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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:

• Poisonous plants, such as poison ivy, oak, or sumac, or touching an object that has had contact with one of these plants.
• Soaps, detergents, shampoos, perfumes, cosmetics, or lotions.
• Jewelry or fabrics.
• New tools, toys, appliances, or other objects.
• Latex gloves. Allergy to natural rubber latex affects people who are exposed to rubber products on a regular basis, especially health care workers, rubber industry workers, and people who have had many surgeries. Latex allergies can cause a severe reaction.

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.