How is the SPF of Sunscreen Determined?

I’m someone with fair skin. Extremely fair skin. Which means during these scorching summer days, I make sure to always have sunscreen with me and apply it generously. But I’ve never fully grasped the meaning behind those SPF numbers, so I reached out to some sun care experts to explain it and to show me the best ways to apply sunscreen, ensuring I can enjoy the summer without turning into a lobster.

Soaking in the sunshine ... the safe way

SPF, or Sun Protection Factor, represents a sunscreen’s effectiveness in blocking UVB rays. This concept was first developed at the Coppertone Solar Research Center in 1972. By 1978, the FDA introduced an SPF testing method based on Coppertone’s approach, as explained by Dr. David Leffell, chief of Dermatologic Surgery and Cutaneous Oncology at Yale.

The SPF number represents the approximate duration a person can stay in the sun without burning after applying sunscreen. For instance, if you typically burn after 20 minutes of sun exposure without sunscreen, applying SPF 30 would theoretically allow you to stay out 30 times longer—around 600 minutes or 10 hours. However, this is not an exact science, as factors like cloud cover, time of day, and the reflection of UV rays from the ground all influence how much UV light reaches you. Hence, it's generally advised to reapply sunscreen every two hours (or more often).

A sunscreen’s higher SPF is determined by its formulation. Dr. Patricia Agin, president of Agin Suncare Consulting, explains, "An SPF 50 might contain slightly more of one or more active ingredients to achieve that higher number. Alternatively, it could contain an additional active ingredient to enhance SPF performance and reach SPF 50."

Regardless of the SPF, improper application can still result in a burn. Let’s now dive into the correct way to apply sunscreen.

How to apply sunscreen

Start with a water-resistant, broad-spectrum sunscreen that protects against both UVB and UVA radiation, and has an SPF of at least 30. Dr. Leffell advises, "Generally, you don’t need sunscreen with an SPF higher than 30 unless you have particularly sun-sensitive skin." (People like redheads, those with light eyes, or anyone who turns pink after just a few minutes of sun exposure—should go for an SPF above 30.)

About twenty minutes before heading to the beach or pool, begin applying sunscreen in a smooth, even layer. Dr. Leffell advises, "Don’t apply it like icing on a cake. I often see patients with thick, uneven sunscreen on their ears, and that’s a sign of improper application." Sunscreen sprays are a great option for achieving that even coat.

Whether using lotion or spray, Dr. Leffell recommends starting with your scalp and face, even if you plan to wear a hat. “Make sure to cover your ears, nose, and under your eyes,” says Leffell. “Then move on to your shoulders and ensure someone helps apply sunscreen to your back where you can’t reach.”

Other commonly overlooked areas that are important to cover include the tops of your feet, the backs of your hands, and your chest. Leffell adds, “It’s become a significant concern for women as they age, especially the chest area. Despite using various treatments for the face, the chest is often neglected, leading to a mismatch between the face and chest appearance as they age.” Sunscreen can help prevent this.

A crucial thing to remember: Water-resistant doesn’t mean waterproof. Leffell suggests, "Always reapply sunscreen every couple of hours when outdoors, and after swimming or sweating heavily, no matter what the label says about being water-resistant."

The easiest way to tell if you’ve applied sunscreen correctly is simple: “You know you’re doing it right if, after applying it the first time, you haven’t experienced a burn,” says Leffell.

Agin provides an important note: "It’s unwise to view sunscreens solely as a means to prolong your time in the sun," she explains. "You must also recognize that even before you burn, your skin is exposed to UV radiation that causes other forms of damage. After 600 minutes, you’ll have accumulated enough UV exposure to cause a sunburn—one Minimal Erythema Dose (MED). However, before that point, skin cells’ DNA and the skin's collagen and elastin structure are already undergoing invisible damage, even without visible burning. This type of harm can occur without any sunburn, so focusing only on sunburn doesn’t fully measure the extent of the damage to your skin."

A previous version of this article was published in 2014.