Why did the world use leaded gas—and why did it take so long to phase out?

gas pumpThe UN reported this week that the world recently reached a major milestone: by ending use of leaded gasoline, which has been a major threat to the environment and public heath. But why were countries using it the in first place, and why did it take so long to phase out? Tetraethyl lead was added to gasoline beginning in 1922 to help engines run…