Now that it is summer and I spend most of my free time outside, I am, not surprisingly, a few shades darker now than during the colder months. I was on the beach with some family members recently, and the conversation turned to our differing abilities to tan. Some in the group had not been in the sun enough to sport a tan yet. Others were well on their way to a nice tan. All agreed they would have more color soon because they planned to spend more time in the sunshine. It got me to wonder about the minutia of what a tan is and why we even tan in the first place. 

UV Exposure And Damage

UV from the sun is classified as UVA and UVB. UVB is considered more damaging than UVA. There is a C, but it does not penetrate the atmosphere. UVB directly damages DNA's structure, while UVA indirectly damages DNA by creating reactive oxygen species, leading to single-strand breaks in DNA and DNA-protein crosslinks. DNA's most significant absorption of UV occurs between 245 and 290 nm, making UVB cause more mutations than UVA. UV is known to suppress our immune system, probably to prevent autoimmune responses to inflammatory products resulting from UV-mediated damage. 

Skin Type

Some people are born with darker skin than others, and this is known as constitutive skin color. It is not influenced by external factors, whereas facultative pigmentation refers to an increase in melanin content in the skin induced by environmental factors like exposure to sunlight or hormones. Some people can tan easily, others not at all. The Fitzpatrick Skin Type, or phototype, has been in existence since 1975. Dr. Fitzpatrick, a dermatologist who used UV light therapy to treat skin conditions like acne, needed a way to determine how much UV light he could use before the patients' skin burned. The official term for damage to the skin caused by light is phototoxicity. 

Type I features pale white skin with light eye color that always burns and does not tan. Type II features fair skin and light eyes, and they burn easily and tan poorly. Type III features darker white skin that tans after an initial burn. Type IV features light brown skin that burns minimally and tans easily. Type V features brown skin that rarely burns and easily tans darkly. Finally, type VI Features dark brown or black skin that never burns and always tans darkly.

Melanin, Melanosomes, and Melanocytes

Most of us have heard of melanin. It is the pigment in our skin responsible for our color. It is a complex of lighter reddish-yellow pheomelanin and darker brownish-black eumelanin. Generally, all skin types contain a higher proportion of eumelanin than pheomelanin. 

Melanin is made in melanosomes found in special cells called melanocytes. Skin pigmentation differences are not caused by variations in the number of melanocytes in the skin. Instead, these variations are caused by differences in melanin-producing activity, the type of melanin produced in melanosomes, their size, number, and packaging. The melanin content of melanosomes can range from 17.9% to 72.3%. Once melanin is made, it ends up in our normal skin cells, called keratinocytes.

Melanin and Protection From UV Light

Scientific research shows that melanin is important in protecting the skin from sun damage. This is supported by epidemiological data, indicating that darker-skinned people have a lower risk of skin cancer caused by sun exposure. Melanin in Black skin is twice as effective at blocking UVB radiation than in White skin. Black skin only allows 7.4% of UVB and 17.5% of UVA to penetrate, while White skin allows 24% UVB and 55% UVA to pass through. Accordingly, individuals with White skin are around 70 times more likely to develop skin cancer than those with Black skin. Melanin, particularly eumelanin, acts as a protective shield by scattering UV light as a physical barrier and filtering UV as an absorber to limit its penetration through the epidermis. 

Supranuclear Caps Versus Melanin Dust

As I mentioned before, melanin migrates to skin cells called keratinocytes. In people with darker skin, melanin forms a layer over the nucleus containing the DNA on the side of the cell facing the outside where light can penetrate, thus protecting sensitive genetic material. The formations are called supranuclear caps and are much more robust in darker skin. On the other hand, in people with skin with less pigment, the melanosomes break down and remain only as "melanin dust" in the upper layers.

Immediate Pigment Darkening

When UV hits existing melanin, it causes oxidation. The newly oxidized melanin migrates peripherally, causing the skin to turn brown within minutes and sometimes days. This is known as immediate pigment darkening or IPD. It is more responsive to UVA and not very protective, which is puzzling. Nobody is sure of its significance.

Persistent Pigment Darkening

Similar to IPD, due to photooxidation, persistent pigment darkening or PPD takes over and lasts 3 to 5 days. 

Delayed Tanning

Delayed tanning, or DT, is the last phase that begins two or three days after exposure. It is totally different than PPD and occurs due to the stimulation of melanin synthesis. This process involves an increase in the number and activity of melanocytes, increased synthesis and transfer, and changes in the packaging of melanosomes. 

When exposed to UVB rays, DT (delayed tanning) can protect the skin from sun damage with an estimated SPF of 3. However, UVA-induced DT does not offer the same level of photoprotection. The intensity of DT varies depending on the amount of UV exposure and the individual's skin tone, typically lasting from 10 days to 3-4 weeks. 

It may take several weeks or even months for the skin to return to its natural color after tanning. UVA-induced DT is less efficient than UVB-induced DT, with an earlier onset that can occur directly after IPD.

Other Factors Involved In Tanning

The tanning process is influenced by various factors, including the direct response of melanocytes to UV light and the synthesis of hormones, cytokines, and growth factors in epidermal cells, which are indirectly affected by UV light. For instance, more than 120 genes regulate pigmentation in mammals. 

Conclusion

The function and survival of many cell types are affected by ultraviolet radiation, which is the main cause of skin tumors such as basal cell carcinoma, squamous cell carcinoma, and malignant melanoma. Skin pigmentation has traditionally been seen as the most important factor in protecting against UV radiation, as melanin absorbs UV rays and has antioxidant and radical scavenging properties.

Sources

Lan, Y., Zeng, W., Wang, Y. et al. Opsin 3 mediates UVA-induced keratinocyte supranuclear melanin cap formation. Commun Biol 6, 238 (2023). https://doi.org/10.1038/s42003-023-04621-8

Brenner M, Hearing VJ. The protective role of melanin against UV damage in human skin. Photochem Photobiol. 2008 May-Jun;84(3):539-49. doi: 10.1111/j.1751-1097.2007.00226.x. PMID: 18435612; PMCID: PMC2671032.