The number on a sunscreen bottle is generated by a standardised test whose conditions differ substantially from how the product is used. Understanding the test explains the gap between labelled and actual protection.
The factor is a ratio measured on skin
Testing involves applying the product to marked areas of a volunteer's back and exposing those areas to graded doses of ultraviolet light.
The dose that produces detectable reddening on protected skin is compared with the dose that produces the same reddening on unprotected skin, and the ratio is the factor.
Because reddening is driven mainly by UVB, the number describes protection against that part of the spectrum rather than against ultraviolet exposure as a whole.
Application thickness is the critical variable
The test specifies a defined quantity per unit of skin area, and that quantity is considerably thicker than most people apply in practice.
Protection does not fall proportionally with a thinner layer; it falls faster, so applying roughly half the test amount delivers substantially less than half the labelled factor.
This is the main reason measured protection in real use falls well below the label, and it is a use problem rather than a product problem.
The scale is not linear
Moving from a low factor to a moderate one removes a large share of the ultraviolet reaching the skin, while further increases add progressively smaller amounts.
High factors therefore represent diminishing returns in blocked radiation, though they do provide a margin against under-application.
The larger practical effect of a high number is that it extends the period before protection is lost, not that it changes protection by the same proportion.
UVA protection is rated separately
UVA penetrates more deeply and contributes to ageing changes and to skin cancer risk without producing much immediate reddening.
Because the main test uses reddening as its endpoint, it captures UVA poorly, and separate methods and marks exist for that part of the spectrum.
Labelling conventions for UVA differ between regions, which is why the same protection can be indicated by quite different symbols on different packages.
Water resistance and reapplication
Water-resistant claims are supported by testing after defined periods of immersion, and the claim expires at the tested duration rather than lasting all day.
Sweating, towel drying and rubbing all remove product, which is why reapplication intervals are specified rather than being a precaution.
Anyone with a history of skin cancer, a photosensitising medication or a large number of unusual moles should be guided by a dermatologist rather than by label figures alone.