Sun Capital: How can I determine the condition of my skin?

The term 'solar capital' refers to the skin's ability to combat the adverse effects of the sun. This varies from one individual to another. Indeed, people with a darker phototype naturally have a larger solar capital, due to the protective role of melanin. By forming a filter around DNA strands, this brown pigment protects genetic material from the mutagenic and carcinogenic effects of UV rays. Melanin can also capture free radicals generated within melanocytes and keratinocytes. However, the protection offered by melanin is not complete, regardless of one's phototype: this pigment can only absorb up to 90% of UV rays that have penetrated the corneal layer. Those not stopped can react with DNA and other cellular constituents and damage them.

The more we expose ourselves, the less effective the protection provided by melanin becomes. This results in a decrease in the solar capital and an increase in the risk of skin disease. Therefore, to protect one's skin, it is essential to learn how to manage one's solar capital and to know where it stands. Certain indicators are unmistakable and alert to a deficit in solar capital.

• Sunburns are becoming increasingly frequent and taking longer to heal.

  Regardless of its size, a sunburn is never harmless and indicates that the skin has been damaged by UV rays. A sunburn is characterised by redness, pain with itching, and sometimes blisters. These symptoms are the result of an inflammatory process causing vasodilation of the blood vessels and a reddening of the skin. By filtering UV rays, melanin normally limits sunburn. However, if you notice that you tend to redden more quickly than before, this means that your defence system and your sun capital are becoming exhausted.

  Similarly, sunburns that last for a long time are indicators of an almost depleted solar capital. Indeed, after a sunburn, the skin initiates a tissue regeneration process that involves, among other things, the recruitment of immune cells and the formation of new blood vessels (neoangiogenesis). When the healing process becomes less efficient, it can be a sign that the skin has been damaged too quickly and excessively.

• An increased sensitivity to the sun.

  Generally speaking, an increase in skin sensitivity to the sun indicates that the skin's defence capacity against UV rays is diminished, meaning that the sun protection factor is quite low. Without going as far as a sunburn, increased sensitivity is manifested by tingling and itching. From a biochemical perspective, these discomforts are explained by the generation of pro-inflammatory cytokines in skin cells under the effect of UV rays.

• An accentuated skin laxity.

  The sun is one of the primary causes of skin ageing . The early onset of wrinkles and fine lines, along with a general sagging of the skin on the face, neck, and hands - areas most often exposed to the sun - is another sign of depleted solar capital. Indeed, when the skin's endogenous antioxidant defences are overwhelmed, the free radicals generated by UV rays activate matrix metalloproteinases, enzymes that promote the degradation of the dermis's protein fibres. The skin then becomes less firm, less elastic, and less supple.

• Pigmentary changes.

  The emergence of brown spots, or solar lentigines, should also serve as a warning that the skin's sun resilience is being depleted. Indeed, oxidative stress is a disruptive factor in the melanogenesis process, leading to an excessive and disorganised production of melanin, which manifests as brown spots or hypopigmentation. More often than not, these pigment changes appear on the backs of the hands or on the face, the areas most exposed to the sun.

• The emergence of small blood vessels on the surface of the skin.

  Another sign of solar capital exhaustion is the development of telangiectasias, these small dilated blood vessels. This characteristic is explained by the stiffening action of UV rays on the blood vessels. Combined with the progressive reduction throughout life of the density of the perivascular connective tissue support, this phenomenon leads to chronic dilation, causing the gradual appearance of red spots and telangiectasias.

To minimise the risks of skin cancers such as carcinomas or melanomas and to preserve one's solar capital, it is essential to protect one's skin from UV rays throughout one's life, particularly during childhood, a high-risk period.

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• CARLISLE K. & others. Histology of sun-damaged human skin. Journal of the American Academy of Dermatology (1988).

• FITZPATRICK T. B. The Validity and Practicality of Sun-Reactive Skin Types I Through VI. Archives of Dermatology (1988).

• BJÖRKLUND S. & co. The Impact of UVB Exposure and Oxidative Stress on the Skin Barrier—A Novel Approach to Assess Sun Protection Factor Using Electrical Impedance Spectroscopy. Sensors (2019).