MedlinePlus defines a skin rash as a condition that "involve[s] changes in the color or texture of the skin." There are many different types of skin rashes, which cause the skin to become red, itchy, scaly, swollen or bumpy. Severe rashes may cause blisters and oozing. Rashes can be categorized by causes, which commonly include allergies, irritants and infection.
Allergy
The medical name for a rash caused by an allergic reaction is allergic contact dermatitis. The American College of Dermatitis notes that these rashes occur when the skin comes into contact with a substance to which a person is allergic. One common cause of an allergic rash is poison ivy or poison oak, which causes the skin to become very red, itchy and swollen, with occasional oozing blisters.
Other common causes of allergic dermatitis are rubber, dyes, drugs, preservatives, fragrances, and nickel. Repeated exposure to the allergic substance can cause the skin to become cracked and leather-like. The specific allergic substance is often diagnosed by the location of the rash, its appearance, and common substances with which the person comes into contact. Doctors can also apply a sample of a variety of allergic substances to the skin in the doctor's office to diagnose the allergic substance.
Infection
Rashes may also be a symptom of bacterial, viral or fungal infection. MedlinePlus reports that common types of a rash caused by infection include: impetigo, an infection common in children which is caused by a bacteria that normally lives on the upper layers of the skin; shingles, a viral infection caused by vericella zoster, the same virus that causes chickenpox; fifth disease, a viral infection caused by human parvovirus B19; and seborrheic dermatitis, which causes dandruff in adults and "cradle cap" in babies.
Irritant
Rashes induced by irritants develop because of the toxic effects that some substances have on the skin. The damage caused by the toxic substance triggers an inflammatory response in the skin that leads to a rash. The Mass General Hospital for Children notes that common irritants that cause rashes include acids and alkalis, which have a very low and very high pH, respectively. In contrast, pure water has a pH of neutral, meaning it is neither high nor low. Other common irritants are solvents, which can actually dissolve the natural oils that serve to protect the skin. The detergents and chemicals common in cleaning solutions are often acidic, alkaline, and/or act as solvents. Certain ingredients in lotions and perfumes are also possible irritants.
