In the 1920s, researchers realized that you can add lead to gasoline to help keep car engines healthy for longer. But while leaded gasoline was good for cars, it was bad for humans.

Leaded gasoline is highly toxic and in addition to causing a number of health problems, it can also cross the blood-brain barrier and accumulate in some parts of the brain, where it can cause a number of problems, including reducing intelligence. According to a new study, exposure to car exhaust from leaded gasoline affected the IQs of over 170 million Americans alive today, costing the country a collective 824 million IQ points.

The findings come from a new study published by Aaron Reuben, a PhD candidate in clinical psychology at Duke University, and Michael McFarland and Mathew Hauer, both professors of sociology at Florida State University. The researchers started from publicly available data on US childhood blood-lead levels and leaded-gasoline use. They then determined the likely lifelong burden of lead exposure of every American alive in 2015. From this, they calculated how much of an intelligence burden this exposure to lead proved to be. While IQ isn’t a perfect proxy to intelligence, it’s still a good population-level indicator.

Previous studies have suggested an association between lead exposure in childhood and a drop in IQ. But when the results came in, even the researchers were surprised.

“I frankly was shocked,” McFarland said. “And when I look at the numbers, I’m still shocked even though I’m prepared for it.”

The results show that over half of all Americans (170 million out of an entire population of 330 million) had clinically significant levels of lead in their blood, resulting in lower IQ levels as adults, as well as a number of potential health problems (such as reduced brain size, greater likelihood of mental illness, and increased cardiovascular disease). The people affected by lead exposure would have each lost, on average, 3 IQ points.

“Lead is able to reach the bloodstream once it’s inhaled as dust, or ingested, or consumed in water,” Reuben said. “In the bloodstream, it’s able to pass into the brain through the blood-brain barrier, which is quite good at keeping a lot of toxicants and pathogens out of the brain, but not all of them.”

Three IQ points may not seem like much, but keep in mind that this is an average for a whopping 170 million people. At its worst, people born in the mid-late 1960s may have lost 6 IQ points on average. At a population level, this is a considerable margin — and even though leaded gasoline was banned in the US in 1996, the effects of the problem are still visible today.

“Millions of us are walking around with a history of lead exposure,” Reuben said. “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.”

Thankfully, the era of leaded gasoline is finally over. Most countries banned it two decades ago, but only last year, in 2021, the era of leaded gasoline was finally over as the last stocks were used in Algeria (which had continued to produce leaded gasoline until July 2021).

Leaded gasoline is a good example of an exciting technology that turns out to be very bad for the environment and for human health. But while leaded gasoline has been phased out, there are plenty of other sources of pollution still affecting our brains, lungs, and hearts.

Journal Reference:  “Half of US Population Exposed to Adverse Lead Levels in Early Childhood,” Michael J. McFarland, Matt E. Hauer, Aaron Reuben. Proceedings of the National Academy of Sciences, March 7, 2022. DOI: 10.1073/pnas.2118631119