Sunscreen, SPF, and UV: What I Actually Tell My Patients

Every spring, my patients start asking the same questions. Is SPF 30 enough, or should I get SPF 100? Mineral or chemical? How often do I really need to reapply? And the most common one: I only got a little pink, that's not really a sunburn, right?

The sunscreen aisle is confusing, and there are strong opinions online about every ingredient. Here's the version I'd give to a patient who lives in San Diego and spends real time outdoors.

Why It Matters More Here

San Diego's climate is one of the reasons people move here, and it's also the reason UV exposure adds up faster than most people realize. Coastal mornings can be cool, overcast, and breezy, which makes the sun feel mild. Patients tell me all the time that it wasn't even hot that day, but heat and UV are different. UV gets through clouds. You can pick up real sun damage on a 68-degree foggy morning at La Jolla Cove or a gray afternoon in Carlsbad.

Sun damage is also cumulative. The brief exposures most people don't bother protecting against, school pickup, a coffee walk, the drive to work with your arm in the window. add up over decades.

What SPF Actually Means

SPF measures how well a sunscreen blocks UVB, the rays primarily responsible for sunburn. It is not a measure of how long you can stay in the sun, even though the labels often imply it.

SPF 30 blocks about 97 percent of UVB. SPF 50 blocks about 98 percent. The improvement from 30 to 100 is smaller than the number on the bottle suggests. What I recommend is broad-spectrum, water-resistant sunscreen with SPF 30 or higher. Broad-spectrum means it also protects against UVA, which drives photoaging and contributes to skin cancer risk. A high SPF without broad-spectrum protection is doing half the job.

The Application Problem

Most adults apply about a quarter to a half of the sunscreen they should be using, so the SPF 50 you think you're wearing is functionally closer to SPF 15. For an adult, you need about an ounce to cover the usual exposed areas. Face and neck alone need about a teaspoon.

Then there's reapplication. Sunscreen is not a one-and-done product. Reapply every two hours when you're outdoors, and immediately after swimming, sweating, or toweling off. Even water-resistant sunscreen breaks down with time and friction.

Beyond the Beach

Sun protection is not just for beach days. The cumulative exposure that drives most skin aging and skin cancer risk doesn't come from two annual vacations. It comes from daily exposure: driving, walking the dog, lunch on a patio, watching a kid's soccer game. A daily facial sunscreen — broad-spectrum, SPF 30 or higher, applied every morning — is one of the highest-return habits I recommend to almost any adult.

The Bottom Line

Sun damage is one of the most preventable health problems we deal with, and sunscreen is one of the simplest tools we have. The patients who do well with this are the ones who made sun protection a default. They use it on cloudy days, in winter, in fall. Over a lifetime, that habit shows up in the mirror and in the conversations we don't have to have in my office.

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