The toxic metal lead has cast a long shadow on our collective well-being. A new study published Wednesday shows that lead exposure during the 20th century significantly worsened Americans’ brain health, likely helping cause many more cases of mental illness that otherwise would have never happened.
Scientists at Duke University and Florida State University conducted the study, building on their past research of lead’s impact on our health. They estimated that childhood lead exposure—particularly during the decades when it was most found in gasoline—has directly contributed to 151 million more cases of psychiatric disorder among Americans over the past 75 years. The findings indicate that lead has been even more dangerous to humanity than we knew.
Car manufacturers began to add lead to gasoline in the 1920s, aiming to reduce wear and tear on the engines. Leaded gasoline eventually became Americans’ single largest source of lead exposure, reaching a peak during the 1960s. But while scientists had long known that heavy lead exposure was bad for us, it became firmly established by the 1970s that even small amounts of lead could be harmful, especially to the brains of developing children.
It would take decades more for lead to be fully phased out of gasoline (1996 in the U.S., but 2020 for every country in the world) and other common products, however. And scientists are still trying to quantify the subtle but meaningful health effects of lead’s constant presence in people’s lives during the 20th century, including the researchers behind this latest study.
The team’s earlier research in 2022 calculated that about half of all Americans alive in 2015 were likely exposed to damaging levels of lead in their childhood, based on population survey data and known levels of leaded gasoline use in the country. Furthermore, they estimated that this lead exposure had collectively lowered Americans’ IQ by 824 million points, or about 3 points per person (those born in the 1960s may have lost up to 6 points).
In their new study, the researchers decided to examine the mental health toll of lead. They cross-referenced their earlier data on Americans’ collective lead exposure with other data estimating how much lead is needed to raise a person’s risk of various psychiatric conditions, including schizophrenia, depression, and attention deficit hyperactivity disorder. From there, they created a proxy score of Americans’ vulnerability to mental illness, quantified as “general psychopathology factor” points.
All in all, they estimated that lead exposure, especially during the peak of leaded gasoline, had added 602 million more points of this vulnerability to Americans living in 2015. More practically, they estimated that lead directly helped cause 151 million more cases of psychiatric disorder. The biggest lead-linked increases were seen with symptoms of anxiety, depression, and ADHD.
“Childhood lead exposure has likely made a significant, underappreciated contribution to psychiatric disease in the United States over the past century,” the researchers wrote in their paper, published Wednesday in the Journal of Child Psychology and Psychiatry.
While these findings are ultimately only an estimate of how harmful lead has been to our mental health, this is far from the first study to blame lead for widespread effects on the population. Other research has found evidence that higher lead levels contributed to higher crime rates during the 20th century by raising people’s tendency toward violent, antisocial behavior, for instance. And given that there isn’t a truly safe level of lead exposure, the researchers say their math might still be underestimating how bad lead has been for our brains.
Lead levels in the environment are thankfully now much lower than they were in the 1960s. But there are still pockets of the country where levels are higher than normal as well as sources that can cause acute clusters of increased lead exposure, such as damaged water systems (aptly seen during the Flint Water crisis) or old houses built before 1978 that begin to peel paint.
According to the Centers for Disease Control and Prevention, about 2.5% of children between the ages of one to five have noticeably higher levels of lead in their blood. And there are many parts of the world where lead regulations are much more lax. The researchers cite recent data from the UN estimating that about 800 million children, a third of the world’s population, are currently being exposed to high levels of lead.
Of course, there are also undoubtedly many people still alive today who have suffered from lead-caused mental illness that wouldn’t have occurred in a better world (not to mention the families or caregivers who have been affected as well). So while the worst of lead’s harms may be over, its impact will continue to loom large for a long time.
