Many rashes are caused by contact with a substance that causes an allergic reaction (contact dermatitis). The rash usually starts within 48 hours after a person is exposed to the irritating substance, but it may not show up for several days. A minor case of contact dermatitis may cause mild redness of the skin or a rash of small red bumps. A more severe reaction may cause swelling, redness, and larger blisters.
Common causes of contact dermatitis include:
Contact dermatitis doesn't usually occur the first time you are exposed to the irritating substance (allergen). After you have had a reaction to the substance, a rash can occur in response to even very small amounts of the substance.