Solar capital: understanding it better to preserve it more effectively.

The solar capital, or tanning capital, refers to the total amount of sun rays that each person can receive throughout their life. It can be likened to a reservoir granted at birth and whose level decreases with each exposure to UVA and UVB rays. Far from being infinite, the solar capital diminishes each time we expose ourselves, thereby increasing the dangers of sun exposure. Indeed, UV rays have a harmful effect on the skin. In addition to causing the disorganisation of the dermis's structural protein fibres, which promotes skin sagging, they attack DNA and can lead to mutations and the formation of cancerous cells.

Inherent to each individual, the sun capital granted at birth largely depends on the phototype. This classification, ranging from I to VI, was developed in 1785 by the dermatologist Thomas FITZPATRICK, differentiating individuals based on their skin's reactivity to the sun. Individuals with a light phototype are more sensitive to the sun than those with a dark phototype, due to the lower quantity of melanin they synthesise. Furthermore, there are two forms of melanin, produced in different proportions depending on the individual: eumelanin, which is quite dark and protective, and pheomelanin, which is lighter.

Individuals with darker skin naturally have a higher sun protection factor. However, it is not unlimited and must be preserved.

Beyond the aesthetic appeal it lends to the skin, melanin primarily plays a defensive role against UV rays and is strongly associated with the concept of sun capital. Indeed, through a process known as "capping", melanin positions itself around the nuclei of keratinocytes to protect the genetic material from the mutagenic effects of UV rays. Moreover, melanin has the ability to neutralise the free radicals generated in skin cells before they can cause damage.

A study conducted by PATHAK and his team focused on phototypes and their defence system against UV rays. This research measured the minimal erythemal doses (MED) and the minimal melanogenic doses (MMD) related to the emission of UVA and UVB rays for each of the phototypes. To clarify, the MED is the smallest amount of light capable of triggering sunburn after 24 hours. It measures photosensitivity. The MMD is the smallest amount of light that allows tanning. The study shows that both MED and MMD increase with phototypes. These results highlight the greater natural protection that darker phototypes possess.

With repeated exposure, the photoprotection system provided by melanin becomes less effective. Several mechanisms are at play, including the attack on melanocytes by free radicals. By destabilising the melanogenesis process, these reactive species alter the sun capital. Its depletion leads to an increased risk of melanomas and carcinomas and increased sun sensitivity. This can result in itching and an acceleration of sunburn. The arms, hands, décolletage and face are frequently affected.

Furthermore, a depleted solar capital is often recognised by the appearance of brown spots on the skin's surface, due to the dysfunction of melanocytes associated with the exhaustion of their pigmentary potential at the level of hair follicles. Other indicators of a low solar capital include telangiectasias, these small dilated vessels due to the stiffening action of UV rays, and the premature loosening of skin structures.

Regrettably, it is impossible to increase one's solar capital, hence the importance of taking care of it. It is recommended to start from the earliest age, as sun exposure during childhood is often responsible for skin cancers in adulthood. Scientists indeed estimate that the risk of melanoma increases by 75% when sun exposure begins before the age of 30. Thinner, drier and synthesising less melanin, the skin of babies and young children is naturally more vulnerable and must absolutely be kept away from direct sun exposure.

Furthermore, certain lifelong habits can have a positive impact on your solar capital. During prolonged exposure, we advise you to protect your skin with covering clothing and a hat. The rest of the time, rely on a good sunscreen with a SPF suitable for your skin type and the season, and frequently reapply. To preserve your solar capital, it is also recommended to avoid exposure between 12pm and 4pm, the time of day when UV rays are most dangerous. Finally, the danger that UV tanning beds pose to the solar capital should not be overlooked.

• PATHAK M. A. & others. Sunlight and melanin pigmentation. Photochemical and Photobiological Reviews (1976).

• 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).