The jump from one sun protection factor to another looks like a proportional increase and is not. The number describes a ratio, and ratios behave in a way most shoppers do not expect.
The figure is a ratio of exposure time
Sun protection factor is measured by comparing how much ultraviolet exposure produces visible reddening on protected skin against unprotected skin.
A factor of thirty means the protected skin required roughly thirty times the exposure to reach the same reaction under controlled laboratory conditions.
It is therefore a multiplier on an individual's own tolerance, which varies widely, rather than a fixed duration in minutes that applies to everyone.
The protection curve flattens quickly
Because the factor is the inverse of the fraction of radiation reaching the skin, the proportion blocked rises steeply at low numbers and then barely moves.
Moving from a low factor to a moderate one produces a large change. Moving from a high factor to a very high one changes the transmitted fraction only slightly.
Which means the difference between the top two numbers on a shelf is far smaller than the difference between the bottom two, despite the larger gap in the figure.
Application thickness dominates the result
The laboratory test applies a specified thickness of product per unit of skin area, and it is considerably more than most people apply in practice.
Protection falls faster than the thickness does, so applying half the tested amount does not give half the stated factor but a good deal less.
A high number applied thinly can therefore deliver less protection than a lower number applied properly, which is the practical failure mode of the whole system.
The number says nothing about the other wavelengths
Sun protection factor is measured against the reddening response, which is driven mainly by shorter-wavelength ultraviolet.
Longer-wavelength ultraviolet penetrates deeper and contributes to other changes in skin without producing the reddening the test measures.
Separate designations exist to indicate protection across both ranges, and they are the part of the label that describes breadth rather than strength.
Reapplication is a physical requirement
Films of product are removed by sweat, water, towelling and ordinary contact with clothing, and some filters degrade as they absorb radiation.
No factor changes this. The protective layer has to be present on the skin, and once it is disturbed the stated figure no longer describes the situation.
People with specific skin conditions or medications that affect sensitivity should be guided by a clinician, since general labelling is not designed to answer individual questions.