Sunscreen is the single most effective anti-ageing and skin-protecting product available, and its label is a small mess of numbers and symbols most people never learn to read. Decoding it takes ten minutes and changes how well you are actually protected — because the highest SPF on the shelf is not necessarily the best choice.
It starts with understanding that sunlight's damaging ultraviolet comes in two relevant flavours, and a good sunscreen has to handle both.
UVA and UVB, the two culprits
A useful memory aid: UVB for Burning, UVA for Ageing. UVB rays are stronger, more seasonal and cause sunburn and much skin cancer risk. UVA rays are present all day, all year, penetrate deeper, pass through glass, and drive much of the wrinkling, sagging and pigmentation people blame on 'getting older'. Both harm skin; you need protection from both.
This matters because the SPF number only measures UVB protection. A sunscreen can have a sky-high SPF and still leave your skin exposed to the UVA rays doing the long-term ageing damage — unless it is also broad-spectrum.
Reading the numbers
SPF measures UVB protection: SPF 30 blocks about 97% of UVB, SPF 50 about 98%. The jump from 30 to 50 is real but small; there is no such thing as total blockage, and very high numbers can lull people into skipping reapplication. 'Broad-spectrum' is the crucial word — it means the product also protects against UVA. Outside the US, UVA protection is often graded separately as PA (more plus signs = more UVA protection) or a circled UVA symbol.
The practical rule: choose broad-spectrum, SPF 30 or higher, and treat that as the floor. A broad-spectrum SPF 30 you apply properly beats an SPF 100 that only guards against burning.
The part everyone gets wrong: amount
The most common sunscreen failure is not the product; it is using too little. Protection is tested at a generous amount most people never apply — roughly a quarter-teaspoon for the face and neck, or the 'two-finger' guideline of two lines of product along your index and middle fingers. Use half that, as most do, and an SPF 50 performs more like an SPF 15.
And reapply: sunscreen wears off with time, sweat and touching, so protection needs topping up roughly every two hours of real sun exposure. Apply enough, apply broad-spectrum, reapply through the day — those three habits matter far more than the exact number on the bottle.
The two wavelengths, and what each one does
Ultraviolet reaching the ground divides into two bands that matter for skin. UVB, at roughly 280 to 315 nanometres, is the shorter and more energetic. It is absorbed mostly in the epidermis, it damages DNA directly, and it is what causes sunburn and drives most skin cancers.
UVA, at 315 to 400 nanometres, is longer and penetrates further, reaching well into the dermis where collagen and elastin live. It causes comparatively little burning, which is precisely why it went unaddressed for so long, and it is the dominant driver of photoageing and pigmentation.
The proportions are instructive. Around ninety-five per cent of the ultraviolet reaching the earth's surface is UVA, and unlike UVB it is present at fairly consistent intensity throughout the day and throughout the year.
It also passes through window glass, which UVB largely does not. The commonly cited photograph of a lorry driver with decades of one-sided facial damage is a UVA photograph.
What the SPF number actually measures
SPF is a ratio measured against sunburn, which means it is overwhelmingly a measure of UVB protection. It compares the ultraviolet dose needed to redden protected skin against the dose needed to redden unprotected skin, under laboratory conditions with a defined quantity of product applied.
The common interpretation — that SPF 30 lets you stay out thirty times longer — is a poor way to use it, because ultraviolet intensity varies through the day, sunscreen degrades, and nobody applies the tested quantity. Treating SPF as a measure of how much gets through is more useful.
On that reading, SPF 15 blocks around ninety-three per cent of UVB, SPF 30 around ninety-seven per cent, and SPF 50 around ninety-eight per cent. The differences look small stated that way, and they matter more than they appear because the residual dose is what does the damage.
Critically, SPF says nothing at all about UVA. A high-SPF product with poor UVA filtering will prevent your sunburn while doing little about the wavelengths that age your skin.
Diminishing returns, and why they are not the whole story
The percentage figures above are why the argument that SPF 100 is a marketing exercise has some force. Going from 50 to 100 moves blocked UVB from about ninety-eight to about ninety-nine per cent, which is not a doubling of anything.
The counter-argument is real-world application. Because most people apply between a quarter and a half of the tested quantity, the protection actually received is far below the label, and it falls off non-linearly rather than proportionally. A high number provides headroom for under-application.
Studies comparing high-SPF and moderate-SPF products in field conditions have found meaningful differences in favour of the higher number, which supports the headroom argument rather than the arithmetic one.
The sensible position is that SPF 30 applied properly beats SPF 50 applied thinly, and SPF 50 applied thinly beats SPF 30 applied thinly. Buy the higher number if it helps, and fix the quantity regardless.
UVA protection, and the labels that describe it
Because SPF does not cover UVA, separate systems exist, and they differ by region in ways that make comparison genuinely difficult. The PPD system measures persistent pigment darkening and produces a number; PA+ through PA++++ is a Japanese-origin grading built on the same measurement.
In the European Union, a UVA inside a circle indicates that UVA protection is at least a third of the labelled SPF, which is a ratio requirement rather than an absolute one. It is a floor, and a product can exceed it substantially without saying so.
In the United States, broad spectrum is determined by a critical wavelength test, and the regulatory framework has permitted fewer modern UVA filters than Europe and Asia, which is why enthusiasts import sunscreens.
The practical implication for a buyer is to look for PA++++ or the circled UVA mark alongside a decent SPF, rather than treating a high SPF alone as sufficient.
Broad spectrum is the more important claim
If a choice has to be made between a very high SPF with unstated UVA performance and a moderate SPF with strong, explicitly labelled UVA protection, the second is the better product for most purposes.
This follows from what each band does. Burning is unpleasant and is a marker of DNA damage, so UVB protection matters enormously. But photoageing, pigmentation, melasma and the slow structural degradation of the dermis are largely UVA, and those are the changes people are usually trying to prevent when they buy sunscreen.
It is also the band you cannot detect. There is no immediate feedback from UVA exposure — no redness, no warmth, no signal that anything happened — which is why a product that leaves it unaddressed can feel entirely adequate for years.
Broad spectrum is therefore not a bonus feature. It is the part of the label that determines whether the product does the job people actually want done.
Filters: mineral, organic and the newer generation
Mineral filters are zinc oxide and titanium dioxide. Zinc oxide covers the full ultraviolet range including long UVA and is well tolerated by reactive skin; titanium dioxide is stronger against UVB and weaker in the UVA range. Both work primarily by absorption, despite the persistent description of them as reflectors.
Organic filters absorb ultraviolet and convert it to heat. The older ones vary considerably in stability — avobenzone provides good UVA coverage but degrades in sunlight unless stabilised, which is why it is formulated alongside octocrylene or other stabilisers.
The newer generation of organic filters, available in Europe, Asia and Australia but not currently in the United States, offer better photostability and broader coverage in more elegant textures. Tinosorb and Uvinul are the names most often encountered.
For a buyer, the useful signal is not mineral versus organic as a category but whether the specific filters listed cover long UVA and whether the formulation is one you will apply generously.
The quantity nobody applies
SPF is tested at two milligrams per square centimetre of skin. For a face and neck that works out at roughly a third of a teaspoon, or the length of your index and middle finger squeezed out from a tube — the two-finger method is a rough guide that gets most people much closer than instinct does.
For a whole body at the beach, the tested quantity is around thirty millilitres, which is a shot glass, per application. Very few people use a bottle at anything like the rate that implies.
The consequence is not proportional, which is the part that surprises people. Applying half the tested quantity does not halve the SPF; the relationship is closer to exponential, and an SPF 50 applied at half rate can behave more like an SPF 7 to 15.
This single variable outweighs almost every other decision in sunscreen selection. Choosing between two good products matters far less than doubling how much of either one you use.
Water resistance, sweat and reapplication
Water resistant means the product maintained its SPF after a defined period of immersion — typically forty or eighty minutes depending on the claim. There is no such thing as waterproof, and the term is no longer permitted in most markets for that reason.
Reapplication every two hours is the standard advice for meaningful exposure, and immediately after swimming, heavy sweating or towelling regardless of how long has passed. Towelling in particular removes a great deal of product very efficiently.
For a day spent indoors away from windows, the two-hour rule matters much less, and treating it as universal is one reason people conclude sunscreen is impractical and abandon it. Matching reapplication to actual exposure is more sustainable than following a rule that does not fit the day.
Sticks, powders and cushion compacts exist because reapplying over makeup is the practical obstacle for a lot of people. They are imperfect and they are considerably better than the reapplication that does not happen.
Visible light, and where the evidence stands
Beyond ultraviolet, visible light — particularly the high-energy blue end — has been shown to induce pigmentation in deeper skin tones and to worsen melasma. This is well enough established to have changed clinical advice for those specific conditions.
Conventional transparent sunscreens do not filter visible light. Iron oxides do, which is why tinted formulations outperform untinted ones for melasma in comparative studies, and why the tint on a sunscreen is doing protective work rather than only cosmetic work.
Screen-emitted blue light is a separate question and the evidence is much weaker. The doses from devices are orders of magnitude below sunlight, and the studies most often cited in marketing used intensities that a phone does not produce.
The defensible summary: if you have melasma or pigmentation in deeper skin, a tinted sunscreen is a genuine upgrade. If you are worried about your laptop, the sun coming through the window beside it is the larger exposure by a wide margin.