Antiseptics prevent the growth of disease-causing micro-organisms without damaging living tissues. Among chemicals used are boric acid, carbolic acid, hydrogen peroxide and products based on coal tar, such as cresol. Chlorhexidines, iodine, formaldehyde, flavines, alcohol and hexachlorophane are also used. Antiseptics are applied to prevent infection – for example, in preparing the skin before operation. They are also used externally to treat infected wounds.