Certain ingredients used in cosmetic products often produce allergic contact dermatitis of the skin. Valencia University Hospital has conducted a review of 740 patients diagnosed over a period of seven years. The cause was cosmetics in 27.3% of these cases.
Contact dermatitis causes itchiness and rashes and even burn-like reactions. The study concluded that the cosmetics that most often caused problems were hair dyes (18.5%), gels (15.7%) and moisturizers (12.7%). Doctors tested the study patients for 46 known allergens. Those most important reactions were caused by methylisothiazolinones (19%), which are used as preservatives, the colourant paraphenylenediamine (15.2%) and perfumes (7.8%).