Buying sunscreen isn’t as straight-forward as you might think. All those bottles at the store seem to be clearly marked. They show the SPF. They list the ingredients. Pick one and you’re good, right? The higher the SPF the better, right?

Well, it turns out that marketing techniques actually make sunscreen labels pretty misleading. Here are 9 things you need to know before you buy your next bottle of sunscreen.

1. What SFP Really Means

   SPF stands for “Sun Protection Factor.” If a bottle of sunscreen has an SPF of 15, it means you can stay in the sun 15 minutes longer than you could without sunscreen before you get a sunburn. If the SPF is 30, you can stay in the sun 30 minutes longer than you could without sunscreen before you get a sunburn. Although this seems to be pretty simple math, it doesn’t work as SPF goes higher than 30.

2. SPF Higher Than 30 Means Very Little

   According to dermatologist Dr. Shari Sperling of Sperling Dermatology, “SPF of 30 protects against 97% of sun rays. Anything over 30 has little additive benefit.” Basically, there’s no point in paying double for an SPF of 50, which protects against 98% of sun rays, just 1% more than SPF 30.

3. There Are Two Types of Sunscreen

   There are chemical sunscreens which use chemicals to protect your skin from the sun. Then there are physical sunscreens which use ingredients like titanium or zinc to protect your skin from the sun. While there is not evidence that sunscreen causes cancer, anyone who is worried about the effects of chemicals in their sunscreen should opt for a physical sunscreen.

4. SPF in Your Makeup Is Not Enough

   Think you’re covered because your foundation has an SPF? Think again. There is not enough sunscreen in your makeup for the SPF to adequately protect your skin.

5. No Sunscreen Is Really Waterproof

   Just because you paid more for a “waterproof” sunscreen does not mean that it won’t rub up on your beach towel. Be sure to reapply sunscreen after swimming no matter what kind of sunscreen you use.

6. When to Apply Sunscreen

   I’ve seen it so many times at the park, at the beach and at amusement parks: parents applying sunscreen to their kids when they are already outside. Sunscreen is not effective the moment it is applied. Ideally, you should apply it 30 minutes before going out in the sun and reapply it every 2 hours.

7. Sunscreen Expires

   If there’s a sale on sunscreen, refrain from stocking up. There is an expiration date on those bottles of sunscreen, and it may not protect your skin after it has expired.

8. Everyone Needs Sunscreen

   Sunscreen is not just for pale people. While some people may say that they tan instead of burn, and other people may think that their skin tone is naturally dark enough that the sun won’t hurt them, everyone still needs sunscreen. Sunscreen protects us from UVA and UVB rays. UVB rays cause sunburns. UVA rays cause skin cancer and premature aging. Even if you don’t get a sunburn, the UVA rays may be damaging your skin if you don’t protect it.

9. How Much Sunscreen Do You Need?

   Every time you slather on the sunscreen, you need to use a shotglass amount of sunscreen. You need that same amount when you reapply sunscreen every 2 hours.