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The use of leaded gasoline in motor vehicles was phased out in the U.S., starting in the 1970s. Tomasz Pietryszek/Getty Images
  • Exposure to car exhaust from leaded gas during childhood took a collective 824 million IQ points away from more than 170 million U.S. adults alive today, a study has found.
  • The researchers estimate that childhood lead exposure has, on average, led to a reduction of 2.6 IQ points per person as of 2015.
  • The research also found that non-Hispanic Black people, individuals with a lower family income-to-poverty-ratio, and those with an older housing age were likely to have higher levels of lead in their blood.

Back in the 1920s, engineers began adding lead to gasoline to reduce engine knocking.

Almost immediately, researchers raised concerns about the health impacts of the gas additive, but it was not until 1973 that the Environmental Protection Agency (EPA) issued its first call for manufacturers to begin a gradual reduction of the amount of lead in gasoline.

Around that same time, car manufacturers began building vehicles with catalytic converters, which reduced pollution and required unleaded fuel.

By 1995, according to the EPA, leaded fuel only accounted for 0.6% of total gasoline sales. The EPA put the final nail in the coffin for gas with lead in 1996 by officially banning its use in on-road vehicles.

Public health officials are well aware that lead poisoning causes severe health problems. It can, according to the EPA, adversely affect the nervous, reproductive, and cardiovascular systems, alongside others.

Infants and young children are especially sensitive to lead exposure. Among this population, exposure may cause behavioral problems, learning deficits, impaired growth, and lowered IQ.

In a new study in the journal Proceedings of the National Academy of Sciences (PNAS), researchers determined the lifelong burden of exposure to car exhaust from leaded gas that every U.S. adult alive in 2015 is now likely to carry.

Dr. Michael McFarland, Ph.D., associate professor of sociology at Florida State University (FSU), wrote the paper along with Matt Hauer, an assistant professor of sociology at FSU, and Aaron Reuben, a doctoral student in clinical psychology at Duke University.

The goal of the study, Dr. McFarland explained in an email to Medical News Today, was “to ascertain exactly how much damage was done.”

In addition to causing cognitive and IQ deficits, the study authors point out that exposure to lead can cause problems with emotional regulation and have physical effects, such as producing deficits in fine motor skills.

“These deficits largely persist across time and, in some cases, worsen and are now hypothesized to put individuals at risk for difficult-to-treat chronic and age-related diseases, including cardiovascular disease and dementia,” they write in the paper.

Dr. McFarland, who studies health disparities, developed an interest in leaded gasoline while reading the book What the Eyes Don’t See: A Story of Crisis, Resistance, and Hope in an American City about the water crisis in Flint, MI.

“It included one chapter on the history of leaded gasoline,” Dr. McFarland wrote to MNT. “[I]t showed approximately 90% of kids had elevated levels [of lead] from 1976–1980. I hadn’t studied the history of lead in the [U.S.], so this was quite shocking.”

As he began to look at the subject, Dr. McFarland discovered that researchers had dedicated an insufficient amount of study to the topic.

“I checked the research and found that we had no idea how many young children had been exposed to adverse lead levels,” he said.

For the study, the team used publicly available U.S. data on childhood blood-lead levels, leaded gas use, and population statistics to determine the likely lifelong burden of lead exposure that every U.S. adult alive in 2015 now carries. From there, the researchers estimated the impact of lead on intelligence by calculating the reduction in IQ points that exposure to leaded gas likely caused.

The researchers report two main findings:

  • Childhood lead exposure among the U.S. population by 2015 was responsible for the loss of more than 824 million IQ points. That number equates to an average of 2.6 lost IQ points per U.S. adult.
  • Estimated lead-linked deficits were greatest for individuals born between 1966 and 1970. This population experienced an average deficit of 5.9 IQ points per person.

“Their estimate for the number of IQ points lost is pretty high,” Dr. Nicholas Newman, a pediatrician and the director of the Environmental Health and Lead Clinic at Cincinnati Children’s Hospital, OH, told MNT.

“But I don’t think [it is] implausible based on the way their work was done,” he said.

Overall, the scientists found a drop in IQ in approximately 170 million U.S. adults. This number of individuals represents close to half of the entire U.S. population, which current estimates put at more than 332 million.

If so many individuals had not had exposure to lead from motor vehicle exhaust, we might be living in a different society today, according to Dr. McFarland.

“We’d definitely be healthier, wealthier, and smarter,” he said.

For their next investigation, Dr. McFarland and the other researchers plan to look at the effects of past lead exposure on brain health in old age.

“Recent research suggests these exposures may increase rates of dementia in older age,” Dr. McFarlandsaid.

The researchers also want to evaluate racial disparities in lead exposure pre-1976. The researchers point to 2021 research that found that non-Hispanic Black people have disproportionately higher blood lead levels at the national level.

Dr. Newman hopes that the study in PNAS will inspire researchers to develop therapies for individuals who had exposure to high levels of lead as children.

“[I]t should be a call for us to be really vigilant in terms of caring for adults and anticipating these problems and then applying what we know that works to try to help them,” he told MNT.