The high numbers found on sunscreen bottles, such as SPF 70 or SPF 100, often lead to consumer confusion. Many people assume that a higher number provides a proportionally greater level of sun protection. To make an informed choice, it is helpful to understand the science behind the Sun Protection Factor (SPF) rating and how it truly translates to defense against ultraviolet radiation. This breakdown reveals whether higher SPF values offer a significant, measurable benefit for daily sun exposure.
Understanding the SPF Rating System
The Sun Protection Factor (SPF) quantifies a sunscreen’s ability to protect the skin from ultraviolet B (UVB) rays, which are the primary cause of sunburn. Scientists determine this number by comparing the amount of UV exposure required to cause skin redness with sunscreen applied versus without it. This comparison is conducted in a controlled laboratory setting, where a precise amount of product is applied to human volunteers.
The resulting SPF number represents a theoretical multiplier of the time a person can stay in the sun before burning, compared to unprotected skin. For example, if a person typically burns after 10 minutes, an SPF 30 product would theoretically extend that time by a factor of 30, to 300 minutes. This calculation is based on ideal lab conditions, which are rarely replicated in real-world application.
The Protection Plateau: SPF 70 Versus Lower Numbers
The protection provided by SPF ratings does not increase linearly, meaning the benefit of higher numbers quickly diminishes. Sunscreen with an SPF 30 rating filters out approximately 97% of UVB rays. Increasing the protection to SPF 50 raises the filtering capacity slightly, to about 98% of UVB rays.
The difference between SPF 50 and SPF 70 is even smaller, with SPF 70 blocking an estimated 98.6% of UVB rays. Boosting the SPF from 50 to 70 only adds about 0.6 percentage points of protection. This negligible increase demonstrates the plateau effect, where the added benefit of using an extremely high SPF number becomes minimal.
Real-World Efficacy: Application and Coverage
The SPF number offers limited real-world protection if the product is not applied correctly. Most people apply significantly less than the amount used in laboratory testing, which immediately compromises the labeled SPF value. For full-body coverage, an adult should use roughly one ounce of sunscreen, an amount equivalent to a standard shot glass.
Proper coverage also requires consistent reapplication, regardless of the SPF number. Sunscreen should be reapplied at least every two hours, or immediately after activities like swimming or heavy sweating. A higher SPF number does not extend the time between these necessary reapplications.
For comprehensive defense, select a product labeled “Broad Spectrum,” which guards against both UVB and ultraviolet A (UVA) rays. While UVB rays cause sunburn, UVA rays penetrate deeper into the skin. UVA rays contribute to premature aging and long-term damage. Since the SPF number only measures UVB protection, the Broad Spectrum label is a necessary indicator of balanced defense.
Why High Numbers Exist
High SPF numbers exist largely due to consumer psychology, where people are conditioned to believe that a larger number signifies a proportionally better product. Companies capitalize on the misunderstanding of the protection plateau, knowing that many consumers will choose SPF 70 over SPF 30, assuming it provides twice the defense. This marketing strategy is effective even though the scientific difference is tiny.
Some global regulatory bodies, recognizing this misleading effect, cap the maximum labeled value at SPF 50+. Regulators are concerned that SPF values exceeding 50 are inherently misleading because they provide a false sense of security. This false confidence can lead users to stay in the sun longer or reapply less frequently, inadvertently increasing their overall UV exposure risk.