Allergy is a common cause of skin rashes. In order to diagnose the cause of such rashes, it is important to consider several factors. The character of the skin rash often gives a clue to its cause. Hives (urticaria) and swelling (angioedema) are often due to immediate reactions to food, medications or contact allergens such as dust and pet dander. Such rashes occur several minutes to two hours after exposure to the allergen, and often disappear within one day. The timing of the outbreak and any suspicious exposures is therefore important. On the other hand, eczematous rashes are often due to chronic or repeated exposure to allergens such as rubber, metals, chemical preservatives or fragrances. The timing of the outbreaks can be very variable, and such skin rashes can appear even years after a patient started to have exposure to the allergens.
Rash on the face is often due to ingredients in skin care and cosmetic products, such as fragrances, preservatives and colour pigments. Eyelid dermatitis is often caused by eye shadow, which often contains cobalt. However, moisturizers, toners, serum and eye creams are often culprits, as are allergens transferred by the fingers, such as nail varnish. Another commonly overlooked allergen is hair dye, which often causes rash along the hairline, the ears, the face and the neck, but not necessarily on the scalp.
Rash on the hands is also common, and is often triggered by rubber, metals, detergents and ingredients in hand soaps and sanitizers. There are also allergens that are specific to certain professions, such as hair dyes and perming chemicals in hair-dressers, chromium in construction workers, colophony and resins in electronic technicians, acrylics in dental technicians etc. Many babies develop eczema. This might suggest a predisposition to eczema due to genetic defects in skin barrier function. These babies are prone to developing food allergy, asthma and other allergies due to increased skin permeability. It is important to avoid using irritants and allergens on their skin, such as fragrances, strong soaps and wet wipes. Regular use of barrier ointments such as Aquaphor might also help to mitigate this problem.