So why is MTBE a problem in the USA?
Leaking petrol storage tanks and spillage have caused MTBE to get into groundwaters in the USA. Although it is not very toxic, it is not very biodegradable either, and has a strong taste and smell, noticeable at the 15 parts per million level. There is now a strong movement to ban it from petrol, in California in particular.
If there is no lead in it, surely unleaded petrol is safe?
There is quite a lot of benzene in unleaded petrol; it makes up 1.4% of normal unleaded (out of 28 % aromatics) and 2.6% in 4 star unleaded (out of 45% aromatics). Not all the hydrocarbons get burned up in the engine so that some gets passed into the air. Benzene is believed to cause cancers, leukaemia and anaemia. Catalytic converters can help reduce hydrocarbon emissions, but not until they are warmed up.