Over 170 million U.S.-born people who were adults in 2015 were exposed to harmful levels of lead as children, a new study estimates.

Researchers used blood-lead level, census and leaded gasoline consumption data to examine how widespread early childhood lead exposure was in the country between 1940 and 2015.

In a paper published in the Proceedings of the National Academy of Sciences on Monday, they estimated that half the U.S. adult population in 2015 had been exposed to lead levels surpassing five micrograms per deciliter — the Centers for Disease Control and Prevention threshold for harmful lead exposure at the time.