Ethanol (70%) and isopropanol (70–90%) are effective skin antiseptics because they denature microbial proteins. Formaldehyde, which also denatures proteins, is too irritating for topical use but is a disinfectant for instruments. Acetic acid (1%) is used in surgical dressings and has activity against gram-negative bacteria, including Pseudomonas, when used as a urinary irrigant and in the external ear. Salicylic acid and undecylenic acid are useful in the treatment of dermatophyte infections.