4.2.3 The hazards of excessive cleaning and the cl...
1.The hazards of excessive cleaning
The purpose of cleaning is to remove excess skin metabolites，contaminated dust and cosmetics.However，too frequent cleaning and excessive use of cleaning products with strong decontamination and exfoliation capabilities will cause damage to the skin barrier，affect normal pH and destroy the micro-ecological balance of the skin.In severe cases，it can induce or aggravate some skin diseases such as dry skin，itching，skin allergies，and eczema.
2.The classification and selection of skin moisturizers
The use of moisturizers after cleaning is an important measure to reduce the adverse effects of the cleansers on the skin barrier and maintain the steady state of the skin barrier.Moisturizers，also known as humectant，can maintain and replenish moisture in the stratum corneum of the skin to prevent skin dryness，or rescue skin that has dried，loss of elasticity and chapped，to make it smooth，soft and elastic again.Moisturizers are divided into natural moisturizers and synthetic moisturizers according to their sources.According to the mechanism of moisturizing，they can be divided into sealing moisturizers and wetting or hygroscopic moisturizers.(https://www.daowen.com)
（1）Sealing moisturizers：They are a type of insoluble lipid substance.By supplementing the stratum corneum with lipid substance，it forms an additional lubricating film layer on the surface of the skin，wraps the stratum corneum，and blocks the loss of moisture from the skin.And the water can continuously diffuse from the inner layer of the skin into the stratum corneum to help maintain a certain water content in it.There are many characteristics of this type of moisturizers，including good moisture absorption and moisturizing capacity which is rarely affected by environmental conditions（e.g.，temperature and humidity），low volatility and low freezing point，suitable viscosity and good affinity to skin，colorless，odorless，tasteless，and good compatibility with other ingredients.Sealing moisturizers mainly have the following 6 categories：①Mineral oils and waxes，such as petrolatum and mineral oil.②Plant and animal fats，such as lanolin.③Silicone oils，such as cyclomethicone.④Fatty acids and esters categories，such as stearic acid and linolenic acid.⑤Phospholipids，cholesterol，such as lecithin and cholesterol sulfate.⑥Plant and animal waxes，such as carnauba and beeswax.
（2）Wetting or hygroscopic moisturizers：The substances of this kind of moisturizers is similar to natural moisturizing factors，and human polysaccharides and proteins in the epidermis，including glycerin，hyaluronic acid，etc.It is a water supplement and enters the inside of skin through pores，or carries moisture into skin when skin moisture is lost，or wraps the unlost moisture to preserve it in the skin.The characteristics of this type of moisturizers are that they can increase the skin water content and reduce the transepidermal water loss to make stratum corneum flexible and softening，as well as attract moisture from the skin deep and dermis tissue into stratum corneum to be easily acquired by cell.
The selection of moisturizers should be based on skin conditions.People with oily skin are recommended to choose more water-soluble moisturizers，while people with dry skin can form a better protective layer on the skin surface with more oil composition moisturizers.It is necessary to apply moisturizers in areas where sebum glands are small and the stratum corneum are thinner，such as eyes，lips and neck.