by John Knutson, M.D.
Contact dermatitis is a term used to describe rashes that develop after direct skin exposure to certain substances. The vast majority of these reactions are irritant reactions caused only by the irritating nature of the substance. About 20% of contact dermatitis reactions are, however, based on a true allergic reaction to the substance. These allergic reactions can be quite severe and cause great discomfort. It can sometimes be quite difficult to identify the inciting substance. A thorough history with physical examination and screening allergy patch skin testing is often required to properly identify the substance so that careful avoidance can be undertaken.
After the initial contact with the allergenic substance, the rash that develops is typically delayed by several days. With subsequent exposure, the rash develops much more rapidly, usually within 12 to 24 hours. The skin becomes red, itchy, swollen, and often blisters develop. Scratching the skin or breaking open the blisters will not spread the rash. Once the affected skin is washed, the rash is not contagious.
Across the U.S., plants from the Rhus group cause more cases of allergic contact dermatitis than all other substances combined. These plants include poison ivy, poison oak, and poison sumac. The leaflets of the poison ivy and poison oak plants often grow in groups of three, giving rise to the warning, “Leaves of three, let them be!”
A number of other plants in Arizona can cause allergic contact dermatitis, including oleanders which are grown as hedges in desert cities. Several weeds, including ragweed, sage, and mugwort, have been implicated. Nickel, found in jewelry, zippers, snaps, belt buckles, and wristwatches, is a very common cause of contact dermatitis. Chrome and mercury are also metals that can cause an allergic reaction. Dyes, perfumes, eye shadow, nail polish, sunscreens, as well as antibiotic creams and other medications, can also cause allergic contact dermatitis.
Avoidance of substances known to cause allergic contact dermatitis is very important. Protective clothing such as long sleeves and gloves can be worn to reduce exposure. Barrier creams (several are available) can also be effective in preventing contact. If one does have contact with such a substance, it is important to wash the skin with soap and water as soon as possible. It is also important to wash clothing and other articles which have been in contact with the substance since many resins will bind to these objects. Wet cool compresses can soothe the skin and antihistamines can help somewhat with the itchiness. The most effective treatment is corticosteroids. For mild reactions, non-prescription hydrocortisone cream can be useful. For moderate and severe reactions, prescription strength creams and oral cortisone pills are often necessary. Unfortunately, there is not an allergy shot that has been found to be effective in preventing allergic contact dermatitis.
Posted in: Dermatitis