cases

Childhood exposure to leaded gasoline via car exhaust over the past 75 years is linked to 151 million excess cases of psychiatric disorders in the United States, new research suggested.

The data revealed that the group most heavily exposed to lead — individuals born between 1966 and 1986, commonly known as Generation X — experienced the biggest increases in mental health issues.

Within this cohort, those born between 1966 and 1970 were affected the most. This timeline, the investigators noted, aligns with the peak use of leaded gasoline during the mid-1960s and 1970s. Specifically for this group, overall mental health issues increase by 0.35 times the average, anxiety and depression by 1.75 times, and attention-deficit/hyperactivity disorder (ADHD) symptoms by 1.17 times.

“Lead exposure across the country has probably played a significant role in worsening mental health in ways that had previously been unappreciated and invisible,” study investigator Aaron Reuben, PhD, assistant professor of clinical neuropsychology at the University of Virginia in Charlottesville, Virginia, told Medscape Medical News.

However, the investigators emphasized that the study only establishes an association and not a causal relationship between leaded gas exposure and subsequent psychopathology.

The findings were published online on December 4 in the Journal of Child Psychology and Psychiatry.

Lead Astray?

Leaded gasoline was first used in the 1920s to stop engine knocking and improve performance. Despite early evidence of neurotoxicity its use continued until 1996 when it was banned.

The investigators noted that over half of the current US population was exposed to adverse lead levels from gasoline in childhood. However, they added the total contribution of childhood lead exposure to the population’s mental health and personality has not previously been evaluated.

For the study, the researchers combined serial, cross-sectional data on blood lead levels (BLLs) from the National Health and Nutrition Examination Survey and integrated it with historical data on gasoline usage, allowing them to estimate childhood BLLs in the United States from 1940 to 2015.

They calculated the impact of lead exposure on mental health using general psychopathology factor points, also referred to as mental illness points, which function similarly to IQ points.

These measures used were based on a prior study led by Reuben in 2019 from a New Zealand cohort and a study by a different group that followed a longitudinal birth cohort in Chicago .

Using these data, the researchers calculated population-level elevations in mental health symptoms on the basis of lead exposure and five key psychiatric outcomes in the US population.

These included general psychopathology, which reflects an individual’s overall liability to mental disorders and was scaled to match IQ scores with a mean of 100 and an SD of 15.

They also evaluated symptoms of internalizing disorders including anxiety and depression and ADHD and standardized them to a mean of 0 and an SD of 1.

The researchers also looked at differences in the personality traits of neuroticism, which relate to emotional instability and conscientiousness and reflect organization and responsibility. Both of these were assessed using similar standardized scales.

151 Million Excess Cases of Mental Illness

Using this approach, the researchers were able to assess the historical and long-term implications of lead exposure on mental health and personality traits in the US population over time.

Results showed that during the peak era of leaded gasoline in the United States, children were routinely exposed to lead levels three to six times higher than the current reference point for clinical concern (3.5 μg/dL of blood), the authors noted.

While the United States banned leaded gasoline in 1996, lead can still be present in water pipes, old paint, and soil. The US Centers for Disease Control and Prevention cautions that no BLLs are safe.

The investigators found that between 1940 and 2015, the US population gained 602 million general psychopathology points, which the investigators said equates to 151 million excess instances of mental disorders that are primarily, but not completely, attributable to early life exposure to leaded gasoline.

“Assuming that published lead-psychopathology associations are causal and not purely correlational, we estimate that by 2015, the US population had gained 602 million General Psychopathology factor points because of exposure arising from leaded gasoline, reflecting a 0.13 standard deviation increase in overall liability to mental illness in the population and an estimated 151 million excess mental disorders attributable to lead exposure,” the researchers wrote.

Specific effects included a 0.64 SD increase in anxiety and depression symptoms, a 0.42 SD increase in ADHD symptoms, a 0.14 SD increase in ADHD symptoms, a 0.14 SD increase in emotional instability (neuroticism), and a 0.20 SD decrease in traits like organization and responsibility (conscientiousness).

These mental health and personality changes were not distributed evenly among the generations, most significant in those born between 1966 and 1986, the investigators reported.

For example, children between 1966 and 1970, the period when leaded gasoline use was at its peak, had BLLs > 5 μg/dL and experienced a 1.75 SD increase in internalizing symptoms and a 1.17 SD increase in ADHD symptoms.

Assess Lead Risk

The study had several limitations. Causality could not be established, and the accuracy of the estimates relied on findings from the researchers’ two previous key studies.

However, the investigators noted that these findings have been replicated across multiple cohorts and settings. Additionally, the results may not be fully generalizable to the entire US population, as one study was based in New Zealand and the other in urban Chicago.

Reuben cautioned that even though gasoline and new paint no longer contain lead, exposure is still possible.

“We saw this most acutely in Flint, Michigan,” when aging water pipes exposed more than 100,000 residents to high lead levels in 2014. “This situation made us aware that thousands of communities are exposed to lead service lines.”

He recommended that physicians consider screening patients for lead exposure — both new and old. Experts estimate that 90% of lead in the body is stored in the skeleton and can be released back into the bloodstream over time, particularly in cases of calcium deficiency, pregnancy, or osteoporosis.

While reversing childhood lead exposure is not possible, Reuben noted that healthy lifestyle choices and multimodal interventions such as medication and therapy can effectively address and alleviate mental illness.

‘Legacy of Lead’

Commenting on the findings for Medscape Medical News, Terrie Moffitt, PhD, who was not involved in the research, said the study “is important because it gives us a crystal ball to see into the future of children living with lead today.”

“It’s called the ‘legacy of lead,’ and what a legacy,” said Moffitt, PhD, professor in the Department of Psychology and Neuroscience, Nannerl O. Keohane University at Duke University in Durham, North Carolina.

Moffitt emphasized that children exposed to lead today often live in areas of poverty and disadvantage, making it difficult to disentangle the potential effects of lead exposure from those of childhood adversity that predispose individuals to mental illness.

“This study tells us about lead’s damage in an era when it was everywhere, not just in poor communities,” she said.

The study was funded by a fellowship from the National Institute of Environmental Health Sciences. Reuben and Moffitt reported no relevant financial relationships.