According to the amount of sebum secretion, the nature of skin is divided. Sebum secretion is influenced by androgen and adrenocortical hormone, which has the function of lubricating skin and hair. Excessive sebum secretion is called oily skin, and the PH value of oily skin is between 4.5 and 5.0. Too little is called dry skin, and the PH value is between 5.6 and 6.6; Sebum secretion is moderate, it is neutral skin, and the PH value is between 5.0 and 5.6. After menopause in women and after 70 years old in men, the amount of sebum secretion will be significantly reduced.
50% of sebum is triglycerides and diglycerides, followed by cholesterol, wax esters and squalene, and carries some resident microorganisms, such as coryneform bacteria, yeast and mites. Sebum is decomposed into free fatty acids by enzymes produced by parasitic Corynebacterium, which can inhibit some pathogenic bacteria such as Staphylococcus, Streptococcus, fungi, etc. However, excessive sebum can stimulate the hair follicle wall and cause inflammation, showing as folliculitis papules. Poor sebum excretion deposits sebum plugs in hair follicles, which is called acne.