SPF stands for Sun Protection Factor, and it measures one thing: how much longer skin takes to redden with the product on, compared with bare skin, under laboratory conditions.
Two consequences follow immediately, and both are widely missed. The numbers are not proportional to how much protection you get. And SPF describes protection against burning only, which is not the same as protection against sun damage.
The numbers are compressed at the top
SPF is a ratio, and ratios flatten out. Converting them into the share of ultraviolet-B blocked makes the shape obvious.
| SPF | UVB blocked | UVB reaching skin |
|---|---|---|
| 15 | 93% | 7% |
| 30 | 97% | 3% |
| 50 | 98% | 2% |
| 100 | 99% | 1% |
SPF 100 is not three times SPF 30. Going from 30 to 100 removes two of the remaining three percent. The steep part of the curve is between 0 and 30, and everything after that is refinement.
This is exactly the same reading error people make with vaccine efficacy percentages: a number describing reduction in risk gets read as a share of something. Worth checking what the denominator is before drawing a conclusion.
The real risk of a very high SPF is behavioural. People who believe they are almost completely protected stay out longer and reapply less, and end up with more exposure than someone using SPF 30 sensibly.
UVA and UVB do different things
Ultraviolet from the sun reaches us in two bands that matter.
UVB is shorter and higher energy. It is absorbed in the outer skin and causes sunburn. It is the band SPF measures.
UVA is longer and penetrates deeper. It causes far less immediate redness, which is why it went underappreciated for years, and it contributes substantially to premature ageing and to skin cancer risk. SPF does not measure it at all.
This is why broad spectrum on the label matters more than a high number. A product marked SPF 50 without broad-spectrum protection can let you spend hours in strong sun without burning while accumulating UVA damage the whole time. In Europe the UVA seal, a circled UVA logo, indicates UVA protection at least a third of the stated SPF.
Clouds do not help as much as they appear to
A substantial fraction of UV reaches the ground on an overcast day. Cloud droplets are large compared with a wavelength of light, so they scatter and diffuse rather than absorb, in the same way that atmospheric scattering spreads light across the whole sky rather than removing it.
The light gets softer, so it feels cooler and the visual cue to seek shade disappears. The ultraviolet is still arriving. Overcast days are a common source of unexpected burns for exactly that reason.
How much to use, which is the actual problem
The rated SPF is measured at 2 milligrams of product per square centimetre of skin. For an adult body that is roughly 30 millilitres, about a shot glass, per application. For a face and neck, roughly a third of a teaspoon.
Most people apply between a quarter and a half of that. Protection does not fall in proportion; it falls faster. Applied at half the tested thickness, an SPF 50 product performs closer to SPF 15 in practice.
Which means the single most effective change available to almost everyone is not buying a higher number. It is using considerably more of the number they already own, and reapplying every two hours, after swimming, and after towelling.
Mineral and chemical filters
Two families, both effective.
Mineral filters, zinc oxide and titanium dioxide, sit on the surface and both scatter and absorb UV. They work immediately, are less likely to irritate, and can leave a white cast.
Chemical filters absorb UV and release it as a small amount of heat. They are cosmetically lighter and generally need about fifteen to twenty minutes to settle before exposure.
Both are regulated and tested. The best sunscreen is the one you will actually apply in the required quantity, which is a more useful criterion than the chemistry.
What SPF does not tell you
- Not a duration. The old "SPF 30 means thirty times longer" is misleading, because it assumes perfect application and ignores sweat, water and rubbing.
- Not water resistance. That is a separate claim with its own test, and "water resistant" means 40 or 80 minutes, not indefinitely.
- Not UVA protection. Look for broad spectrum, separately.
The short answer
SPF is a burning number, it flattens out above 30, and it says nothing about the band responsible for most of the long-term damage. Broad spectrum, SPF 30 or higher, applied in the amount the test assumed, and reapplied, beats a bigger number applied thinly. This is general information rather than medical advice; a dermatologist is the right source for anything specific to your skin.