When you shop for sunscreen, what do you look for? Most people prioritize the SPF rating, which they perceive to mean as the amount of protection a product offers.

And it's true that SPF ratings explain how much of the sun's rays a formula can block. The thing is, most sunscreen rubs or sweats off before those numbers make a difference. So, no matter the strength of your sunscreen, applying (and reapplying) properly is the most important factor for healthy skin.

Which means there's more to your bottle of sunblock than the biggest numbers on the label. In fact, there are a few things that dermatologists recommend looking out for when buying sunscreen. To keep your skin healthy and protected from the sun, look out for these factors.

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Choose Products That Are Non-Comedogenic

Non-comedogenic basically means that a product aims to let your pores breathe. Unfortunately, a lot of sunscreens are greasy and sticky, clogging up your skin and keeping it from breathing and even sweating. But this is the number-one piece of advice from dermatologists: keep your skin clean and let it breathe. Even if you're applying acne products or oil-blotting formulas, you still need to go bare-faced often so your skin can heal.

So skip sunscreen products with known comedogenic ingredients like cocoa butter, coconut oil, soybean oil, and other "oily" components if you have problem skin, to begin with. Ladies and gents with dry skin might benefit from the moisturizing features of such ingredients, but proceed with caution or you might wind up with a mid-vacay breakout.

Don't Trust "Waterproof" Or "Sweatproof" Products

Every sunscreen type, strength, and variety must be reapplied regularly. There's really no such thing as a waterproof sunscreen, says the American Academy of Dermatology (AAD). Some products will last longer than others, sure, even if you're sweating or swimming. But the maximum length of time you should go between applications is 80 minutes.

You should also reapply after swimming regardless, or if you're sweating profusely. Toweling off is another reason you'll need to reapply, even if you don't feel like your skin is bare. Better to be safe than sorry, derms note.

Skip The Spray-On Types (Unless You Want To Be Crispy)

Dermatologists want you to apply your sunblock properly. And that doesn't mean a light spritz the way you would put on perfume. While spray-on sunblocks are great because they don't get your hands all gross, a light dusting isn't enough to protect your skin. You need about an ounce of sunblock, says the AAD—that's about a shot glass' worth—to cover your entire body.

Of course, you also have to rub the sunscreen in and let it soak into your skin. If you dip in the water too soon, it will slide right off. And if you step out from under the shade too prematurely, you could even burn before the sunblock sets in.

Think About Ray Types When Reading Labels

A lot of sunblock products advertise UV protection. But there are two types of UV rays: ultraviolet A (UVA) and ultraviolet B (UVB). Both types can cause skin cancer, say dermatologists. UVA rays primarily contribute to skin aging. UVB rays are the ones that cause burns most often. Plus, UV damage is cumulative, says Skincancer.org, which means the more times you burn or leave your skin exposed, the higher your odds of developing skin cancer and other issues.

For the best protection, you should pick a formula that is "broad spectrum"—that means it protects against both types of sun damage. Then again, derms also recommend staying out of the sun during the hottest times of the day and generally whenever the rays are super bright. There are other types of UV rays that can also be damaging, and long-term exposure to the sun can cause other health problems (like dehydration!).

Plus, it's still not known whether sunscreens are 100 percent effective in preventing skin cancer. Only time will tell whether our magic formulas offer true protection while we're lounging on the beach.

Avoid Labels That Feature Ingredients With Unknown Safety Levels

The Environmental Working Group (EWG) ranks the safety of consumer products, covering mostly cosmetics, personal care products, soaps, and other household items. They also rank the safety of sunscreens, and their rankings are based on the safety of the individual ingredients in each formula.

But as the EWG explains, the Food and Drug Administration says there's really only enough information to definitively rate two sunblock ingredients for safety. Those ingredients are zinc oxide and titanium dioxide. Other ingredients—like oxybenzone or chemical filters—don't have enough research on the books to recommend them (or not).

Basically, skincare and food and drug experts say that you should aim for products with as few ingredients as possible, prioritizing those with skin-protecting ingredients like zinc oxide and titanium dioxide.