Gasoline used in automotive applications was leaded from the 1920s through the 1970s. The United States shifted to unleaded use in the 1980s. Leaded fuel offered improved resistance to detonation, which allowed engine manufacturers to use higher compression engine designs that produced higher power. Due to the environmental impact of leaded fuels, coupled with the need for unleaded fuels for modern catalytic converter equipped vehicles, leaded fuel has been phased out in many parts of the world.
Leaded gasoline offers two advantages over unleaded gasoline -- improved resistance to detonation, and improved protection for engine valves. A lead additive, typically tetraethyl lead, is added to the fuel as an additive. The better resistance to detonation (which is when fuel prematurely detonates in the engine cylinder and cases damage) allowed engines to be designed with higher compression levels, which generated more power.
The move to unleaded fuels occurred due to the dangers of lead exposure, and the need to reduce pollution output of automobiles. Modern fuel companies still add a variety of additives to unleaded fuel, but they no longer include tetraethyl lead.
Evidence began to mount from the 1920s that the lead poisoning from leaded fuels was a real danger. As lead particles are heavy, the lead tends to settle close to the tailpipe, which means the lawns and sidewalks adjacent to public roads became increasingly dangerous places for children to play.
Catalytic converters, which were mandated in the United States from 1975 forward, are emission control devices. The platinum matrices within the converters can become fouled by lead additives present in leaded fuels.
