4.2.1 Skin type and significance of skin cleaning
1.Skin type
The skin varies greatly between races and individuals.And there are many methods to classify skin types，most of which are based on skin hydration，sebum secretion，skin pH value and skin reactivity to external stimuli.Under physiological conditions，the skin can be divided into the following four types（Figure 4-4）.
（1）Neutral skin：Also known as ordinary skin，and it is the ideal skin type.The water content of the stratum corneum is about 20％and the pH value ranges from 4.5 to 6.5.The skin is soft，delicate and elastic，with moderate sebum secretion.This type of skin has strong adaptability to external stimuli.
（2）Dry skin：The water content of stratum corneum is less than 10％and the pH value is greater than 6.5.The skin of this type is dry and lack of oil because of less sebum secretion.The skin lines are fine and pores are not obvious.People with dry skin tend to feel tight after washing face，and are sensitive to external stimuli（e.g.，climate and temperature changes）.The skin is also prone to chapping，scaling and wrinkles.The development of dry skin is related to genetic factors，as well as environmental factors and inappropriate habits，including overexposure of wind and sunlight and excessive use of alkaline detergents.
（3）Oily skin：Also known as fatty skin，and it is more common in young and middleaged people and obese people.The water content of stratum corneum is about 20％and the pH value is less than 4.5.A distinctive feature of this skin type is the strong sebum secretion resulting in greasy and shiny appearance，easy to adhere to dust and large pores.The color of oily skin is often darker，while skin elasticity is great and not easy to wrinkle.And it's generally not sensitive to external stimuli.Oily skin is mostly related to hyperleydigism，high-fat diets and fragrant condiments，and is prone to skin diseases（e.g.，acne and seborrheic dermatitis）.(https://www.daowen.com)
（4）Combination skin：It is a skin type in which neutral skin，dry skin or oily skin coexist.This skin type is mostly characterized by oily skin in the central part of the face（i.e.，forehead，nose，nasolabial fold，and chin），while neutral or dry skin in the bilateral cheeks and temples，etc.The skin and hair characteristics of the trunk are generally consistent with that of the head and face.The hair of oily skin is always oily，while the hair of dry skin is always dry.
2.Significance of skin cleaning
The surface of the skin is often adhered by physiological dirt and exogenous dirt.The former case includes metabolites produced，secreted or excreted by the human body，including aging and shed cells，sebum，sweat，mucous membranes and excretions of the lumen.The latter case includes microorganisms，environmental pollutants，residues of various cosmetics and topical drugs.In addition，there are pathological dirt on the skin surface of patients with dermatosis，such as scales，pus，scab，and increased sweat.These skin dirt block pores and hinder the normal physiological functions of the skin and mucous membranes.Therefore，it is of great significance to clean the skin regularly，which is the basic method to maintain skin health.Moreover，skin cleaning can also stimulate blood circulation to improve skin health.