Silicon is the second most abundant element in the earth's crust, comprising around 28% of it. It is not found in its elemental form but occurs mainly as oxides and silicates. In contrast to carbon, silicon-silicon bonds are uncommon. Natural silicon-carbon bonds are extremely rare but they can be created synthetically. Silicone is incorporated into medicines; used in food processing (for example, canning and ready meals); used in a wide range of medical devices; used as putty and sealants. The use of silicone oils in food processing and food contact can give rise to systemic exposure to small chain silicone components which are known to be absorbed through the gastrointestinal tract. Silicone is used in domestic and personal products such as cleaning solvents, handcream, hair and skin products, and antiperspirants. It may be absorbed orally or through the skin and absorption can be measured on a scale from 'minimal' to 'well'.