Sunscreen is one of the few cosmetic categories regulated as a medicine or over-the-counter drug in several markets, which means its central claim is defined by a test rather than by a marketing department. Understanding what that test measures explains why the difference between two high SPF numbers is smaller than it looks.
What SPF measures
The sun protection factor is derived from a standardised laboratory test of how much longer treated skin takes to redden compared with untreated skin. The test applies a fixed, generous quantity of product. Real-world application is typically far thinner, which reduces the protection achieved below the labelled figure.
Quantity beats number
Under-application is the most common reason protection falls short. Public health guidance emphasises applying a generous layer, covering easily missed areas, and reapplying — not selecting a higher number to compensate for a thin coat.
Broad spectrum
Sunburn is driven largely by one part of the ultraviolet spectrum; longer-wavelength ultraviolet contributes to other forms of skin damage. A broad spectrum designation indicates the product has passed a test covering that wider range. Where the designation is absent, the SPF figure alone does not imply it.
Water resistance is time-limited
- Water resistance claims are stated for a specific tested duration, not indefinitely.
- Towel drying removes product mechanically, whatever the claim.
- Expiry dates apply; degraded product should not be relied on for a full season.
- Shade, clothing and timing remain part of the guidance from every major health authority, not alternatives to it.
This article does not recommend or rank a sunscreen. For skin-cancer risk, medication interactions or an existing skin condition, a clinician's advice supersedes any published guide, including this one.
