The skin supports its own ecosystems of microorganisms, including yeasts and bacteria, which cannot be removed by any amount of cleaning. Estimates place the number of individual bacteria on the surface of one square inch (6.5 square cm) of human skin at 50 million though this figure varies greatly over the average 20 feet² (1.9 m²) of human skin. Oily surfaces, such as the face, may contain over 500 million bacteria per square inch (6.5 cm²). Despite these vast quantities, all of the bacteria found on the skin’s surface would fit into a volume the size of a pea.^[3] In general, the microorganisms keep one another in check and are part of a healthy skin. When the balance is disturbed, there may be an overgrowth and infection, such as when antibiotics kill microbes, resulting in an overgrowth of yeast. The skin is continuous with the inner epithelial lining of the body at the orifices, each of which supports its own complement of microbes.

Oily skin is caused by over-active glands, that produce a substance called sebum, a naturally healthy skin lubricant.^[1] When the skin produces excessive sebum, it becomes heavy and thick in texture. Oily skin is typified by shininess, blemishes and pimples.^[1] The oily-skin type is not necessarily bad, since such skin is less prone to wrinkling, or other signs of aging,^[1] because the oil helps to keep needed moisture locked into the epidermis (outermost layer of skin).

The negative aspect of the oily-skin type is that oily complexions are especially susceptible to clogged pores, blackheads, and buildup of dead skin cells on the surface of the skin.^[1] Oily skin can be sallow and rough in texture and tends to have large, clearly visible pores everywhere, except around the eyes and neck.^[1]

The goal of treating oily skin is to remove excess surface sebum without complete removal of skin lipids.^[1] Severe degreasing treatment can foster an actual worsening of sebum secretion, which defeats the aim of the cleansing.^[1] A method of cleansing oily skin is to wash with a solution of a mild synthetic detergent^[1] (see: surfactant) containing no oils, waxes or other lipid agents that could aggravate the oily condition of the skin, sometimes combined with a toning lotion. Such a product removes the oily residue and debris from the skin surface. Some cleansing products have lower concentrations of hydroxy acids, which remove dead cells from the upper levels of the stratum corneum.^[1] Those products should be used on a regular basis to work adequately.^[1] A light moisturizer may be included in a product to counteract any drying effects of the cleanser.^[1]