The question of whether an SPF 70 sunscreen is significantly better than an SPF 50 is a common source of confusion for consumers. Sun Protection Factor (SPF) is the standard measure of a sunscreen’s ability to protect the skin from ultraviolet B (UVB) rays, which are the primary cause of sunburn. Higher numbers suggest greater protection, but the relationship between the number and the actual benefit is not a simple linear scale. Understanding what the SPF number truly represents is the first step in clarifying this difference.
The Meaning Behind SPF Numbers
The SPF rating is a laboratory measure indicating the relative amount of time it would take for UVB rays to redden skin protected by sunscreen compared to unprotected skin. For example, if your bare skin would typically burn in 10 minutes, an SPF 30 product theoretically allows you to stay in the sun for 30 times longer, or 300 minutes, before burning. A more direct way to understand the SPF number is by the percentage of UVB rays it filters out. An SPF 30 sunscreen blocks approximately 97% of UVB radiation, while moving up to an SPF 50, the filtration increases slightly to about 98% of UVB rays. The protection does not increase proportionally with the SPF number because the scale is non-linear. This concept of diminishing returns is where the comparison between SPF 50 and 70 becomes particularly relevant.
Marginal Gains and Practical Protection
An SPF 50 sunscreen blocks roughly 98% of UVB rays, while an SPF 70 sunscreen blocks approximately 98.5% to 98.6% of those same rays. This difference suggests the higher number offers a negligible gain in protection for the average user. This slight increase is often why regulatory bodies in some countries have considered capping SPF labeling at 50 or 60. The concern is that very high SPF numbers, such as 70 or 100, can mislead consumers into believing they are receiving significantly more protection. This false sense of security may cause people to stay in the sun much longer or skip reapplication, ultimately increasing their overall UV exposure. The primary benefit of a higher SPF like 70, in practical terms, may be to compensate for the fact that most people do not apply enough sunscreen. By starting with a higher number, the user might still achieve an effective level of protection closer to the intended SPF 50, even with a lighter application.
Factors That Matter More Than High SPF
The effectiveness of any sunscreen is far more dependent on how it is used than the final digit on the label. The most significant factor is the quantity applied to the skin. To achieve the labeled protection, a person needs to apply about two milligrams of sunscreen per square centimeter of skin, which translates to a full ounce (a shot glass full) for the entire body. Most people apply only a fraction of this amount, often achieving a real-world SPF that is only one-third to one-half of the number printed on the bottle.
Reapplication is another factor that overrides the importance of a high SPF number. Sunscreen ingredients break down, rub off, or are diluted by sweat or water, requiring reapplication every two hours, or immediately after swimming or heavy sweating. Finally, the protection must be “Broad Spectrum,” an indication that the product guards against both UVB rays (which cause sunburn) and UVA rays (which cause photoaging and contribute to skin cancer). The SPF number measures only UVB protection, making the presence of broad-spectrum coverage a more significant indicator of overall skin health benefit than the difference between 50 and 70.