July 02, 2018
Radiation energy emitted from the sun is a form of ultraviolet rays (UV). There are two types of rays that reach earth, UVA and UVB.
UVA rays (the dominant tanning ray) have a longer wavelength and can penetrate two layers beneath the skin into the dermis. UVA have long been known to play a major part in skin aging and wrinkling (photoaging). Recent studies, however, show that UVA damages skin cells in the basal layer of the epidermis, where most skin cancers occur.
UVB, the chief cause of skin reddening and sunburn, tends to damage the skin's more superficial epidermal layers. It plays a key role in the development of skin cancer and a contributory role in tanning and photoaging.
Broad spectrum sunscreens protect against both UVA and UVB rays; however, SPF only measures a sunscreen’s ability to filter UVB rays.
SPF is a measure of how much solar energy (UV radiation) is required to produce sunburn on protected skin (i.e., in the presence of sunscreen) relative to the amount of solar energy required to produce sunburn on unprotected skin. It is not directly related to time of solar exposure but to amount of solar exposure.
It is a common mistake to assume that the duration of effectiveness of a sunscreen can be calculated simply by multiplying the SPF by the length of time it takes to suffer a burn without sunscreen, because the amount of sun exposure a person receives is dependent upon more than just the length of time spent in the sun.
Generally, it takes less time to be exposed to the same amount of solar energy at midday compared to early morning or late evening because the sun is more intense at midday relative to the other times. Solar intensity is also related to geographic location, with greater solar intensity occurring at lower latitudes. Because clouds absorb solar energy, solar intensity is generally greater on clear days than cloudy days. Additionally, other factors such as your skin type, amount of sunscreen applied and reapplication frequency influence the amount of solar energy exposure.
Sunbathers often assume that they get twice as much protection from SPF 100 sunscreen as from SPF 50. The SPF scale is not linear and in reality, the extra protection is negligible. Properly applied SPF 50 sunscreen blocks 98 percent of UVB rays; SPF 100 blocks 99 percent. When used correctly, sunscreen with SPF values in the range of 30 to 50 will offer adequate sunburn protection, even for people most sensitive to sunburn.
Mineral sunscreens are made with zinc oxide and titanium dioxide. These minerals sit on the surface of your skin and physically reflect sunlight away from the skin. They can be thick and take a little longer to rub in but mineral sunscreens offer many benefits:
Chemical sunscreens absorb UV rays into the skin. Then, by a chemical reaction, transform UV rays into heat and release that heat from the skin. While chemical sunscreens are easier to apply than mineral sunscreens, they have many downsides:
Use a broad spectrum mineral sunscreen.
Stay away from aerosols, you are likely not applying a thick enough coating and it is not something you want to inhale.
Reapply every 2 hours for maximum protection. Everyday activities, friction with clothing, water sports, and sweat can all reduce the amount of sunscreen on your skin.
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