External Medicine
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Tanning/Sunscreen
Ultraviolet Light
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Topical suncreens are products designed to block ultraviolet radiation from reaching skin cells. They do this by absorbing or reflecting ultraviolet radiation. Sunlight is composed of numerous wavelengths of light inlcuding longer wavelengths like infrafred light, which produce heat, and shorter wavelengths like ultraviolet radiation, which can cause damage to skin cells. Visible light is also emitted by the sun, and is a longer wavelength than ultraviolet, but shorter than infrared. Ultraviolet light can be broken up into categories based on wavelength, from longest to shortest: UVA, UVB, and UVC.
Ultraviolet A (315-400nm)
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95% of the UV from the sun is UVA. Doses of UVA from natural sunlight are rarely enough to induce sunburn.
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Longest UV wavelength, penetrates deep into dermis. Longer wavelengths are poor at inducing sunburn. 360nm light is 1000x worse at causing erythema than 300nm light. Deeper penetration results in more profound effects on collagen and elastin fibers which reside in the dermis. UVA increase progerin, causes deletion of mitochondrial DNA, and increases cathepsin K which degrades elastin.
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Responsible for immediate tanning, a result of oxidation and redistribution of melanin without skin thickening or hyperkeratosis (less effective tan at preventing subsequent sun damage).
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UVA rays are more consistent in their strength with respect to the angle of the sun, meaning their strength does not vary much based on the time of day or year. They also can pass through window glass (UVB is largely filtered out).
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The chromophore for UVA is porphyrins (which are also chromophores for visible light)
Ultraviolet B (280-315nm)
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Intermediate in wavelength, penetrates to superficial dermis. Induces sunburn.
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Responsible for delayed tanning (peaks around 3 days after exposure), result of increased melanocytes, arborization of melanoctyes, melanin synthesis, transfer of melanosomes to keratinocytes, and epidermal thickening and hyperkeratosis (5-10x more protective than UVA tan).
Ultraviolet C (200-280nm)
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Most filtered out by atmosphere, not reaching earths surface. Largely irrelevant at ground level altitudes.
