Past Century of Lead Exposure Lowered IQ Score of Half of Americans, Study Finds

Intelligence quotient—or IQ—tests look at everything from a person’s cognitive abilities to education levels, presenting a supposedly standardized marker to assess individuals. However, like many other broadly distributed tests, a myriad of factors determine a person’s IQ score.

A recent study from the Proceedings of the National Academy of Sciences (PNAS) found that over 170 million Americans—about half of the nation’s population—experienced high levels of lead exposure in childhood. As a result, they lost a total estimate of 824 million IQ points since the 1940s.

There is no longer lead in our gasoline, but a large percentage of US water pipes contain lead. This research could further inform city planners and public health experts as they strive to keep the American public safe from toxic metals.

History of Lead Exposure

In 1921, automotive engineers were searching for a solution to engine knocking in cars—a phenomenon that could hurt the engine and lower the car’s efficiency. They discovered lead, a natural, now understood to be toxic, metal was the answer. As a result, leaded gasoline became the primary source of fuel in the United State

However, it wasn’t long before the first signs of lead’s health ramifications emerged. In 1924, the Surgeon General temporarily restricted leaded gasoline after suspicion that 15 refinery workers had died from lead poisoning. Yet, the investigating committee didn’t find enough evidence to substantiate it, and the Surgeon General allowed leaded gas at voluntary standards.

Thanks to the Clean Air Act’s passage, this reliance on leaded gas began to change in 1970. The federal government created the Environmental Protection Agency (EPA), which started phasing lead out due to its increasingly understood health ramifications. Four years later, the EPA took further steps by requiring at least one unleaded gasoline option compatible with cars created in 1975 or later. They were equipped with catalytic converters—a feature lead would damage.

However, it wasn’t until 1996 that the EPA entirely banned the use of leaded gasoline in on-road vehicles. As of July 2021, leaded gasoline is not available anywhere in the world.

Lead's Effect on the Body and Brain

So, how can lead lower IQ points and why was it so necessary to remove it? “Lead is a metal and chemical element that is found naturally in the environment. Unlike many other elements, lead has no biological role in the human body,” says Dr. Kelly Johnson-Arbor, medical toxicologist and co-medical director of the National Capital Poison Center. Instead, lead acts as a toxin. 

According to Centers for Disease Control and Prevention, short-term lead exposure can cause issues such as memory loss, abdominal pain, constipation, headaches, and appetite loss. Long-term exposure can lead to depression, nausea, irritability, high blood pressure, kidney disease, and impotence. Certain people have a higher risk associated with lead exposure.

Children fall in that group as they absorb ingested lead at a rate of four to five times that of adults. Lead exposure can cause children to have brain development issues, causing issues such as a lower IQ score, smaller attention span, and antisocial behavior. It can also cause children to have hypertension, enter a coma, experience convulsions, or pass away.

“There is no known ‘safe’ level of lead in the human body, and even relatively low lead levels are associated with lower IQ scores and decreased academic progress in children,” says Johnson-Arbor. “Because of this, many states require that all children undergo blood testing at a young age. Currently, a blood lead level of 3.5 mcg/dL is the accepted threshold limit for blood lead in children.” 

In pregnant people, high levels of lead exposure causes a higher chance of miscarrying, stillbirth, low birth weight, and premature labor.

The Institute for Health Metrics and Evaluation (IHME) reported that in 2019 alone, lead exposure worldwide caused an estimated 900,000 deaths and 21.7 million healthy life years lost. Low- and middle-income countries felt the brunt of this issue. In the same year, lead exposure caused 62.5% of global cases for developmental intellectual disability that didn’t have a clear cause.

These factors are critical to keep in mind when looking at lead’s impact on IQ points. It’s also necessary to look at the disparities in lead exposure. As one of the study's researchers points out, Black children tended to have higher levels of lead exposure than white children.