A new study published Monday in the Proceedings of the National Academy of Sciences found lower IQs in adults who had been exposed to leaded gasoline.
The study found that more than 170 million American adults were exposed to high lead levels in childhood, lowering the IQ of the average person approximately 2.6 points. In total, researchers estimated lead is responsible for the loss of more than 824 million IQ points in Americans as of 2015.
Researchers used blood-lead level, census, and leaded gasoline consumption data to determine how widespread lead exposure was between 1940 and 1915, The Associated Press reported.
The study was done by researchers at both Florida State University and Duke University, and found that 90% of children born in the U.S. between 1950 and 1981 had higher levels of lead in their blood than the threshold defined by the Centers for Disease Control and Prevention, the AP reported.
Certain segments of the population saw higher impact than others, with people born in the 1960s and 1970s seeing the highest loss of IQ, with up to six to seven points lost per individual, NBC News reported. Researchers said the lead exposure largely came from inhaling auto exhaust.
“I frankly was shocked,” Michael McFarland, lead author of the study and a sociology professor at Florida State University told USA Today. “And when I look at the numbers, I’m still shocked even though I’m prepared for it.”
Lead was added to gasoline beginning in 1921, when researchers as General Motors discovered adding it to gasoline could improve engine performance, NPR reported. At the time, it was already known that lead was dangerous, but researchers working for the auto companies and oil companies said the general public would not be harmed by low levels of exposure through gasoline, NPR reported. The Environmental Protection Agency began phasing out leaded gasoline in 1973, and it was fully banned in the United States in 1996, NPR reported.
McFarland described the study’s results to The Associated Press, saying the findings were “infuriating” because the dangers of lead were long-known.
The current blood-lead value that triggers clinical concern is 3.5 micrograms per deciliter, USA Today reported. “We found over 4.5 million Americans had levels over 30 — 10 times what’s considered alarming,” Aaron Ruben, the study’s co-author and Ph.D. candidate at Duke University, told USA Today.