Sun protection factor is printed on every sunscreen and widely misread as a strength rating. It is a ratio derived from a specific test, and the conditions of that test explain most of the confusion.
The number is a ratio of doses
The factor is measured by applying sunscreen to a patch of skin, exposing it to a controlled ultraviolet source, and finding the dose that produces just-visible redness.
That dose is divided by the dose required to produce the same redness on unprotected skin, and the resulting ratio is the stated factor.
It therefore describes how much more ultraviolet energy the skin can receive before reddening, not how long a person may stay outside.
Redness measures only part of the spectrum
Skin reddening is caused mainly by ultraviolet B, so a test based on redness measures protection against that band specifically.
Ultraviolet A penetrates deeper and contributes to ageing and to cancer risk without producing much immediate redness, so it is largely invisible to the test.
Broad-spectrum labelling and separate rating systems exist to cover this gap, which is why the factor alone is an incomplete description of protection.
The relationship flattens quickly
Because the scale is a ratio of doses, the proportion of ultraviolet blocked rises steeply at low values and then approaches a ceiling.
Moving from a low factor to a moderate one produces a large change in transmitted radiation, while moving from a high factor to a very high one changes little.
This is why regulators in several regions cap the number that may be displayed, since larger figures imply differences that do not exist.
Test application is thicker than real use
Laboratory testing applies a standardised quantity per area of skin that is considerably more than people apply in practice.
Protection falls faster than the shortfall in quantity would suggest, so a thinly applied high-factor product can deliver protection well below its label.
Reapplication matters for the same reason, since sunscreen is removed by sweat, water and rubbing regardless of its rated water resistance.
Filters work by two different routes
Mineral filters sit on the surface and reflect or scatter radiation, while organic filters absorb ultraviolet energy and release it as heat.
Both are effective, and the choice between them turns on skin sensitivity, cosmetic appearance and formulation stability rather than on protection level.
Anyone with a history of skin cancer or a photosensitive condition should have protection specified by a dermatologist rather than chosen from a shelf.