Exposure to lead in car exhaust may have caused a population-wide drop in IQ, according to a new study from researchers from Florida State University and Duke University.
Lead is a known neurotoxin that can cause developmental delays, seizures, learning disabilities, and hearing loss, along with symptoms such as fatigue and irritability. Until the mid-1970s, lead was a ubiquitous gasoline additive. It stabilized gas and prevented fuel from igniting too early and causing engine knocking and engine damage. With the passage of the Clean Air Act in 1970, the Environmental Protection Agency began phasing out the use of lead in gasoline, resulting in a total ban on the additive for “on-road vehicles” in 1996, though by that point, only 0.6% of gasoline contained lead.
For the new study, researchers examined archival census data and records of blood lead levels in children born from 1940 to 2015. They also looked at the rate of use of leaded gasoline. They discovered that 90% of children born between 1950 and 1981—a total of about 170 million people—had blood lead levels greater than 5 micrograms per deciliter. In 2021, the Centers for Disease Control and Prevention (CDC) lowered the threshold for acceptable blood lead levels from 5 micrograms per deciliter to 3.5 micrograms per deciliter. This means that the vast majority of Baby Boomers and Gen Xers exceeded the acceptable amount of lead in their blood as children.
This level of lead exposure correlates to an IQ drop of 2.6 points per person, or a cumulative 824 million points for the entire population. For those born in the 1960s and early 1970s, when leaded gas use was at its highest, the loss could be even higher—up to 7 points.
Once a person is exposed to lead, the effects can linger, so researchers are concerned about how this early childhood exposure translates to brain health in old age. “Millions of us are walking around with a history of lead exposure,” study co-author Aaron Reuben, a clinical psychology Ph.D. candidate from Duke University, said in a statement.. “It’s not like you got into a car accident and had a rotator cuff tear that heals, and then you’re fine. It appears to be an insult carried in the body in different ways that we’re still trying to understand but that can have implications for life.”
Reuben told USA Today that not all children were exposed equally, and that those who lived on busy streets or in urban areas may have been exposed to higher levels of lead from car exhaust than those who did not. “It’s hard to know if you’re one of those Americans but if you grew up near lead emitting, you might just take a proactive approach,” he said. “Let your primary care physician know that it’s a concern, and it could motivate additional surveillance of conditions that could arise later in life.”
Lead exposure is not only a historical problem. The neurotoxin is still found in contaminated water supplies, paint, and even food today. Half of U.S. children have been shown to have at least some lead in the blood, and over the years, lead has been discovered in popular fruit juices as well as foods marketed for babies and young children.