How does solar radiation function?

The sun emits infinite light rays of various wavelengths, from the shortest to the longest. It also emits cosmic rays, gamma rays, X rays, ultraviolet (UV) rays (including UVC, UVB and UVA rays), visible radiation, infrared radiation (IR) and radio waves.

The most dangerous part of a solar ray is filtered by the atmosphere. Two-thirds of this ray reach the earth. Cosmic rays, gamma rays, X rays and UVC rays, incompatible with life, do not reach its surface. The rays that make their way to us (UVB, UVA, visible and infrared rays) influence the body. Infrared rays give off heat; these rays make skin feel hot, as opposed to UVB and UVA rays that are invisible and cold but still have a large biological effect.

Why is sun protection necessary?

Solar radiation has destructive effects on the skin that range from solar erythema (sunburn) to accelerated skin ageing and skin cancer. Sun exposure is the leading cause of skin cancer, which is radiation’s most serious consequence. But you must also keep in mind that there are other risks, such as heat stroke, sun stroke, photosensitivity, ophthalmia and sun allergy.

  • The epidermis stops 85% of UVB rays; only 15% reach the dermis.

The quantity of UVB rays depends on the season, latitude, time of day and altitude. These are the rays that cause tanning and sunburn, which is the skin’s natural alarm indicating it’s receiving too many UVB rays. UVB rays have a carcinogenic effect over the long-term. .


  • UVA rays penetrate deeply into the skin ; nearly 50% of them reach the dermis.

The sun emits these rays year round and they pass through clouds and windows. They are responsible for skin ageing (photoageing) and cause free radicals to form. These molecules attack cell structures and DNA. They have a carcinogenic effect, but to a lesser degree than UVB rays.

Sunbrun skin graphic

Key points

Different areas diffuse varying amounts:

- Snow reflects 80% of UV rays, sand 15% and water 25%.

- Even 40 cm below water, UV radiation exposure is still about 40% of what it would be on the surface.

UVA and UVB rays cause skin cancer because they weaken the skin’s immune defences, triggering significant oxidative stress and creating lesions on the DNA that can cause cancer cells to appear.

Not all skin types respond the same way to solar radiation because each individual has his/her own photosensitivity, characterised by his/her phototype. A phototype is the quality of a subject’s response to the action of the sun’s rays. There are six phototypes determined based on skin tone, hair colour, whether there are ephelides (freckles) and the individual’s tendency to get sunburns or a tan.

  1. Phototype I: very fair skin (red head), always burns, never tans, many freckles.
  2. Phototype II: fair skin, always burns, may get a light tan, several freckles.
  3. Phototype III: fair to olive skin, sometimes burns, always tans (light to medium tan), may have a few freckles.
  4. Phototype IV: olive skin, rarely burns, always tans (dark tan), no freckles.
  5. Phototype V: brown skin, never burns, always tans (very dark tan), no freckles.
  6. Phototype VI: black skin, never burns, no freckles.

The lighter your phototype (phototypes I and II especially, as well as III), the more you need to use high photoprotection.

Your skin - hands skin colors

How does tanning work?

Tanning is mainly triggered by UVB rays under the “natural” sun. It is a way for the skin to adapt and defend itself against the sun but is not a total shield. Being tanned often provides protection from sunburn but should not encourage excessive exposure, which has long-term effects (skin cancers in particular). 

Exposure to the sun activates melanocytes, which produce melanin to protect themselves from the sun. Melanin migrates towards the skin’s surface. At the same time, the skin thickens (through an increase in horny layer thickness), which is an additional protective factor along with pigmentation. Remember that UVA rays trigger immediate pigmentation that only lasts a few hours. This is not a tan, but oxidation that becomes visible right after sun exposure. UVB rays are the only ones responsible for a real tan, which begins to develop two to three days after exposure and reaches its maximum three weeks after this exposure. 

Photoprotection products are designed to filter both UVB and UVA rays, with an appropriate balance between the two. Their filtering power is indicated by an SPF (Sun Protection Factor). This SPF should be chosen based on your phototype and the intensity of the sun’s radiation, which varies based on location (sea, mountains, etc.).
. SPF 6 = low protection SPF 15 = moderate protection SPF 30 = high protection
. SPF 50+ = very high protection

Choose a product whose texture you like, which will encourage you to apply it more regularly. You should reapply every two hours for it to be most effective. In addition, some formulas are water resistant or speed up tanning to reduce exposure time.

Skin graphic - sun protection

Key points

According to experts*, 80% of facial ageing is caused by sun exposure! The ageing linked to this exposure is called photoageing.

Its signs include deep wrinkles, pigment spots, and telangiectasias (visible dilation of small blood vessels below the skin). This type of accelerated ageing affects all frequently-exposed areas, such as the face, back, hands, forearms and décolleté. UVB and UVA rays drastically alter skin cells, destroy collagen and elastin, and can cause cancerous tumours to develop.

There are so many good reasons for adopting common-sense habits to deal with the sun and protect yourself whenever you are exposed.

*Gilchrest BA et al, Effect of chronologic aging and photoaging: an overview., J Am Acad Dermatol, 1989; 21(3 Pt 2): 610-3

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