Understanding SPF: What the Numbers Actually Mean

SPF, or Sun Protection Factor, measures how well a product protects skin from UVB rays, the primary cause of sunburn. An SPF 30 product, when applied correctly, filters out roughly 97% of UVB rays, while SPF 50 filters roughly 98%. The difference is smaller than the numbers suggest, which is why consistent, generous application matters more than chasing the highest SPF available.

It's also worth understanding that SPF only measures UVB protection. Look for "broad-spectrum" on the label, which indicates the product also protects against UVA rays, linked to premature ageing and deeper skin damage. Reapplication is just as important as the number itself. Most sunscreens lose effectiveness after two hours, or sooner after swimming or sweating, so timing matters as much as the formula you choose.

Finally, application technique is critical. Most people apply only a quarter to half of the recommended amount. To get the SPF on the label, you need to apply approximately two milligrams of sunscreen per square centimeter of skin. For the face and neck, that's about a nickel-sized amount. Don't forget often-missed areas like the ears, back of the neck, and the tops of your feet.