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.[9]
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.[10][11] The cyclomethicone D4 is bioaccumulative[12] D4 can cause liver damage, has an estrogenic effect and is known to impair fertility in mammals.[12]
In the European Union, D4 and D5 have been deemed hazardous as per the REACH directive. D4 is regulated as a pollutant in Canada.[10]
D5 and D4 have attracted attention because they are pervasive. A scientific review in Canada in 2011 concluded that "Siloxane D5 does not pose a danger to the environment."[13]