The most common skin allergies include contact allergic dermatitis and urticarial reactions (hives). Contact allergic dermatitis can have a variety of presentations, mostly commonly appearing as red, scaly itchy marks that can blister and weep in severe cases. The distribution of this rash on the body can often give clues regarding its underlying cause. Nickle in jewellery and belt buckles is a common cause. There are many other allergens that cause contact dermatitis and certain occupations such as hairdressers and nurses are at increased risk of developing these reactions.
Another common contact allergen is the preservative Methylchloroisothiozolinone (MCI) and Methylisothiozolinone (MI), also sometimes referred to as Kathon. This allergen is a preservative found in many bathroom products including shampoos, conditioners, washes, wet wipes, cosmetics, sunscreens and moisturising creams. Some paints also contain this preservative. Patch testing is the formal method of allergy testing for contact allergic dermatitis. The repeat open application test (ROAT) is a self-directed way of testing a product/allergen to see whether you react. This involves applying a small amount of a suspect product on the inner forearm twice daily for a week to see whether an itchy rash develops.
Urticarial reactions, or hives, can occur from a variety of different insults to the body. This rash can appear like mosquito bites or at times like ringworm without the dry scaly changes. An individual lesion usually lasts less than 24 hours and does not leave any stains behind. Certain foods and medications can be identified as a cause in some cases, but a definite cause may not be found in others. Urticaria usually resolves within six weeks from the time the rash first occured. If urticaria persists for more than six weeks, certain investigations will have to be performed. In cases where no cause can be found, the term chronic idiopathic urticaria is used. Long-term antihistamines and other medications may be used to keep this condition under control.