Leaded gasolin was containing a substance known as tetraethyl lead or TEL, a compound of lead in liquid form originally added to petrol to increase its octane rating. A side effect of adding TEL is that a layer of lead compounds forms on the valve faces of the engine, retarding wear. Lead is very toxic and lead compounds in exhaust gases escape into the atmosphere causing pollution. Impacts on human health are widely documented. This led to the introduction of lead-free petrol. Lead Replacement Petrol was introduced to address the issue of valve wear.