Complex sentence: The last major innovation to expose humans to contact with lead was the addition of tetraethyllead to gasoline as an antiknock agent, a practice that originated in the United States in 1921; it was phased out there, and in the European Union, by 2000.
Split into simpler sentences: The last major innovation to expose humans to contact with lead was the addition of tetraethyllead to gasoline as an antiknock agent, a practice that originated in the United States in 1921. It was subsequently phased out in the United States, and in the European Union, by 2000.