Why do hands and feet crack in winter?

In winter, many people's hands and feet are prone to cracking and bleeding, which is very painful and affects the appearance. When the cracks are pulled deep into the skin due to activities, there is obvious pain, and activities often cause bleeding and enlargement of the cracks. This symptom often occurs on the fingertips, the side edges of the knuckles, the side edges of the feet, and the heels. This condition of dry and cracked hands and feet is medically called hand and foot chapped skin (Figure 1), which is a common skin disease in autumn and winter.

Figure 1 Chapped hands and feet

Why are chapped hands and feet more likely to occur in winter? Apart from pathological trauma, aging and other factors, the reasons why most people get chapped hands and feet in winter are as follows.

1. Structure of the skin

First, let's get to know the skin. The skin is the largest organ in the human body. It covers the surface of the body and is in direct contact with the external environment. It has the functions of protection, excretion, regulating body temperature, and sensing external stimuli. It accounts for about 16% of body weight. The skin area of ​​an adult is about 1.2~2.0 square meters. The thickness of the skin varies from place to place. The back, neck, palms and soles of the feet are the thickest, and the armpits and face are the thinnest (it seems that the saying "thin skin" makes sense, Figure 2). The average thickness is 0.5~4.0 mm.

Figure 2 Human skin organs

The skin consists of two layers: the epidermis and the dermis. The epidermis is a stratified squamous epithelium, while the dermis is a dense connective tissue with many elastic fibers and collagen fibers, so it is elastic and tough. Under the skin is the subcutaneous tissue, which is a loose connective tissue with a large number of fat cells. The skin also has many appendages such as hair, sweat glands, sebaceous glands, and nails (Figure 3).

Figure 3 Anatomical structure of the skin

The skin is the most important armor to protect our body. It protects the subcutaneous organs and tissues from damage by mechanical, physical, chemical and other harmful factors in the external environment, and prevents the loss of various nutrients, electrolytes and water in the tissues.

Regarding the natural barrier function of the skin, in 1983, American scholar Peter proposed a "brick wall theory". It says that the "brick wall structure" composed of "lipids" and "natural moisturizing factors" between stratum corneum cells and cells, with a "sebum membrane" attached to its surface, together form a natural protective barrier for the human body. The sebum membrane is composed of "oil" secreted by sebaceous glands, "water" secreted by sweat glands, and "natural moisturizing factors" secreted by the epidermis. There are many sebaceous glands in the human skin, which secrete a lot of oil, which can moisturize, smooth and soften the skin to a certain extent, ensuring that the skin has a certain degree of elasticity; it can also protect the human skin to a certain extent when it is impacted by the outside world.

So, the meaning of "brick wall structure" is that the epidermal stratum corneum cells are like "bricks", and the "lipids" and "natural moisturizing factors" between the stratum corneum cells are like "mud" that tightly connect the stratum corneum cells together.

The barrier function of the skin is very important to human health. First, it prevents the loss of moisture from the human skin, that is, the moisturizing function; of course, it also prevents external moisture from easily entering the human body. For example, even if we swim or take a bath for an hour, most of our skin will not feel swollen, let alone "water in the brain". This is the role of the skin's waterproof barrier. Second, it prevents bacteria, fungi and viruses on the surface of the skin from entering the human body and causing diseases. If a part of the body is injured, even if it is just a small wound, a large number of bacteria and viruses that are everywhere in nature will take advantage of the opportunity to enter and induce various inflammations and adverse reactions.

2. Reasons why chapped hands and feet occur in winter

Next, let's talk about why winter is prone to chapped hands and feet. The key word is winter. There are four seasons in a year, spring, summer, autumn and winter. The difference between winter and winter is that the climate characteristics of winter are "dry" and "cold"! In winter, the north wind blows and the snowflakes float! The cold wind blows, the weather is cold, the skin is dry and wrinkled. The difference between summer and winter hands is quite big (Figure 4).

Figure 4: The difference between hands in summer and winter is quite big

The cause of dry and cracked skin in winter is related to sebaceous glands. Sebaceous glands are unevenly distributed on the human body. They are more distributed on the scalp and face, so these places tend to produce "oil", while the hands and feet, especially the palms and heels, have almost no sebaceous glands. The palms and soles of the feet do not have sebaceous glands and cannot secrete oil, so they do not tend to produce "oil".

The sebaceous glands have a characteristic that the higher the temperature and the warmer the weather, the more oil they secrete. In warm seasons, it often flows out through the pores automatically, constantly "refueling" and lubricating the skin to prevent it from drying out.

As winter approaches, the temperature gradually drops, and the secretion of sebaceous glands also decreases. In addition, cold air often invades in winter, and the heat and oil of the hands and feet will evaporate quickly, aggravating the degree of dryness of the skin, which is manifested as hardening of the stratum corneum. Many people always feel that their hands and feet are dry, rough, astringent, lack elasticity, and are very uncomfortable to touch. In worse cases, they may even crack and bleed. The normal stratum corneum contains about 20% water. When the water content is less than 10%, the skin becomes dry, less tough, and prone to cracking. Generally speaking, people who are prone to chapped hands and feet are those who often engage in outdoor physical labor.

3. Biomechanical mechanism of chapped hands and feet in winter

Next, let's analyze why dry and wrinkled skin in winter is prone to chapped hands and feet. What is the biomechanical mechanism behind it? Let's analyze it from several aspects.

(1) Mechanism of stress concentration

Stress concentration refers to the phenomenon that the maximum stress value in a local area of ​​a structure or component is higher than the average stress value. It often occurs in sharp corners, holes, notches, grooves, and rigid constraints and their neighborhoods (Figure 5). The locally increased stress value decays rapidly as the distance from the peak stress point increases. Stress concentration can cause brittle materials to break; it can cause fatigue cracks in brittle and plastic materials. There are such notches on various packaging bags in life, some are serrated and some are semicircular. Their function is to produce stress concentration and facilitate tearing open the package.

Figure 5 Stress concentration occurs in components with inner hole and outer groove defects

Similarly, the skin on the hands and feet becomes rough and wrinkled in winter, forming higher skin hillocks and deeper skin grooves (Figure 6). Due to the stretching of the skin, stress concentration will occur in the deep valleys of the skin grooves, which can easily lead to cracks extending and forming chapped skin.

Figure 6: Chapped skin is more likely to occur in the furrows of the skin

(2) Effects of temperature and humidity on the mechanical properties of skin

As mentioned above, when the temperature is high and the humidity is high, the skin has more "oil and water", and vice versa. The skin mechanical property curve (that is, the stress-strain curve) is shown in Figure 7. Obviously, the skin with more "oil and water" and moist has good ductility, while the dry skin breaks without much deformation.

Figure 7 Stress-strain curve of skin stretch

Ductility is represented by a professional indicator, that is, elongation. Elongation is the percentage of the total deformation of the gauge length after the specimen is stretched and fractured to the original gauge length. It is also the strain value at the far right end of the curve represented by the horizontal axis in Figure 7.

For example, when pulling noodles, if the noodles are chewy, they can be pulled very long without breaking; while noodles without chewiness, or without elasticity and toughness, are easily broken and difficult to stretch. The elongation of the skin is closely related to the microscopic arrangement direction, tightness, and distribution uniformity of the elastic fibers and collagen fibers inside it.

The skin in winter is dry and wrinkled. The arrangement direction of the fiber bundles in the skin is not easy to deform to a state parallel to the main stress direction. The arrangement is tight and uneven. The fibers at different levels deform asynchronously when subjected to stress, which makes them unable to resist external forces synchronously. Some fibers will break first when subjected to large stress, thus expanding the cracks and eventually forming macroscopic cracks. When I was a child, I often played on the cracked mud blocks on the cracked riverbed. Listening to the "crunch" and "crunch" sounds of the mud blocks breaking, I enjoyed the pleasure of destruction (Figure 8). After the riverbed dried and cracked, the elasticity and toughness of the mud became worse, and it could not withstand tensile and compressive deformation, so it was easy to brittle fracture and make a sudden fracture sound. This will not happen with soft mud. This is the effect of elongation on the mechanical properties of materials.

Figure 8 The cracked mud in the riverbed is designed to make noise when you step on it.

In addition, the strain rate will produce a "strain stiffening" effect on the material, which will also affect the mechanical properties of the skin, thereby affecting the occurrence of chapped hands and feet in winter. I won't go into details here. The mechanical properties of the skin show strong nonlinearity, anisotropy, and viscoelastic response, and vary depending on the body part, age, gender, ultraviolet radiation and other conditions. Even a normal person cannot resist the dryness and weakened elasticity of the skin caused by aging; time is a butcher's knife, and one has to sigh!

4. Ways to prevent chapped hands and feet in winter

According to the above introduction, it is not difficult to understand that the way to prevent chapped hands and feet in winter is mainly to keep warm and moisturize. Therefore, the following skin care measures are more or less used by everyone.

(1) Keep your skin warm. Winter clothing often wraps the body tightly, but the hands and feet may be exposed. The hands and feet are far from the heart, and may not be adequately supplied with blood, making it difficult to maintain a constant temperature. If you do not take measures to keep warm, it is easy to cause chapped hands and feet. Therefore, when going out or working outdoors, you should wear thick, warm shoes, socks and gloves.

(2) Pay attention to moisturizing the skin. In order to remove the oil on the hands and feet, we are accustomed to using detergents. The surfactants in the detergents can easily remove the "oil and water" on the surface of the skin, causing the skin to dry out. Therefore, in winter, you should wash your hands as little as possible; after washing your hands, you should dry them in time and apply non-irritating liquid oils or skin care creams, such as vaseline, lanolin, cod liver oil ointment, silicone cream, vegetable oils, or commercially available skin care ointments, oil-in-water formulations, creams, ointments, etc., in order to keep the skin moisturized.

(3) A good and inexpensive skin care secret. In my opinion, wearing a pair of rubber gloves when working or washing is very effective in preventing chapped hands and feet (Figure 9). It is simple and convenient, and most importantly, it has a good moisturizing and skin care effect.

Figure 9 Affordable and good skin care secret recipe

I heard the old saying from the old people: Men's hands should be soft, so that they can have money even if they have no money; women's hands should be hard, so that they can have money even if they have no money. Here, "soft" means soft and delicate, while "hard" means rough and dry. Don't believe it. Whether it is "male hands" or "female hands", or "male football" or "female football" (don't mention "football", it's not about "football" here), they all need to keep warm and moisturized in winter to prevent chapped hands and feet. With moisturized hands and feet, touching yourself is a pleasure.

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Why do hands and feet crack in winter?