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When you buy a sunscreen, what makes you buy one brand over another?

For many people, the reasons they choose aren鈥檛 the ones most likely to keep them safe from the sun.

In a in the journal JAMA Dermatology, researchers looked at the top 1% of sunscreens being sold on Amazon.com as of December 2015 (65 out of 6,500). By 鈥渢op,鈥� I mean the ones that had the highest number of reviews and got at least 4 out of 5 stars. When they looked at the brands, they found that a full 40% of those top 65 didn鈥檛 meet the sunscreen guidelines of the American Academy of Dermatology (AAD).

The AAD recommends that sunscreens:

• Be broad spectrum (meaning they protect you from both UVA and UVB rays).
• Have an SPF (sun protection factor) of at least 30, which blocks about 97% of the sun鈥檚 rays (note: there is no evidence that anything above an SPF 50 protects better than SPF 50).
• Be water resistant, so that they are more likely to stay on if you get sweaty or go in the water.

The whole point of sunscreen is to, well, screen you from the sun 鈥� and prevent not just sunburn and premature signs of aging, but also skin cancer. If you get five sunburns in your lifetime, you will . This can be particularly important in children, as sunburns early in life can . So when choosing a sunscreen, it鈥檚 important to choose one that will best screen you 鈥� and your child 鈥� from the sun.

But in the study, that wasn鈥檛 the top reason people chose sunscreen. When the researchers analyzed the comments, they found that what was most important to buyers was what the researchers called 鈥渃osmetic elegance,鈥� that is, how well the sunscreen rubbed in to the skin, and how it felt on the skin. That was rated more important than how the sunscreen actually worked.

Now, looking all white and streaky, or feeling sticky, are clearly not that desirable 鈥� and the researchers did comment that sunscreen manufacturers should try to increase the 鈥渃osmetic elegance鈥� of their products so that people were more likely to use them. But given that it is estimated that there will be , and that 10,130 people will die of the disease, it鈥檚 important to keep perspective on what is truly desirable.

It鈥檚 important not just to choose the right sunscreen, but also to use it the right way. Here are :

• Apply sunscreen before you go outside. It can take about 15 minutes for it to be absorbed into the skin 鈥� if you wait until you get outside, you will be unprotected for that time.
• Use enough. You need about an ounce of sunscreen (roughly the amount in a shot glass) to cover your skin.
• Cover all the bare skin. This is where a lot of people mess up. Remember your back, the back of your arms, legs, ears and neck, the tops of your feet. For hard to reach places, you may need help or a spray (never spray near the eyes or mouth 鈥� inhaling it is not a good idea). Here鈥檚 a tip for parents of small children: put the sunscreen on when they are naked, before you put on their suit. For people with thinning hair (or babies who don鈥檛 have much hair yet), use a broad-brimmed hat.
• Reapply at least every 2 hours, sooner after swimming or sweating a lot.

So choose carefully 鈥� and use carefully. It can literally save lives.