A Rash or Skin Allergies?

An allergist breaks down the difference in reactions, and when it's something more serious that warrants a doctor visit

Medically Reviewed

“Doc, I recently developed an itchy rash on my arms and legs. I changed body soap days before it started. Is my eczema flaring up, or am I allergic to the soap?”

Allergists are routinely confronted with this question, but first, let’s go over some basic concepts and terminology. Skin rashes occur as a result of the body’s surface layer attempting to protect itself from something harmful.

Collectively, conditions associated with skin inflammation are referred to as “dermatitis.” Eczema is a form of dermatitis that can be allergic or non-allergic. The allergic form is called “atopic dermatitis” and can be triggered by food allergies. Non-allergic forms of eczema frequently occur in the winter and are associated with excessive drying of the skin. Chemicals and other irritants can aggravate both types of eczema.

An allergic skin rash results from a previous contact with an allergen and involves the immune system. The initial contact is not painful or itchy and may have occurred days or months prior to the rash. “Sensitization” is the term used to describe the initial immune response, which is silent and sets you up for the allergic reaction that occurs after contacting the trigger again. Allergic rashes are often raised, resulting in bumps or welts (hives) and almost always itch.

Eczema is a common disorder that often begins in childhood and is associated with patches of red, rough-textured skin that is very itchy. The skin lesions generally occur in the creases of major joints, such as the elbows and knees. Some people experience outbreaks all over the body, including the face, neck, back, and chest.

Atopic dermatitis, the allergic form of eczema, is a chronic condition that is hereditary and commonly accompanied by other allergic disorders, such as allergic rhinitis (hay fever) or asthma. If a rash suddenly appears in an older child or adult, it may not necessarily be atopic dermatitis, which is often a more prolonged and recurring skin rash.

A first-time, short-lived skin rash is more likely the result of contact with an irritant or chemical. New soaps, shower gels, or moisturizers may irritate the skin of some individuals. As stated above, people who have a history of eczema tend to be more sensitive to some chemicals in hygiene products, laundry detergents, and clothing (especially new, unwashed clothes).

Contact dermatitis is a delayed form of skin reaction that may occur after contact with certain plants (for example poison ivy), metals (such as nickel). or chemicals (such as formaldehyde). Many other substances may cause contact dermatitis in susceptible people. The rash is associated with mild-to-moderate itching but often people complain of a burning sensation. The rash may appear several hours after contact with skin, sometimes the next day, and persist for several days.

If you have an idea of what caused the rash, avoid further contact with it. If it has resulted in contact dermatitis, it may take several days or even weeks to completely clear and warrant prescription medication. Contact dermatitis may persist and keep reappearing in new areas of the body because of the continued presence of the trigger on clothing, furniture, toys, utensils, or equipment. It’s important to carefully clean any contaminated items that may bare small amounts of the residue from the trigger. Meticulous cleaning is often required when the culprit is poison ivy.

In the above case, the patient had recently changed body soap. The problem went away after going back to his previous brand of soap. He was asked to write down all the ingredients of the new soap associated with the rash for future reference. If the rash recurs, the list will be used to cross-check for potential triggers that may be associated with other hygiene products. He applied hydrocortisone cream (over-the-counter) to the affected areas for seven days after showering, and the rash subsequently faded.

Final words

Skin rashes may occur as a result of many different causes, some of which were not discussed. Eczema is a chronic relapsing dermatitis that may be aggravated by allergic and/or irritant triggers. If the rash and symptoms are mild, avoidance of the trigger, accompanied by washing and moisturizing the skin may be enough.

If the skin rash is severe, prolonged, or there is burning, consult your family doctor. A referral to an allergist or dermatologist may be considered. Identifying and avoiding the trigger is the most important take-home lesson.