What Does SPF 100 Mean? Actually, not at all what you think.

If you’ve ever stood in the sunscreen aisle, you’ve probably seen bottles boasting SPF 50, 70, even 100. It’s tempting to think that higher is always better. But as a dermatologist and skin cancer surgeon with over two decades of experience, I can tell you: the SPF rating system is not what you think—and it’s far from perfect.

Let’s break down the real story behind that number on your sunscreen bottle.

1. SPF Only Measures UVB Protection—Not UVA

SPF, or Sun Protection Factor, is a rating based solely on how long it takes for your skin to turn red (a sign of UVB damage) when exposed to sunlight. It does not measure protection against UVA rays, which penetrate deeper, accelerate aging, and are just as responsible for skin cancer as UVB.

2. No Required UVA Rating in the U.S.

Here’s a major flaw: in the United States, there is no required product rating for how well a sunscreen protects you from UVA damage. This means a product labeled SPF 100 could be offering you very little defense against the rays that cause wrinkles, brown spots, collagen breakdown, and—most importantly—skin cancer.

3. Both UVA and UVB Are Harmful

Both types of ultraviolet radiation—UVA and UVB—cause sunburns, skin cancer, premature aging, and even bruising (solar purpura). Yet, the SPF number only tells you about UVB. That’s like locking your front door but leaving the back door wide open.

4. Most of the Sun’s Rays Are UVA

Here’s the kicker: 85% of the ultraviolet radiation that reaches the earth’s surface is UVA. So, if your sunscreen isn’t blocking UVA, you’re missing the majority of the threat.

5. Chemical Sunscreens: Environmental and Health Concerns

Many chemical sunscreen ingredients—like oxybenzone and octinoxate—are not only less effective at blocking UVA, but they also harm coral reefs and aquatic life. Some have even shown cancer-causing effects in laboratory animals. This is not the kind of “protection” I want for my patients or the planet.

My 22 Years’ Worth of Meaningful Skin Protection Recommendations:

1. Choose Mineral-Based Sunscreens (Zinc Oxide):
Look for zinc oxide as the active ingredient, with an SPF of 30-50. Zinc physically blocks both UVA and UVB, offering true broad-spectrum protection. And just as important: pick a formula that feels good on your skin, so you’ll actually use it.

2. Nourish and Hydrate Your Skin:
Start and end your day with products that build your skin barrier and neutralize oxidative damage. Antioxidants, hydration, and barrier repair are your best friends for long-term skin health.

3. Cover Up:
No sunscreen is perfect. Wear protective clothing, a full-brimmed hat, and sleeves when driving. Don’t be afraid to use a sun umbrella if you’re going to be out for long periods.

Bottom Line:

SPF 100 is not a magic shield. It’s a misleading number that gives a false sense of security. Real protection comes from understanding what’s in your sunscreen, how it works, and how you use it. Be smart, be safe, and don’t get skin cancer.