rationalskepticism.org

The exact underlying cause of aquagenic urticaria is poorly understood. As of 2016, the main scientific ideas about the cause are that the person is reacting to tiny amounts of an unknown substance dissolved in the water, or that the water interacts with or combines with an unknown substance present in or on the skin, and that the person's immune system is reacting to this compound. Despite the common name water allergy, scientists do not believe that this condition technically represents a true allergy to pure water.