Contact dermatitis is inflammation of the skin caused by contact with a substance that causes an allergic reaction. A minor case of contact dermatitis may cause mild redness of the skin or a rash of small red bumps, while a more severe reaction may cause swelling, redness, and larger blisters.

A rash usually starts about 48 hours after exposure to the irritating substance. Common causes of contact dermatitis include:

• Poisonous plants, such as poison ivy, oak, or sumac.
• Soaps, detergents, shampoos, perfumes, cosmetics, or lotions.
• Jewelry or fabrics.
• New tools, toys, appliances, or other objects.
• Latex gloves.

The location of the rash may provide a clue about the cause.

Contact dermatitis does not usually occur with the first exposure to the irritating substance (allergen). Once a reaction to the substance occurs, a rash can occur in response to even very small amounts of the substance.