Chlorine is one of the elements most frequently found in nature; it is even more abundant than carbon. Chlorides, i.e. salts containing chlorine, are one of the few raw materials which will not be exhausted within the foreseeable future. Naturally-occurring chlorine compounds are present in our blood, skin and teeth, and chlorine in the form of hydrochloric acid has an important part to play in the digestive process. There are also organic compounds present in nature which contain chlorine; marine algae, for example, produce about 5,000,000 tonnes of methyl chloride annually (ie: around 15 times more than yearly industrial production). In total, more than 2,000 natural organic chlorine compounds have already been identified.