The specific characteristics of baby's skin.

Compared to an adult's, a newborn's skin is thin, delicate, and vulnerable. The skin barrier gradually develops over the first few years of life and will not fully perform its functions until the age of six. Only then can it be physiologically compared to an adult's skin. Before this age, baby skin has its own unique characteristics.

A newborn's skin absorbs external compounds such as pollution particles, bacteria, and allergens much more easily than an adult's skin, making it particularly vulnerable. Indeed, although the overall structure of a baby's skin is the same as that of an adult, consisting of an epidermis, dermis, and hypodermis, their epidermis is 20% thinner than that of adults.

Furthermore, their stratum corneum, which is the upper layer of the epidermis that protects us from environmental influences and retains moisture, is 30% thinner than that of an adult. Similarly, the corneocytes, the cells of the stratum corneum, in infants are 20% smaller than those in adults, indicating a faster cellular turnover in babies.

The hydrolipidic film of the skin is still forming during the early years and remains delicate, making it less effective in performing its barrier function.The absence of this acidic mantle has an impact on the pH of baby skin: it is more neutral than that of adults. However, the natural acidity of adult skin serves a very practical purpose: it helps to neutralize alkaline aggressors, such as certain surfactants, inhibits bacterial growth, and maintains the skin microbiota.

A pH level below 7 also promotes the synthesis of epidermal lipids, playing a major role in retaining water in the skin's superficial layer. Lastly, it's worth noting that sebaceous glands, which are responsible for sebum production, and sweat glands, which are responsible for sweat formation, are less active in infants. These biological differences make newborns' skin more prone to skin dryness.

Note : Contrary to what one might think, the low hydration of babies is not due to a deficiency of NMF compounds (Natural Moisturizing Factors). These are all the hygroscopic substances responsible for maintaining hydration in the epidermis.

UV rays are naturally harmful to all skin types, but particularly to baby skin. To defend against the sun's rays, the skin secretes melanin, a pigment that gives it a tanned hue. Indeed, melanin can act by enveloping the nucleus of the keratinocytes in the horny layer to form a veil that protects the DNA from the mutagenic and carcinogenic effects of UV rays. It can also directly capture free radicals, which are responsible for damage to various cellular constituents.

However, the process of melanin synthesis is not fully developed in babies. Up until the age of 3, children's skin has a lower concentration of melanin than that of adults. Consequently, it is more sensitive to ultraviolet rays and photodamage. That's why it's very important to protect them from the sun's rays.

• ATHERTON D. & MILLS K. What can be done to maintain the health of a baby's skin? RCM Midwives (2004).

• KOLLIAS N. & al. Infant skin physiology and development during the first years of life: a review of recent findings based on in vivo studies. International Journal of Cosmetic Science (2010).

• HACHEM J. P. & others. Infant Epidermal Skin Physiology: Adaptation After Birth. British Journal of Dermatology (2012).

• LOW K. Y. & WALLACE M. Prevalence of potential contact allergens in baby cosmetic products. Clinical and experimental dermatology (2019).