We constantly underestimate the importance of our skin. Support for our social interactions, organ of touch, key to our thermoregulation, first protective barrier against external attacks (chemicals, pathogens, UV) and against dehydration… The services it provides us with are legion, and the least of its effects should be taken seriously. Especially one of the most common, also underestimated effects: sunburn.
To understand all the consequences of this painful redness, let’s go back to the biology of our skin – its structure, but also the effects of the radiation received.
Our skin is made up of three compartments, the outermost of which is the epidermis. This is made up of several superimposed layers of cells where several types of “appendages” are inserted, such as the sebaceous glands, responsible for the secretion of sebum (an oily substance made up of lipids) and the hair follicles which produce hair and the hair. Its main cells are keratinocytes and melanocytes, which are responsible for protection against ultraviolet (UV) rays through the production of a pigment, melanin.
The second compartment of the skin is the dermis. This tissue is located under the epidermis, which it supports. It is the dermis that provides skin elasticity. It includes many cell types, such as fibroblasts (supporting cells), immune cells, blood vessels and nerve endings.
Finally, the hypodermis and the deepest skin compartment. Highly vascularized, it is essentially composed of fat storage cells, adipocytes, which govern thermoregulation.
A category of solar radiation is capable of penetrating our skin: these are the famous ultraviolet or UV rays.
Ultraviolet radiation is invisible radiation with a wavelength range between 100 and 400 nanometers (nm, or one billionth of a meter). UV is classified into three categories, based on its wavelength: UV-A, UV-B, and UV-C. The latter are the most energetic, but they are stopped by the ozone layer.
UV-A is radiation with a wavelength between 315 and 400 nanometers. These are the ones that penetrate deepest into the skin. Although they are responsible for skin aging, they contribute little to sunburn, because the absorption of their energy by the DNA housed in the heart of our cells is less efficient. Nevertheless, UV-A stimulates the formation of free radicals, very reactive molecules which can have deleterious effects in the body by reacting with other compounds.
UV-B rays (whose wavelength is between 280 and 315nm) pass through the epidermal layer. They are the ones responsible for sunburn.
There are several levels of sunburn severity.
A first level of severity is defined by redness of the skin, which appears between 6 and 24 hours after exposure. This coloration is due to the dilation of the superficial blood vessels. It disappears with a simple finger pressure on the skin. This type of sunburn disappears after a week and leads to increased flaking: the skin “peels”.
A second level of damage is characterized by the appearance of blisters filled with a clear liquid, circumscribed by a painful red zone. These blisters are a sign that the second degree of superficial burn has been reached. The healing of such sunburns takes about two weeks and leaves no visible sequelae.
Finally, the third level is reached if the skin under the blisters is pale and the lesions are not very sensitive. This is then a deep second degree burn with damage to the epidermis and dermis. The nerve endings are damaged and healing will take longer: it will take at least 35 days.
UV radiation releases energy in our skin which will induce cell damage. UV can have various effects on our body.
One of these effects is an immunosuppressive effect. If a little exposure to the sun allows the synthesis of vitamin D, essential for the absorption of calcium, excess vitamin D limits the proliferation of lymphocytes, a family of essential cells of our immune system.
UV rays also alter the number and morphology of Langerhans cells. Located in the epidermis, these cells present to the T lymphocytes the markers taken from the foreign cells, in order to launch the alert and trigger an immune response in the event of an invasion.
Langerhans cells also activate NK cells (Natural Killer, lymphocytes that play an important role in the fight against tumors, antiviral and antibacterial). They also cause the release of histamine by mast cells in the dermis, which triggers itching.
Finally, UV rays – and more particularly UV-A rays – are responsible for the formation of free radicals, energetically unstable atoms or molecules (in this case, reactive oxygen species or ROS). However, these ROS are able to damage the macromolecules present in our cells: lipids, proteins… and DNA.
This is because the “bricks” that make up the DNA molecule are good at absorbing the energy of UV radiation. Consequence: local lesions occur on the DNA molecule, which leads to the dissociation, at the level of these lesions, of the two strands that compose it. Repair mechanisms are rapidly activated to restore DNA text and structure. Sometimes errors are made during the repair of DNA lesions leading to the appearance of mutations which will have long-term consequences (see next paragraph).
In the short term, the skin peels. This is the consequence of the renewal of the epidermis: the skin separates from the layers of cells that are dead or damaged by sun exposure. It is then necessary to drink a lot, and to hydrate our skin well.
The long-term consequences come from the accumulation of damage produced throughout life by UV radiation. UV-A and UV-B do not have, as we have mentioned, quite the same effects.
UV-A acts as photosensitizers. They facilitate the formation of free radicals, which induce, by accumulating, oxidative stress. This stress in turn leads to the destruction of the network of extracellular macromolecules (such as collagen and elastin), constituting the framework of the skin. The long-term consequence is the premature aging of the skin with the appearance of brown spots and a loss of elasticity.
UV-B causes damage to DNA, which leads to an accumulation of mutations (errors in the genetic text) linked to ever-increasing repair defects as our sun capital decreases (see below) . The development of skin cancer is one of the most well-known consequences. There are two main types: melanomas and carcinomas.
Melanoma, originating from melanocytes, has seen its incidence increase steadily over the past 50 years and represents 2 to 3% of all cancers. Carcinomas, arising from keratinocytes, are 15 to 20 times more common than melanomas. Their prognosis is much better in case of early treatment.
It’s important to remember that even though it’s not visible, UV damage starts at an early age. In order to better understand what is happening under our skin throughout our lives, the notion of “sun capital” is important.
According to the CNRS definition, the sun capital corresponds to the time of exposure to UV rays that our skin can withstand throughout our lives. It corresponds to “all the means of defense of the skin against the harmful effects of the sun”.
Sun capital is acquired at birth. Non-renewable, it depends on the phototype of each individual. Six in number, the phototypes correspond to the different skin types, classified according to their reaction to sun exposure, sensitivity and protective power.
The more the skin suffers sun-related damage, the more the sun capital decreases – like a reservoir in which each exposure draws a little. As you expose yourself, you increase the risk of injury. However, when the latter have been too numerous, they end up not being able to be all repaired.
There is no formula to calculate the sun capital, however dermatologists evaluate it from questionnaires.
Sunburn is not just a summer hazard. If they generally heal quickly in appearance, the long-term consequences are not insignificant: our sun capital is not renewable, it is important to protect yourself well from childhood and adolescence.
Some compounds may have photosensitizing effects. This is particularly the case of various drugs, such as ibuprofen, antibiotics of the quinolone class, tetracyclines and sulfonamides as well as certain treatments against hypercholesterolemia thus sensitize to the sun: even for moderate exposure, one can develop sunburn-like effects. Moreover, these effects are cumulative and dose-dependent.
You should also be wary of certain cosmetics, especially AHA-based creams (natural acids extracted from sugars, fruits or plants, as well as retinol). When applied to the skin, the skin is left with increased sensitivity to the sun. In case of exposure, it is therefore advisable to use them in the evening rather than during the day…
Our vigilance in relation to the sun must be constant. Other tips to keep in mind: avoid exposure between 10 a.m. and 4 p.m., wear SPF 50 sunscreen when you go out in the sun, and renew your apply regularly…
But sunscreens are not a panacea: they are indeed responsible for invisible pollution, since 25,000 tons of residues of sunscreen products reach our bathing waters annually, threatening aquatic ecosystems all over the planet. The best protection, for us and the environment, is therefore clothing!
Soline Estrach, Research Fellow, Inserm and Chloé Feral, Research Director, Inserm.
