Two types of irritant contact dermatitis are described: the acute and the cumulative toxic contact dermatitis. The acute contact dermatitis causes many different lesions on the skin. The most frequent irritants are acids and alkaline solutions. Chemical burns by hydrofluoric acid are the most dangerous of all injuries caused by acids and need special treatment. Cumulative toxic dermatitis is often observed on the back of the hands and forearms after exposure of several weeks or months. Repeated skin contact by harmless products can also cause cumulative toxic dermatitis. Xerodermatitis is the most frequent type of cumulative toxic dermatitis. Phototoxic reactions of the skin are not caused by immunologic factors, and they are only observed at sun-exposed areas. Drugs can cause frequently phototoxic reactions. The lesions on the UV-A-exposed skin are mainly erythema and blisters.