Nyda Express Against Lice & Nits Spray is designed to kill head lice and their eggs using a physical mode of action rather than chemical insecticide (knock down).; NYDA® express contains two dimethicones (silicone oils), which seep into the breathing holes of lice and eggs, blocks their oxygen supply which rapidly suffocates and kills them.; This physical mode of action means that even head lice and eggs which have built up a resistance to chemical treatments (knockdown resistance) are still eliminated.