Dermatitis is the inflammation of the skin, both external and internal. It is characterized by redness, swelling, heat and pain or itching - any part of the body may be affected. 

The genital areas and the exposed areas such as the eyelids, forearms, face and neck are more prone to it.

The cells of the epidermis (the surface layer of the skin) are normally protected from damage by the tightly packed squamae of keratin of the horny layer. The elasticity of keratin varies with its water content. This water content can be reduced by evaporation or by removal of the lipid with which it retains moisture. Substances which produce inflammation of the epidermis or dermatitis by mechanical or chemical disruption of the horny layer are called irritants.

Degreasing agents like soaps, if used too frequently over a short time, will cause dryness, redness, fissuring and irritation of the skin in almost everyone.

The appearance of dermatitis varies according to its severity and the stage of its evolution. The first symptom is erythema or redness. This is usually followed by swelling of the skin due to edema (excessive fluid retention). Vesicle may appear thereafter. In a case of their rupture, their bases exude serum. This condition is known as weeping dermatitis. Later, the serum dries up to form crusts. In some people, the disease seems to come and go without any great change in the skin itself.

Chemical substances that reach the skin from outside or from inside through the bloodstream. About 100 different plants are known to be capable of causing dermatitis in susceptible persons. The onset is usually acute and begins an hour or two after contact. Dermatitis may be caused by external contact with mineral irritants. This includes industrial dermatitis which arise on the hands or forearms which actually come in contact with the irritant.

Certain medications applied externally such as atropine, belladonna, carbolic acid, iodine, mercury, penicillin, sulphonamides, sulphurs, tars and turpentine sometimes cause dermatitis. Other substances are hair dyes, bleaches, skin tonics, nail polish, perfume, wool, silk, nylon, floor-wax and various detergents. Deficiency of vitamin A and pantothenic acid, and nervous and emotional stress can also cause it.