The skin's function is to block noxious chemicals in the environment from entry into the body. It does a very good job most of the time, however, occasionally, a chemical causes an allergy: an itching, red rash. The chemical may be something that came in contact with the skin, like poison ivy, or the substance may have been ingested in the form of a drug or a food additive, or possibly it was inhaled. When the skin reacts in this way, gentle treatment is in order, since the skin is injured. Cleansing with a soap substitute such as Cetaphil is recommended to avoid further drying with a soap. Cortisone creams and moisturizers may be prescribed by your dermatologist to stop the itching and speed healing.