Table 1.

Summary of normal values of basic physiological indexes of skin

Indicators		Equipments	Methods	Normal value	Influence factors
Skin temperature		Contact or infrared instrument	Compare and interpret the international standard ISO 9886	0–2 years old: 36.4–38.0 °C; 3–10 years old: 36.1–37.8 °C; 11–65 years old: 35.9–37.6 °C;  > 65 years old: 35.8–37.5 °C	Environmental temperature; Blood heat; Blood flow; Effects of metabolism of epidermis and related appendages
Skin pH		Skin pH tester	N/A	Whole body: 4.2–6.1 (except the former), showing a Gaussian distribution trend; Newborns: higher than adults 1 day after birth, reaching adult status 4 days after birth; Sweat: 4.0–6.8 Oral mucosa: about 7, the highest in hard palate Armpit, perineum, interdigital (toe) area: about 7	Skin cleaning method; Sweat; Degree of occlusion; Presence of special microorganisms; Circadian rhythm
Skin-type evaluation	Fitzpatrick's skin type	Color difference tester	Quantify skin color based on three primary color stimulation values (L *, a *, b *)	> 55°: very shallow 55° to 41°: shallow 41° to 28°: medium 28° to 10°: tanning 10° to − 30°: brown  < − 30°: black	Sunlight exposure
	Chinese skin type	N/A	N/A	Neutral: the water content of the stratum corneum is normal (10–20%), the sebum secretion is moderate, the skin is tight, elastic, and the surface is smooth, moist and delicate Dryness: the moisture content of the dermis is less than 10%, the sebum secretion is low, the skin is dry, desquamate, delicate and lusterless, the skin color is dark, and it is prone to small wrinkles and pigmentation Oiliness: the water content of the dermis is normal or reduced, the sebum secretion is vigorous, the skin surface is oily and shiny, the pores are thick, and acne and folliculitis are prone to occur	Moisture content of skin dermis; Oil secretion
Complexion evaluation	Skin erythema	Narrow-wave skin spectrophotometer, Spectrophotometer, Mexameter®, Dermacatch (Colorix)	Measure hemoglobin and express erythema as hemoglobin index EI	Relative value, there is no relevant report on the value range. The smaller the value is, the thinner and more sensitive the skin is	Ambient and local temperature (affecting the expansion and contraction of blood vessels); Ambient light
	Skin melanin		Measurement of melanin index MI	0–999, no unit, the higher the value, the higher the melanin content in the skin
	Skin pigmentation	N/A	N/A	Chloasma, freckles, ADM, PIH	N/A
Daylight tolerance	Minimum erythema dose	Solar ultraviolet simulator	The emission light source takes the UVB (290—320 nm) as the main peak, the power is 500W, the irradiation site is the skin of the unexposed area (back, chest or forearm flexor side, etc.), the shortest exposure time is 15 s, and the observation is conducted in sufficient natural light for 24 h	Take the data of normal people as a control	Individual differences; Skin color; Irradiation site; season; Spectrum and intensity of light source
	Minimum persistent pigment darkening	Solar ultraviolet simulation system, ultraviolet phototherapy instrument, solar protection index tester	UVA and UVB are dominant (290—400 nm), and the output rate of light sources below 290 nm and above 400 nm is less than 2%	Take the data of normal people as a control
Epidermal evaluation	Degree of stratum corneum exfoliation	Glue coating plate, different sizes of D-Squame tapes, etc	Semi-passive collection method, standardized tape sticking method, mechanical forced collection method and forced desquamation method	N/A	N/A
	Epidermal friction	Three-axis quartz force plate, skin friction instrument	N/A	Any unit (au)	Material Science; Temperature; Positive force; Contact surface area; Age; Body parts; Stratum corneum water content
	Skin glossiness	Skin glossmeter, GonioLux (Orion Concept) and Translucymeter (TLS 850, Diagron Ltd)	N/A	Usually difficult to quantify	Skin color; Light reflection; Skin surface texture
	Surface texture, roughness		Optical profile measurement technology	Usually difficult to quantify
Skin barrier functions	Transepidermal water loss	Skin moisture loss tester (Tewameter TM210) and Evaporimeter	Subjects wait in a constant test environment	Fluctuate greatly	External humidity; Temperature; Atmospheric pressure
	Stratum corneum water content	GPSkin Barrier	N/A	N/A	N/A
Dermal evaluation	Microcirculation system	RCM, OCT, MSOT	Detected by immunohistochemistry, specific lymphoid markers include hyaluronic acid receptor-1, Prox-1, podoplanin, VEGFR-3, etc	The first form: subcutaneous vascular network consists of papillary ring and horizontal vascular plexus in the superficial dermis; The second form: the vascular network around the hair follicle that nourishes the sebaceous gland hair follicle; The third form: periglandular vascular network	N/A
	Lymphatic vascular microcirculation				N/A
	Skin elasticity	Dermal Torque Meter; Skin tester—Cutter SEM575	Use the torsion parallel to the skin surface as the stress; Use the suction (vacuum) perpendicular to the skin surface as the stress	N/A	Content of skin collagen, elastin and natural fat
Dermatoglyphs	Finger / toe print, palm print	Impression method; Chalk—carbon ink—transparent tape method; Impression method for patients with hand and foot deformities, ninhydrin-monosodium glutamate method for occult dermatoglyphic display, etc	Finger / Toe print: bow pattern, dustpan pattern and bucket pattern Palm print: three areas of the palm (greater thenar, lesser thenar, interdigital area), trigeminal, seven trigeminal and palm fold	N/A	Related to individual vulnerability
Skin appendages	Sweat glands	Chromatograph, etc	Direct weighing method, water evaporation quantitative method, sweat staining visualization method, special organic sweat compound staining method, modeling method and skin capacitance mapping imaging method	N/A	Related to temperature, emotion and taste stress
	Hairs	Clinical commonly used photographic equipment, hair mirror, etc	Full scalp photography technology, contrast-enhanced hair image analysis technology, etc	Hair quantity: about 80–100 hairs/cm2 at the top of the head, about 120–180 hairs/cm2 at the back of the pillow, and 15–20 hairs/cm2 at the back of the hand The shape of the hairline of the forehead: L, M, C and U, and the two special types are F and V	N/A
	Surface sebum	Sebumeter	Spectrophotometric detection	N/A	N/A
Nails	Thickness	Calipers, 20 or 25 MHz ultrasonic imaging	N/A	N/A	N/A
	Roughness	Impression	Computer imaging analysis or optical profilometry	N/A	N/A
	Color	Spectrophotometer, wood lamp	N/A	N/A	N/A
	Nail fold	Capillarioscope	Check the structure of superficial microvessels	Physiological state: the capillary loop is hairpin, with two parallel longitudinal vessels; Pathological changes: branching vessels, increased length of vascular loop, huge capillaries, parallel structure disorder and microaneurysm	N/A
	Nail bed	RCM, OCT, MRI	Perspective, etc	N/A	N/A

EI Erythema index; MI Melanin index; ADM Acquired dermal melanosis; PIH Post-inflammatory pigmentation; UVA Long-wave ultraviolet; UVB Medium-wave ultraviolet; RCM Reflectance confocal microscopy; OCT Optical coherence tomography; MSOT Multispectral photoacoustic imaging; Prox-1 Rat homeobox gene transcription factor 1; VEGFR-3 Vascular endothelial growth factor receptor 3; MRI Magnetic resonance imaging