SPF Doesn't Mean What Most People Think It Means
When shopping for sunscreen, it's easy to assume that a higher SPF number means dramatically better protection.
SPF 100 must be twice as good as SPF 50, right?
Not exactly.
What SPF Actually Measures
SPF stands for Sun Protection Factor, and it measures protection against UVB rays—the type of ultraviolet radiation primarily responsible for sunburn.
What SPF doesn't tell you is how well a sunscreen protects against UVA rays, which are linked to premature aging, pigmentation, and long-term skin damage.
That's why broad-spectrum protection is just as important as the SPF number itself.
The Surprising Difference Between SPF Numbers
Many people are surprised to learn how small the difference is between higher SPF values.
- SPF 30 filters about 97% of UVB rays
- SPF 50 filters about 98%
- SPF 100 filters about 99%
While higher SPF can provide additional protection, the increase is much smaller than most people expect.
How Sunscreen Is Tested to Calculate SPF
In laboratory testing, sunscreen is applied at 2 milligrams per square centimeter of skin (2 mg/cm²), the standardized amount used in SPF testing, and then exposed to controlled UV light. Researchers then measure how much longer protected skin takes to burn compared to unprotected skin.
The catch? Most people don't apply nearly as much sunscreen as the laboratory testing requires. Add in sweating, swimming, towel drying, and everyday wear, and real-world protection is often lower than the number on the bottle suggests.
The Secret Formula
SPF is calculated by comparing how much UV radiation causes redness on sunscreen protected skin versus unprotected skin. During testing, researchers identify the minimal erythema dose (MED), which is the smallest amount of UV exposure that produces noticeable redness. The formula is: SPF = MED on protected skin ÷ MED on unprotected skin. For example, if protected skin reddens after 500 units but unprotected skin reddens after 10 unites, the sunscreen would have an SPF of 50.
However, broad spectrum is a different testing, called critical wavelength testing. It tests the wavelength range that a particular active ingredient protects against, for example UVA and UVB. Want to learn more about the different wavelengths? → https://sunmudsunscreen.com/blogs/news/can-you-tan-while-wearing-sunscreen
What Matters Most
The biggest sunscreen mistake usually isn't choosing the wrong SPF. It's not applying enough.
Using a broad-spectrum sunscreen, applying it generously, and reapplying throughout the day are often more important than chasing the highest SPF number available.
The SUNMUD Difference
At SUNMUD, we use non-nano zinc oxide to provide broad-spectrum protection against both UVA and UVB rays. Our formula also tests SPF after being soaked for 80 minutes to ensure the water resistance is the best it can be. Because effective sun protection isn't just about a number on the label—it's about helping protect your skin from the full spectrum of UV damage.
The Bottom Line
SPF is important, but it only tells part of the story.
A higher SPF doesn't automatically mean dramatically better protection, and even the highest SPF won't work as intended if you don't apply enough.
The best sunscreen is the one you apply correctly, reapply regularly, and actually enjoy using.
Protect your skin with broad-spectrum mineral protection from SUNMUD → https://sunmudsunscreen.com/?srsltid=AfmBOoqOEXkrf1Gx7_QfCo5UzAlWfHjxIiql2AJrZMcanhukFLrA6YlX
References
Diffey, B. L. (2002). What is SPF? Photodermatology, Photoimmunology & Photomedicine, 18(4), 159–160. https://doi.org/10.1034/j.1600-0781.2002.018004159.x
European Commission. (2006). Commission Recommendation of 22 September 2006 on the efficacy of sunscreen products and the claims made relating thereto (2006/647/EC). https://eur-lex.europa.eu/legal-content/EN/TXT/?uri=CELEX:32006H0647
International Organization for Standardization. (2024). ISO 23675:2024 Cosmetics—Sun protection test methods—In vitro determination of sun protection factor (SPF).
International Organization for Standardization. (2024). ISO 23698:2024 Cosmetics—Sun protection test methods—Measurement of sunscreen efficacy by diffuse reflectance spectroscopy.
U.S. Food and Drug Administration. (2021). Sunscreen: How to help protect your skin from the sun. https://www.fda.gov/drugs/information-drug-class/sunscreen-how-help-protect-your-skin-sun
U.S. Food and Drug Administration. (2021). Questions and answers: FDA announces new requirements for over-the-counter (OTC) sunscreen products marketed in the U.S. https://www.fda.gov/drugs/understanding-over-counter-medicines/questions-and-answers-fda-announces-new-requirements-over-counter-otc-sunscreen-products-marketed-us
Stanfield, J. W. (2018). Understanding Sunscreen Testing and Efficacy. Journal of Cosmetic Dermatology.
