The figure printed on a sunscreen is the output of a standardised laboratory test rather than a description of everyday performance. The gap between the two arises mainly from how much product is applied.
The number describes a ratio, not a percentage
The factor is measured by comparing how much ultraviolet exposure produces visible reddening on protected skin against the amount required on unprotected skin.
A factor of thirty means thirty times the exposure was needed to produce the same reddening, which corresponds to blocking most but not all of the relevant radiation.
Because the relationship is a ratio, the difference between successive factors narrows considerably at the upper end of the range.
The test application is thicker than normal use
The standard protocol applies a fixed quantity per unit of skin area, and that quantity is considerably more than most people use in practice.
Protection does not scale in proportion to the amount applied. Using half the tested quantity reduces the effective factor by substantially more than half.
This is the main reason measured real-world protection falls well below the labelled figure, and it applies regardless of which product is chosen.
The factor only describes one part of the spectrum
The reddening the test measures is caused predominantly by shorter-wavelength ultraviolet, so the number reflects protection against that band.
Longer-wavelength ultraviolet penetrates more deeply and contributes to ageing changes and to some skin cancers without producing much reddening.
Protection across the full range is indicated separately, through terms such as broad spectrum or through a star or circle marking depending on the region.
Filters degrade and are removed during wear
Some organic filters break down as they absorb ultraviolet energy, which reduces protection over hours of continuous exposure.
Physical removal matters more in practice, since sweating, swimming, towelling and ordinary contact with clothing all take product off the skin.
Water resistance claims are tested for a specified immersion period rather than indicating that a product remains in place indefinitely.
Why coverage errors dominate the outcome
Studies observing application find that certain areas are missed consistently, particularly around the ears, hairline, eyelids and the tops of the feet.
Uneven thickness produces a patchwork of protection levels, and the least covered area determines where damage occurs.
Anyone with a history of skin cancer, photosensitivity or a condition affected by sun exposure should follow clinical advice rather than general guidance.