Petrol with lead in it will produce more toxic gases when it is burned in a car engine. These toxic gases will then include some lead from the lead in the petrol. Lead pollution in city air is VERY bad for health of the people.
Once lead gets into the body from breathing it or eating/drinking it in food or water, it stays there. It can accumulate (eventually) to poisonous levels and cause many problems. The only way to get lead out of a person is by a doctor treatment, which is difficult.
This is why most countries encourage unleaded petrol for cars and trucks, especially in high traffic areas like big cities. Some places even ban leaded petro for cars and trucks, or don't allow it to be sold, because of the air pollution problems.
Planes still use it, however, because it's necessary because of how their engine valves work .
