How Dimethicone Kills Lice
Our lice neutralizer is a proprietary blend of polydimethylsiloxane (dimethicone) oils. Chemically and physically it is similar to olive oil, and is considered a “generally regarded as safe” ingredient by the FDA. Dimethicone is a common ingredient in some hair products.
It kills the lice by blocking the spiracles, which are pores in the side of a louse through which it breathes. Lice immersed in water will not die, because the high surface tension of water prevents it from penetrating the spiracles. Adding an emulsifier or surfactant, such as a detergent, doesn’t work for two reasons. One is that water is permeable to oxygen, the other is that the water will run right back out again.
An ideal liquid to suffocate lice would have low surface tension, to enable it to penetrate the spiracles, high viscosity, so it won’t run out, and low permeability to oxygen, so the lice can’t just breathe right through it. Kerosene is one such liquid, and in fact was the medically recommended treatment until the late 1960s. Olive oil is another such liquid, and is what we used in the early days at Rapunzel’s.
The problem with these oils is that they take a long time to kill the lice, around eight hours. This is because the lice can go into a sort of suspended animation where their oxygen requirements are so low that they don’t need to breathe.
Silicone oils were first observed to work better than vegetable oils some time in the late 1970s, with the first patent being issued in 1979. For many years the mode of action was unknown, and is still not well understood. If lice can hold their breath for eight hours, how can silicone oils kill them in ten minutes?
Until recently, the best guess was that the silicone oils somehow penetrate deeper into the respiratory tract than the organic oils do, making them more effective at cutting off the oxygen supply. But recent research has shown that another mechanism may be responsible. After a blood meal, a louse must excrete water. They do not have a conventional excretory system. Instead they excrete water through the same spiracles they use for breathing. The silicone oils block this excretion, and cause death by osmotic pressure on the gut.
Exactly why the silicone oils behave this way and the organic ones do not is still not well understood, and further research is needed in this area.
US patent 4146619, March 1979. Linear siloxane polymers have been found to exhibit pediculicidal and/or ovicidal activity.
Heukelbach J, Pilger D, Oliveira FA, Khakban A, Ariza L, Feldmeier H. “A highly efficacious pediculicide based on dimeticone: randomized observer blinded comparative trial.” BMC Infect Dis. 2008 Sep 10;8:115.
Ian Burgess, “The mode of action of dimeticone 4% lotion against head lice, Pediculus capitis.” BMC Pharmacol. 2009 Feb 20;9:3.