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What’s Important?

  1. The water content of the SC should be greater than 10%.

  2. The formation of fine wrinkles is associated with low water content in the skin.

  3. Increasing water consumption will increase the water content of skin, especially in those who consume less than 2,500 mL/day of water.

  4. The microbiome affects skin hydration and skin hydration affects the microbiome.

  5. Relative humidity below 30% can lead to dry skin unless the patient has high levels of sebum production. The optimal relative humidity for skin hydration is 60%.

  6. Increased humidity increases skin elasticity.

  7. An elevated pH impairs barrier homeostasis and SC cohesion, decreasing skin protection from the environment.

What’s New?

  1. Cell phone-attached devices to measure skin hydration are inaccurate due to humidity and ambient air flow.

  2. Use of moisturizers in newborns decreases the development of atopic dermatitis, food allergies, and asthma.

What’s Coming?

  1. The effects of cannabinoids on the skin barrier and skin hydration are unclear.

Dry skin, also known as xerosis, is due to genetics, environment, and lifestyle factors. It can be so mild that it is hardly noticed or so severe that it leads to skin breakdown, severe itching, and infection. Mild dry skin is a condition that affects many patients, especially women over 50. Fine wrinkles occur on the skin when water content is low,1 resulting in billions of dollars a year spent worldwide on moisturizing skincare products. It is important, therefore, for dermatologists, medical providers, and cosmetic scientists to understand the underlying causes of dry skin and how current therapies treat this condition.

There are so many products on the market to treat skin dryness that one can become easily overwhelmed. The preponderance of them treat the symptoms rather than the underlying causes. This chapter will discuss what is known about the etiology of dry skin with an eye toward elucidating issues that must be understood in order to identify the most effective products or the ones best suited to particular skin types. Moisturizers are discussed in Chapter 43.


Dry skin is characterized by the lack of water in the stratum corneum (SC). Water is the major plasticizer of the skin; when levels are low, skin becomes less resilient and cracks and fissures can occur.2 For the skin to appear and feel normal, the water content of the SC must be greater than 10%.3 The increase in transepidermal water loss (TEWL) that leads to dry skin results when a defect in the permeability barrier allows excessive water to be lost to the atmosphere (Box 12-1). This barrier perturbation is caused by several different factors such as anionic detergents, acetone and other solvents, water with high levels of calcium, hot water, and frequent bathing (Chapter 40, Cleansing Agents). Skin hydration is assessed using several types of measurements including TEWL and capacitance (...

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