Whiteheads are a combination of oils, sebum and cellular fragments that produce firm to hard plugs within hair follicles.  They are closed from the skin's surface by cellular debris at the follicle opening.  Because they are closed and have not contact with oxygen, they do not oxidize or turn brown, as blackheads do.  They form a light or yellow-white lump and are called milia (or milium, singular).    When bacteria is added to these plugs, the condition can lead to acne, especially cystic acne.

Whiteheads are promoted by excessive cellular exfoliation, which quickly clog or block the follicles.  Some skin specialists believe individuals with frequent and multiple blackheads and whiteheads produce sebum that is drier than normal and conducive to forming firm plugs.  Sebum contributes to skin oils and people with dry sebum may have dry skin and this may be especially true of people with chronic whiteheads.

The sebaceous glands normally produce sebum with linoleic acid, an essential fatty acid.  This type of sebum has a liquid viscosity.  It is calming to the skin and does not promote follicular irritation that leads to plugs.  When linoleic acid is not available, the sebaceous glands produce sebum with oleic acid.  This oleic form of sebum is irritating to the skin and promotes plugs.  Oleic sebum is much firmer than linoleic sebum and is prone to producing hard plugs rather than flowing out of the follicle.  This causes blackheads and whiteheads and both of these conditions may lead to acne infections.