We asked a dermatologist the most common questions about SPF -- here's what she said

We all know how important protecting our skin from the sun during the summer is -- but there are a few big questions that have us scratching our heads each year when it comes to SPF.

Everyone wants to be able to enjoy the sunshine safely, so we turned to a board-certified dermatologist, Dr. Rita Linkner of Spring Street Dermatology, to find out what's the real deal around different sunscreens.

AOL: Is there a big difference between SPF 15 and SPF 50? What do the different numbers actually indicate?

Dr. Linkner: The most important part of a sunscreen label is the SPF or the sun protection factor level which gives you the interval time frame of which the sunscreen will prevent you from developing a burn from UVB light. The most simple way of thinking of it is an SPF 15 filters out 93 percent of UVB light while a SPF 30 filters out 97 percent of UVB light.

I always recommend a SPF level at or around an SPF 50, as that's the magical number where you are maximizing your UVB protection while using a formulation that is easy enough to rub in and will promote you to use the right quantity of sunscreen, liberally. As SPF levels increase beyond 50, the formulations tend to be thicker and leave a chalkier appearance, and most people will under compensate that white chalkiness by not utilizing the right amount of sunscreen with each application. Broad spectrum refers to a sunscreen that blocks both UVA and UVB light. UVA is responsible for aging the skin. UVB light is responsible for burning the skin. The ++s refer to UVA protection.

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Which ingredients should we look for when shopping for SPF? Are there any ingredients we should avoid?

I prefer mineral based sunscreens like Colorscience Sunforgettable Total Protection SPF50 and Alastin's Hydratint SPF 36 that contain physical blockers also known as mineral sunscreens, which use titanium and zinc oxide as the active ingredients. Mineral sunscreens are less irritating than chemical ones!

When looking for a sunscreen, choose products that are paraben-free, oil-free, and fragrance-free. Avoid sunscreens that integrate insect repellent -- sunscreen should be applied frequently, every two hours, whereas insect repellent is used much more sparingly than that.

Does sunscreen really expire?

Sunscreens are formulated to last up to 3 years, however if they are exposed to light or heat, they lose their effectiveness -- like food, sunscreens can expire.

First, never use a sunscreen after the expiration date. If there is a change in consistency or smell to the formula, it is time to throw it away. Once opened, sunscreen generally expires faster than the date on the tube. I recommend my patients throw out their sunscreen after they’ve used it for a season as you should be using it all year long not just in the summertime.

Should I be using a different sunscreen on my body and on my face?

Yes, as sunscreens for the face differ from sunscreens formulated for the body in that they can be formulated in a way that's oil free and calming so as to not exacerbate acne and clog pores.

If you're in New York, Dr. Linkner has two offices in NYC!

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