Catalytic converters are designed to clean up the exhaust fumes from petrol-driven vehicles, which are otherwise the major threat to air quality standards in congested urban streets and on motorways. Converters remove carbon monoxide, the unburned hydrocarbons and the oxides of nitrogen. These compounds are damaging to human health and the environment in a variety of ways. The converter is attached to the vehicle' s exhaust near the engine. Exhaust gases pass through the cellular ceramic substrate, a honeycomb-like filter. While compact, the intricate honeycomb structure provides a surface area of 23.000 square metres. This is coated with a thin layer of platinum, palladium and rhodium metals, which act as catalysts that simulate a reaction to changes in the chemical composition of the gases. Platinum and palladium convert hydrocarbons and carbon monoxide into carbon dioxide and water vapour. Rhodium changes nitrogen oxides and hydrocarbons into nitrogen and water, which are harmless.