Frequently Asked Questions About Sunscreen

Hello, my name is Dr. Thomas Knackstedt, and I’m a board-certified dermatologist and board-certified, fellowship-trained Mohs surgeon at Peak Skin Center. In this video, I want to talk a little bit about sunscreen.

There are many questions about how sunscreen works, what kind you should be using, and whether you really need to reapply it. I thought it would be easiest to walk through all of that in a short video.

To understand sunscreen, you first need to understand how the sun affects the skin. In addition to visible light, ultraviolet rays from the sun reach our skin. These rays include ultraviolet A, or UVA, and ultraviolet B, or UVB. They have slightly different wavelengths. You can think of UVA as being primarily responsible for skin aging, and UVB as being primarily responsible for sunburn. Together, UVA and UVB contribute to the development of skin cancer.

When you look at a bottle of sunscreen, there are several things you’ll notice. Some of the information is regulated by the FDA and required on the label, while other terms are not regulated and can be harder to interpret.

The most obvious thing to look at is the SPF rating. SPF stands for sun protection factor, and it refers specifically to how much ultraviolet B, or UVB, light you’re protected from. SPF does not measure UVA protection.

An SPF 15 sunscreen blocks about 93% of UVB rays, while SPF 30 blocks about 97% of UVB rays. There is no sunscreen that provides 100% protection. That’s why additional measures, such as sun-protective clothing, can be very helpful in protecting your skin.

Sunscreens generally fall into two categories: physical blockers and chemical sunscreens. Physical blockers typically contain zinc oxide or titanium dioxide. If you look at the back of the bottle, these ingredients should be listed under the active ingredients. Physical sunscreens are also known as mineral sunscreens.

If a sunscreen does not contain zinc oxide or titanium dioxide, then it is considered a chemical sunscreen.

There are also several terms you may see on sunscreen labels that are not strictly regulated. For example, there is no such thing as a completely waterproof sunscreen. Any sunscreen will eventually wash off with prolonged exposure to water. The FDA does allow sunscreens to be labeled as “water resistant” for a specified amount of time, which can be helpful if you’re spending a lot of time outdoors.

Labels such as “sport sunscreen” or “baby sunscreen” are not formally defined. Often, baby sunscreens contain ingredients that are better suited for sensitive skin and tend to rely more heavily on physical blockers like zinc oxide or titanium dioxide. These products are often a good option for people with sensitive skin.

Regardless of the type of sunscreen you use, reapplication is very important. Many people are good about applying sunscreen before going outside, but reapplying it—especially during activities like golfing or swimming—can be easy to forget. In general, sunscreen should be reapplied every two hours, or sooner if you’ve been swimming, showering, or drying off with a towel.

I hope this has helped you better understand what you’re seeing on your sunscreen bottle and how to choose and use sunscreen more effectively.