Shelves full of lotions, sticks, sprays. Does it really matter which sunscreen you grab?

Turns out, it does. For one thing, a recent study revealed that popular doesn't mean protective.

According to a July study in the JAMA Dermatology journal, 40 percent of the best-rated sunscreens on Amazon didn't meet guidelines of the American Academy of Dermatology. The less-impressive sunscreens didn't resist water or sweat well, according to doctors at the Northwestern University Feinberg School of Medicine.

And what about that $24-an-ounce bottle? The study found prices varied up to 3,000 percent — among products that provided the same protection.

What else to consider, from chosen activity to bottle blurbs to seek out:

That number: Do you really need the ever-increasing SPF numbers? The American Academy of Dermatology recommends 30 or higher. Its data show that sunscreen with SPF 15 filters 93 percent of the UVB rays. At SPF 30, that jumps to 97 percent. There's no SPF that filters 100 percent of rays. That's why doctors also encourage finding shade and wearing a hat.

Waterproof sunscreen is not a thing: Yep, you read that right. The FDA prohibits sunscreens from saying “waterproof” or “sweatproof” because no such product exists — sweat and water wash away sunscreen. So don't believe a bottle that promises those words, although some might say “water resistant” or “sport.” That means it stays effective for 40 to 80 minutes in the water.

Is baby sunblock really that different? If a bottle says “baby,” it generally means the sunscreen contains only titanium dioxide and zinc oxide as active ingredients, which are less likely to irritate children's skin, says the American Academy of Dermatology.

Consider your plans: Yes, you should choose a different sunscreen for working out versus lazing at the beach.

“Active” or “sport” sunscreens are usually designed to be more resistant to sweat.

For example, Neutrogena's sport face sunscreen doesn't have that tingling-eye problem when sweat combines with sunscreen, and it goes on pretty lightly.

Look for this on the label: “Broad spectrum.” It's a label the U.S. Food and Drug Administration has defined, and it means that the sunscreen protects from both UVA and UVB rays. In other words, this is the protection that helps prevent skin cancer, aging and, of course, sunburn, according to experts.

Is it OK to shop in the clearance aisle?

The American Academy of Dermatology points out that the FDA requires sunscreens to maintain their original strength for at least three years.

So throw it out — or don't buy that clearance-aisle lotion — if it's past the expiration date on the bottle.

And it recommends writing the purchase date on the bottle, so you remember next summer. Any change in color or consistency? Toss it.

And about that bottle combining bug spray and sunblock sunscreen. think again.

The American Academy of Dermatology doesn't recommend combining the two, because you should be applying (and reapplying) sunscreen far more often than you need the bug spray.

The Mayo Clinic, for example, says to apply insect repellent no more than every six hours. You should apply sunscreen three times during that period.

Spray, stick or lotion? The Mayo Clinic lays out pros and cons.

A stick might be easy to apply around the eye, or a gel might help with a hairy chest.

Lotions are easy for large applications.

And if you use a spray, stay away from the wind — spraying your limbs in the wind might not result in full coverage. With spray, the American Academy of Dermatology suggests spraying into your hands, then applying, to avoid inhaling fumes.

Final thoughts. Of course, apply sunscreen liberally, and put it on at least 15 minutes before you go outdoors.

The American Academy of Dermatology recommends a blob the size of a shot glass. Reapply every two hours.