Contact dermatitis is dermatitis caused by contact with an exogenous (outside) chemical (not food) interacting with the skin. If these chemicals cause an allergy they are referred to as allergens. There are however two pathological processes by which chemicals can damage the skin – irritant contact dermatitis and allergic contact dermatitis. In practice they often occur together resulting in the dermatitis.
Irritant contact dermatitis typically affects the hands and is an important cause of occupational skin disease. The most common irritants are prolonged or frequent contact with water (‘wet work’) soap, detergents and organic solvents. There is considerable individual variation in the threshold for skin irritation.
Allergic contact dermatitis (e.g. allergy to metals or fragrance chemicals or other cosmetic ingredients) involves a specific memory driven immune response, whereas in irritant contact dermatitis a non-specific inflammatory response is caused.
Some chemicals are recognized as having a greater potential to cause sensitisation (the capacity to cause an allergic response) and then on re-exposure to cause an allergic response in the skin resulting in dermatitis. These chemicals include many that we expose ourselves to on a daily basis e.g. cosmetic ingredients, metals plants etc. Therefore, allergic contact dermatitis can be a cause of someone’s eczema in its own right or can complicate another type of dermatitis e.g. atopic dermatitis, seborrhoeic dermatitis or discoid eczema.
Rarely it is possible to become allergic to the ingredients in treatment creams themselves, so that the treatment itself can perpetuate the dermatitis. This may be suspected if the eczema does not respond to treatment as expected.
If allergic contact dermatitis is suspected, then your dermatologist is likely to refer you for patch tests. This is the only test available for identifying if an allergen is causing or contributing to your eczema.