There are 16 active ingredients approved by FDA . 14 of them are organic (“chemical”) and 2 of them are inorganic (“mineral”)
Sunscreen loses its efficacy as a function of both time and UV exposure, whether it is a “chemical” or a “mineral” sunscreen.
There are 16 active ingredients approved by FDA for sunscreen. 14 of them are organic (also called “chemical”) and 2 of them are inorganic (also called “mineral”). Sunscreen has been shown to wear off with time, as explained in the paper by Diffey and his colleagues. Whether “chemical” or “mineral”, sunscreen has also been shown to be unstable under UV exposure -- as demonstrated by Gonzalez and her colleagues.
Sunscreen is the go-to sun protection method these days and comes with the promise of protection from those harmful UV rays and painful sunburns. To top it off, a sunscreen has to meet FDA standards for sun protection factors (SPF) to be offered on the market. So what are you waiting for? Throw some on and slide out into the sun! Sunscreen has got you covered.
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There’s a but.
Directions for sunscreen? What could be so complicated? The thing is, while it may seem self-evident, what we are missing by skipping the label is that sunscreen is tested only at the thickness recommended by the FDA: 2mg for every square centimeter of skin. That’s enough to fill a shot glass! And unfortunately, most of us apply between one-fourth and one-half that amount. Our sunscreen products can’t protect as intended if we're only using half as much as we're supposed to!
Clinical trials show that even people who are sensitive to sunlight don’t wear enough sunscreen. People who are photosensitive often comment that sunscreen products are of little benefit, but showing them proper technique has proven to improve both coverage and satisfaction. Without it, most people tend to neglect many exposed areas, such as the back and sides of the neck, temples, and ears. In general, people tend to overestimate sunscreen's ability to protect us.
All is not lost. Further research has shown that all we need to do to drastically improve the performance of sunscreen is to apply another layer. Although manufacturers recommend reapplying sunscreen within 2-3 hours, sunscreen is twice as effective if you reapply within 20-30 minutes. Applying sunscreen right before sun exposure, and then again right after, will allow coverage that goes deeper than the skin’s surface grooves and maximizes its ability to protect.
Just as a rough wall needs a second coat of paint for sufficient coverage, the skin needs another coat of sunscreen for adequate protection. A layer of sunscreen may not be the sun-shield we had hoped for, but a second one just may be.