Low grade, less stable, silicone using chemical''filers''may contain these contaminants. As a result, contaminants are subject to leaching. This can be harmful to the user.
To check silicone quality, give it the''pinch test''. If the pinched silicone turns white, it's more likely to contain low grade filers or at the least lower grade silicone. Higher grade silicone which adheres to the FDA standard should not change color when given the pinch test.