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Light Energy and Sunscreens
By: Peter T. Pugliese, MD
Posted: March 26, 2009, from the April 2009 issue of Skin Inc. magazine.
page 5 of 13
Next, let’s take a look at a few of the adverse reactions from UVB and UVA.
UVB and adverse skin reactions
An excess exposure to UVB radiation can cause sunburn and some forms of skin cancer. In humans, eye and immune system damage are common effects of sunlight. UV photons harm the DNA molecules of living organisms in different ways.1 In one common form of UV damage in DNA, adjacent cytosine bases bond with each other, forming a dimer and causing the DNA molecule to change into an abnormal shape so it can no longer function properly. See Figure 5. This is bad enough, but the big problem occurs when the DNA code must be replicated in this strand of DNA. The enzyme polymerase will read the dimer as “AA” and not the original “CC.” This will cause the DNA replication mechanism to add a “TT” on the growing strand rather than a “GG.”h Thus, a mutation is produced known as a classical C-T mutation, which can result in cancer. An interesting aspect of this mutation is that it carries a UV signature commonly seen in skin cancers, and the research scientist can recognize it as being caused by the direct DNA from UVB. Malignant melanoma is believed to be caused by the indirect DNA damage (free radicals and oxidative stress). This can be seen from the absence of a UV-signature mutation in 92% of all melanoma. Figure 6 shows the depth of penetration of UVA, UVB, UVC and infrared into the skin.
UVA and adverse skin reactions
The major acute effects of UV irradiation on normal human skin are sunburn, inflammation, erythema, tanning, and local or systemic immunosuppression. UVA and UVB can both damage collagen fibers by fragmenting them and cross-linking them, both conditions that accelerate skin aging.2 UVA and UVB can destroy vitamin A in the skin, causing loss of proliferative cell control. UVA penetrates deeply into the skin, but it does not cause sunburn or erythema, which may lead the public to think it is harmless, but nothing is further from the truth. Although UVA may not damage DNA directly like UVB since it has less energy, it can generate highly reactive chemical intermediates, such as hydroxyl and oxygen radicals, which really can damage DNA.3 Some scientists associate the absence of UVA filters in sunscreens with the higher risk of melanoma found in individuals who use sunscreens.4–6
Melanin is derived from the amino acid tyrosine. The two major forms of human melanin are eumelanin, a brown-black melanin or regular melanin; and pheomelanin, which is a red-brown melanin seen in individuals with red hair and freckles.i The production of melanin in human skin—known as melanogenesis—is stimulated by UVA and UVB, and produces a darker color of the skin, known as a tan. Tanning is true melanogenesis. UVB triggers CPD-DNA damage, increases melanin production and may cause a sunburn, both caused by the direct DNA damage. UVB is reduced by most sunscreens; however, it produces brown spots or moles, and contributes to aging.
UVA can cause preexisting melanin to be released from the melanocytes and activates the oxidation of melanin, causing an immediate tan to appear. Melanoma seems to be associated with UVA.7 Although UVA contains fewer energetic photons, there is more UVA in sunlight and more of it each season. Melanin is able to absorb 99.9% of UV radiation and then dissipates this energy as heat by a process of ultrafast conversion of the absorbed photon energy to heat. It is this property of melanin that makes it a very good photo-protectant.