The use of leaded gasoline was introduced in the 1920s despite known health risks, with General Motors engineer Thomas Midgley Jr. discovering the additive tetraethyl lead to reduce engine knocking. The effects of lead exposure from gasoline are still being felt, particularly in children, with links to lower IQ, behavioral problems, and violent crime. Leaded gasoline was phased out in the US in the 1970s, but its legacy remains. The use of leaded gasoline in aircraft continues to be a concern, with efforts to transition to unleaded fuel ongoing.