Unleaded petrol is petrol manufactured without the addition of a substance called anti-knock, a compound containing lead that was originally put into petrol to prevent knocking in the engine and to promote smooth running. Unleaded petrol has a slightly lower octane rating than leaded petrol, but is has other compounds added so that performance is not really impaired. It has the advantage that its use does not release lead compounds into the atmosphere from exhaust fumes.

There was a great deal of worry that the release of large amounts of exhaust fumes in busy city roads was producing an unacceptably high level of lead at street level. Lead causes toxic effects, particularly in the brain that are irreversible and children were at risk.

Unleaded petrol is now widely in Europe and other countries and all new cars are made to take it rather than leaded petrol. Cars fitted with a catalytic converter, a device fitted to the exhaust to reduce emissions of other dangerous compounds, cannot run on anything other than cars with unleaded petrol.