Cyclomethicones are a group of methyl siloxanes, a class of liquid silicones (cyclic polydimethylsiloxane polymers) that possess the characteristics of low viscosity and high volatility as well as being skin emollients and in certain circumstances useful cleaning solvents.Unlike dimethicones, which are linear siloxanes that do not evaporate, cyclomethicones are cyclic: both groups consist of a polymer featuring a monomer backbone of one silicon and two oxygen atoms bonded together, but instead of having a very long "linear" backbone surrounded by a series of methyl groups (which produces a clear, non-reactive, non-volatile liquid ranging from low to high viscosity), cyclomethicones have short backbones that make closed or nearly-closed rings or "cycles" with their methyl groups, giving them many of the same properties of dimethicones but making them much more volatile. They are used in many cosmetic products where eventual complete evaporation of the siloxane carrier fluid is desired. In this way they are useful for products like deodorants and antiperspirants which need to coat the skin but not remain tacky afterward.Most cyclomethicone is manufactured by Dow Corning.Cyclomethicones have been shown to involve the occurrence of silanols during biodegration in mammals.The resulting silanols are capable of inhibiting hydrolytic enzymes such as thermolysin, acetylcholinesterase, however the doses required for inhibition are by orders of magnitude higher than the ones resulting from the accumulated exposure to consumer products containing cyclomethicones.
Because silicones are heavily used in biomedical and cosmetic applications, their toxicology has been intensively examined. "The inertness of silicones toward warmblooded animals has been demonstrated in a number of tests." With an LD50 in rats of >50 g/kg, they are virtually nontoxic.
Cyclomethicones are ubiquitous because they are widely used in biomedical and cosmetic applications and can be found at high levels in American cities and can be toxic to aquatic animals in concentrations often found in the environment.The cyclomethicones D4 and D5 are bioaccumulative in some aquatic organisms, according to one report.However, Norwegian researcher Nicholas Warner cautioned that findings about bioaccumulation have been largely based on laboratory studies, while field studies of bioaccumulation are still contradicting each other. "Even if the concentrations of siloxanes we havefound in fish are high compared to concentrations of classical contaminants like PCBs, several other studies in the Oslo fjord in Norway, Lake Pepin in the US and Lake Erie in Canada have shown concentrations of siloxanes to decrease as you proceed higher up in the food chain. This raises several questions about which factors influence the bioaccumulation potential of siloxanes."
In the European Union, D4 and D5 have been deemed hazardous as per the REACH regulation. Canada regulates D4 under a pollution prevention plan.A scientific review in Canada in 2011 concluded that "Siloxane D5 does not pose a danger to the environment."