How Does Sunscreen Work? Filters, Factors and What the Number Means
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Sunlight carries, alongside the light that can be seen, a band of ultraviolet that the eye cannot detect and the skin cannot ignore, and it does two things: it burns, within hours, and it damages the DNA in skin cells in ways that show up as wrinkles in decades and as cancer in some. Sunscreen is a film of chemistry that stands between the two. The first commercial versions appeared in the 1930s, the number on the bottle was invented in 1962, and the arguments about what to put in it have kept regulators busy since.
The two ultraviolets
The ultraviolet that reaches the ground comes in two bands. UVB, the shorter and more energetic, is absorbed in the outer skin and is what burns; it also drives the production of vitamin D and does the most direct damage to DNA, and its strength varies with the season, the time of day and the latitude. UVA, longer and less energetic, is about twenty times as abundant, passes through cloud and window glass, penetrates deeper into the skin, and damages the collagen that keeps skin elastic, which is most of what ageing skin is; it also contributes to cancer. A sunscreen must handle both, and for decades most handled only UVB, which is why the label now says broad spectrum.
How the filters work
Two kinds of ingredient do the work:
- •Organic, or chemical, filters: molecules such as avobenzone, octocrylene and oxybenzone with rings of carbon that absorb ultraviolet photons, jump to an excited state and drop back by releasing the energy as a little heat, over and over; each absorbs a particular band, so several are combined to cover UVA and UVB
- •Mineral, or physical, filters: fine particles of zinc oxide or titanium dioxide that mostly absorb ultraviolet as well, with some scattering; they cover a broad band, are less likely to irritate, and were once thick and white until the particles were made small enough to vanish
- •Stabilisers: avobenzone, the main UVA absorber, breaks down in sunlight and must be protected by other ingredients
- •The base: the cream, lotion or spray that spreads the filters into an even film, which matters as much as the filters
What the number means
The sun protection factor is a ratio measured on people. Volunteers are exposed to a lamp with and without a measured layer of sunscreen, and the SPF is how many times more ultraviolet is needed to redden the protected skin than the bare skin; SPF 30 means thirty times, which is to say the screen is letting through about a thirtieth of the burning rays, or blocking 97 percent, while SPF 50 blocks 98 percent and SPF 15 about 93. The scale flattens quickly, and the difference between 30 and 50 is smaller than it looks. SPF measures UVB only, since it is defined by burning; UVA protection is rated separately, by a star system in Britain, a circled UVA mark in Europe, or the broad-spectrum test in America. The catch is the layer: the test uses two milligrams per square centimetre, about a shot glass for a body, and most people apply a quarter to a half of that, which turns an SPF 30 into something nearer 5 to 10.
How to use it
The rules follow from the physics. Apply enough, which is more than feels natural, a teaspoon for the face and neck and a shot glass for the body; apply it fifteen minutes before going out so it forms a film; reapply every two hours and after swimming or sweating, since the film wears and water-resistant means it survives forty or eighty minutes in the water, not all day; and use it on the days that do not look sunny, since UVA comes through cloud. Sunscreen is the third line of defence after shade and clothing, and the Australian campaign that halved that country's melanoma rate in the young, Slip Slop Slap, put it in that order. It does not, at any factor, allow unlimited time in the sun, and a tan is not protection but the skin's record of damage already done.
The arguments
Two disputes recur. Oxybenzone and octinoxate were banned in Hawaii from 2021 on evidence that they harm coral, which is contested but has moved manufacturers to other filters; and traces of several organic filters are found in the blood after heavy use, at levels whose significance is unknown and being studied, which has pushed some users to mineral screens. The other side of the ledger is clear. Melanoma is among the fastest-rising cancers in fair-skinned populations, ultraviolet is its main cause, and regular sunscreen use halves the rate in trials that have followed people for a decade. Vitamin D, which UVB makes in the skin, is little reduced by real-world sunscreen use, since nobody applies it perfectly, and a supplement is cheaper than a burn.
The takeaway
Sunscreen works by filtering ultraviolet before it reaches the skin, with organic molecules that absorb the photons and shed the energy as heat and mineral particles of zinc or titanium oxide that absorb and scatter it, combined to cover the burning UVB and the deeper, ageing UVA. The SPF is a ratio of how much UVB it takes to burn through the screen, measured at a layer most people never apply, so the practical rules are more cream, earlier, reapplied, and behind shade and clothing rather than instead of them.