For many decades gasoline had a lead additive in it as an octane booster. While ethanol could have been used at the time, it was not chosen on purpose. Lead is very toxic, and it wasn't just affecting those who came in close contact with it. The lead was getting into the air as well, and causing a major public health concern. The EPA decided to crusade against it in 1976 with the total phase out completed in 1986.