Burning fuels give off waste gases which cause , which damages the and can bring on asthma attacks.

Tetrethyl lead was used as an additive to to make it burn smoothly. The lead is very . Therefore has been banned from petrol.

Carbon monoxide and nitrogen dioxide are both poisonous and unpleasant gases. Cars now have catalytic converters to get rid of these , turning carbon monoxide into carbon , and nitrogen dioxide into nitrogen. The is rhodium, a rare and expensive metal.

The catalytic converters only work when . If you put leaded petrol into a car with a catalytic converter, a layer of would form over the catalyst. The catalytic converter would no longer work; it has been "poisoned". New catalytic converters are expensive.