Especially in Winter when heat inversion does not allow the pollution to be dispersed. Carbon monoxide Carbon monoxide is generated by incomplete combustion of the fuel caused by the lack of oxygen. This is a toxic gas which is odourless and colourless, the presence of which in traffic jams may reach high levels. Hydrocarbons Organic products that are produced by incomplete combustion of carbon-based fuels. At the present time the maximum level of lead present in that fuel is set at 0.15 grams of lead per litre of petrol.
Sulphur oxide Gases from the combustion of certain liquid fuels which contain sulphur.
Nitrogen oxide A gas resulting from the reaction of oxygen and nitrogen in the air in combustion processes through the effect of heat and pressure. In a petrol-driven engine, the mixture of air and fuel is introduced by aspiration, pressurised and lit by a high voltage electrical charge. So as to prevent this mixture from exploding in the compression stage in the cylinder, a substance must be added to prevent detonation. Traditionally the most widely used one has been tetraethyl lead. The problem is that combustion produces lead and lead oxide which are emitted into the atmosphere through the exhaust pipe and have a pollutant effect as well as possible effects on health.