When some substances come into contact with skin, they may cause a rash called contact dermatitis. Some of these reactions are the result from allergies and irritants. Often, it is difficult to tell the difference between these two types of reactions. The hallmark of contact dermatitis is that it occurs mostly where the offending agent such as a plant or chemical comes in contact with the skin. There is two type of contact dermatitis: irritant and allergic contact dermatitis.
Irritant contact dermatitis is often more painful than itchy, and is the result of an offending agent that actually damages the skin with which it comes into contact. The longer the skin is in contact—or the more concentrated the agent—the more severe the reaction. Water with added soaps and detergents is the most common trigger. Thus, it is not surprising that these reactions appear most often on the hands. Individuals with other skin diseases, especially eczema are most susceptible.
Allergic contact dermatitis is best exemplified by the itchy, red, blistered reaction. Unlike irritant contact dermatitis, which occurs within minutes of coming into contact with an offending agent, allergic contact dermatitis reactions can occur 24-48 hours after contact. Once a reaction starts, it takes 14-28 days to resolve, even with treatment.
The most common experience is touching a plant in the “rhus’ family (Poison ivy, poison oak or poison sumac) You can have similar reaction from touching other items with which the plant has come into contact, including cloths, yard tools or the family dog. However, once your skin has been washed, you cannot get another reaction from touching the rash or blisters.
Other agents that frequently cause allergic contact dermatitis include nickel, perfumes and fragrances, dyes, rubber (latex) products and cosmetics including hair dye. Some ingredients in medication applied to the skin also cause allergic reaction; most commonly Neomycin. Rarely Benadryl and cortisone cream can cause it. Patch tests are sometime done to help identify the cause. To prevent the reaction from recurring, make sure to avoid contact with the offending substance.