Healthy skin provides a barrier between the inside of the body and the outside environment. A rash means some change has affected the skin.

Rashes are generally caused by skin irritation, which can have many causes. A rash is generally a minor problem that may go away with home treatment. In some cases, a rash does not go away or the skin may become so irritated that medical care is needed.

In adults and older children, rashes are often caused by contact with a substance that irritates the skin (contact dermatitis). The rash usually starts within 48 hours after contact with the irritating substance. 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. The location of the rash may give you a clue about the cause.

Contact dermatitis does not always occur the first time you are in contact to the irritating substance (allergen). Once you have had a reaction to the substance, a rash can occur in response to even very small amounts of the substance. Contact dermatitis is not serious, but it is often very itchy. 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. 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 multiple surgeries. Latex allergies can cause a severe reaction.

Rashes may occur with viral infections, such as herpes zoster; fungal infections, such as a yeast infection (Candida albicans); bacterial infections, such as impetigo; and sexually transmitted diseases (STDs). Rashes may also occur as a symptom of a more serious disease, such as liver disease, kidney disease, or some types of cancer.

Rashes may also appear after exposure to an insect or a parasite, such as the scabies mite. You may develop a rash when you travel to a rural area or go hiking or camping in the woods.

A rash may be a sign of a chronic skin problem, such as acne, eczema, psoriasis, or seborrheic dermatitis. Other causes of rash include dry, cold weather; extremely hot weather (heat rash); and emotional stress. Emotions such as frustration or embarrassment may lead to an itchy rash.

The need for medical treatment often depends on what other symptoms are present. A rash that occurs with other symptoms, such as shortness of breath or fever, may mean another problem, such as a serious allergic reaction or infection.

Review the Emergencies and Check Your Symptoms sections to determine if and when you need to see a doctor.