Ultraviolet (UV) light is electromagnetic radiation with a wavelength from 10-400 nm, which can be classified as UVA, UVB, or UVC light. Both UVA (320-400 nm) and UVB (290-320 nm) light penetrate the atmosphere and reach earth’s surface. UVA rays play a role in skin aging and wrinkling while UVB rays can cause skin cancer. Light with even shorter wavelengths, including most UVC rays are absorbed by the ozone layer and do not reach the earth. To avoid the deleterious effect of sunlight on the skin, sunscreens were designed to minimize the solar light transmission in the UVA and UVB regions.
Sunscreen can be divided into two types based on different mechanisms of action: physical sunscreen and chemical sunscreen. Physical sunscreens contain inactive mineral ingredients, such as titanium dioxide or zinc oxide nanoparticles, which deflect or absorb UV rays via band-gap absorption dissipate them as harmless IR radiation. Chemical sunscreens contain chromophores, such as oxybenzone, octinoxate, octisalate, and avobenzone, which absorb UV rays similarly. Sunscreens marketed as "sensitive" are more likely to contain physical sunscreens which can look like a white cast over the skin. Heavy sunscreens in cosmetics can cause an effect called "flashback" which results in a person's face looking ghostly white in pictures as a result of flash photography.
Sunscreens are commonly rated by a sun protection factor (SPF). This factor corresponds to the proportion of light that will reach the skin with the specified thickness of application of the sunscreen. This means that SPF 50 sunscreen lets in 1/50th or 2% of damaging rays while SPF 80 sunscreen lets in 1.25% of harmful rays meaning that increasing the SPF beyond a certain point has diminishing returns. It is also important to note that a higher SPF number does not increase the sunscreen's longevity.
Avobenzone is one of the most common organic sunscreen ingredients. Its existence as a mixture of the enol and keto forms in the ground state enables it to absorb a wide range of wavelengths with absorption maximum of 357 nm, which offers protection from UVA rays. Most sunscreens contain a mixture of compounds to cover a “broad spectrum” of wavelength light. Skin contact to pure avobenzone could lead to inflammation characterized by itching, scaling, reddening, blistering, pain and/or dryness. Getting sunscreen in the eyes may cause redness, irritation, tearing, pain or eye damage. When exposed to chlorinated water or UV light, avobenzone can degrade into different organic compounds such as aromatic acids, aldehydes, phenols and acetophenones. These can increase the risks of adverse health effects.
https://cosmeticelementreview.wordpress.com/2015/10/06/feature-our-essential-sunscreen/
https://www.skincancer.org/prevention/uva-and-uvb
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