For years a distinctive and intensely pruritic form of hives has been noted in certain people as a result of heat, emotion, or exertion. The numerous small lesions have been labeled "cholinergic urticaria." Critical wetting of the sensitive areas of the skin surface, which triggers cholinergic urticaria, can be achieved by a few minutes immersion in a water bath, contact with distilled water, or sweating itself. Water reacts with a component of the sebum or sebaceous gland to produce a histamine liberator which is absorbed with subsequent discharge of histamine from the perifollicular mast cells. This produces pruritus, axon-reflex erythema, and follicular urticaria. Inert oil coating of the skin and systemic antihistaminics have been found effective in preventing attacks.
Shelley WB, Rawnsley HM. Aquagenic UrticariaContact Sensitivity Reaction to Water. JAMA. 1964;189(12):895-898. doi:10.1001/jama.1964.03070120017003