Allergic Contact Dermatitis
Allergic contact dermatitis is a special pattern of reaction that occurs when your skin comes into contact with a foreign substance (allergen) that your skin is sensitive or allergic to.
The development of the allergic reaction is related to the “sensitising potency” of the allergen and various host factors.
Allergens vary in their potential to induce allergy, eg the chemical urushiol in poison ivy can sensitise most people; nickel sensitises 10% to 20% of women, while many other allergens sensitise a smaller minority.
The susceptibility of an individual to sensitisation is related to genetic factors.
The hallmark of allergic contact dermatitis is that the rash occurs almost exclusively at the site where the offending agent or allergen comes into contact with the skin. Common locations include the face, neck, hands and feet. The rash usually has features of redness, swelling, itching and exudation of fluid.
The common allergens that cause allergic contact dermatitis are nickel, rubber additives, fragrance, chromium, paraphenylenediamine (PPD) and colophony.
Nickel is the most common allergen worldwide. It is present in alloy metal jewellery, keys, coins, belt buckles, clothing studs and metal spectacle frames.
Ear piercing significantly increases the risk of developing nickel sensitivity.
Common sites of the rash seen in nickel sensitivity are the earlobes (related to jewellery), around the navel (related to clothing studs and belt buckle), the back (related to brassiere fasteners) and around the wrists (related to watch straps and buckles).
Allergy to rubber products is usually due to the chemical additives used in the manufacture of the rubber rather than the latex itself. Dermatitis from rubber additives is common in the rubber and tyre manufacturing industry. Domestic contact is normally from rubber gloves. Rubber chemicals in shoes can also cause a dermatitis of the feet.
Fragrance materials are a common cause of allergic contact dermatitis. Fragrances have widespread use. They are found not only in perfumes and cosmetics but also in hair and skin care products, laundry products, cleaning products and toothpaste. It is more common for the source of fragrance exposure to be a skin or hair care product rather than a cologne or perfume itself.
Chromium is found in cement, plaster, anti-rust paint and chromium plating. It is a common cause of allergic contact dermatitis in masons and construction workers. The rash usually occurs on the back of the hands and the forearms.
Paraphenylenediamine (PPD) is commonly found in hair dyes and causes a dermatitis on the scalp, ears, face and neck in many individuals who use these dyes. PPD is also found in black rubber products like rubber boots, diving suits and rubber hoses.
Colophony or rosin is obtained from the sap of pine trees. It is mainly used in the manufacture of adhesives, glues, paper and printing ink. Dermatitis due to adhesive tape is fairly common and is usually due to colophony.
Apart from the allergens discussed above, there are numerous substances that can cause an allergic contact dermatitis. Sometimes the cause and effect relationship is not obvious.
For instance, nail varnish is a fairly common cosmetic sensitiser. The culprit is the formaldehyde resin in the lacquer base of the nail varnish. Since the nail is made of inert material, no reaction occurs on the nail or nail bed.
However, sites that are often touched by the nails, for example the eyelids, neck and chin, can develop a rash due to sensitivity to the resin. Thus a rash suspected to be allergic contact dermatitis should be carefully assessed by a doctor.
Sometimes special skin allergy tests called “patch tests” may be necessary to try to identify the allergen. If an allergen can be identified, avoidance of contact with the substance is of great importance in our endeavour to clear the rash and prevent its reoccurrence.
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