Dr. Henrik Larsson, Sun Protection and Photobiology Editor.
SPF is the most widely recognised number in skincare, printed on every sunscreen bottle sold in Europe, yet what it measures and how it translates to real-world protection is poorly understood by most consumers. SPF stands for Sun Protection Factor, and it quantifies a single thing: how much longer skin coated with the product can be exposed to UVB radiation before reaching the minimal erythemal dose — the point at which the skin begins to redden. It says nothing about UVA protection, nothing about how long the product remains effective on the skin, and nothing about the amount most people actually apply.
- SPF measures UVB protection only: you need a separate broad-spectrum or UVA rating to know whether the product shields against the longer wavelengths that drive photoageing.
- The relationship is not linear: SPF 30 blocks approximately 96.7 per cent of UVB, SPF 50 blocks about 98 per cent — the difference is far smaller than the numbers suggest.
- Laboratory conditions do not match real use: SPF is tested at 2 mg per square centimetre of skin; most people apply less than half that amount.
How SPF Is Measured
The SPF test protocol, standardised by ISO 24444, involves applying sunscreen to the backs of human volunteers at a precise density of 2 mg/cm², exposing the treated skin to a solar simulator, and measuring the UV dose required to produce just-visible reddening. The same measurement is taken on unprotected skin. The ratio between the two doses is the SPF value.
This test is rigorous but narrow. It uses a controlled UV source, not natural sunlight. It applies the product at a thickness most consumers never achieve. And it measures only one outcome — erythema — which is driven primarily by UVB wavelengths between 290 and 320 nanometres.
UVA Protection: What SPF Does Not Cover
UVA radiation (320 to 400 nm) penetrates deeper into the skin than UVB and is the primary driver of photoageing — the loss of collagen, elastin degradation and pigmentary changes that accumulate over years of sun exposure. UVA also contributes to skin cancer risk. SPF does not measure UVA protection.
In Europe, the UVA logo — a circle containing the letters UVA — indicates that the product’s UVA protection factor is at least one-third of its labelled SPF. This is a minimum standard, not an indicator of strong UVA protection. A product with SPF 30 and the UVA logo guarantees a UVA-PF of at least 10 — modest by modern standards. Some newer formulations achieve UVA-PFs that approach or match their SPF values, but this information is rarely printed on the packaging.
The Application Gap
The single largest factor determining how much protection you actually receive is how much sunscreen you apply. The SPF test uses 2 mg/cm², which translates to approximately 1.25 ml (a quarter teaspoon) for the face alone. Studies consistently show that consumers apply between 0.5 and 1 mg/cm² — one quarter to one half of the tested amount.
Because the relationship between application thickness and protection is exponential, not linear, applying half the tested amount does not give you half the SPF. It gives you roughly the square root. An SPF 50 product applied at half thickness provides protection closer to SPF 7. This is the single most important fact about sunscreen that most people do not know.
Chemical Versus Mineral Filters
Sunscreen filters fall into two categories: organic filters (commonly called chemical) that absorb UV radiation and convert it to heat, and inorganic filters (mineral) that scatter and reflect UV light. Both are effective. The distinction matters for formulation, cosmetic elegance and, in some cases, skin tolerance.
Zinc oxide and titanium dioxide, the two approved mineral filters, provide broad-spectrum protection and are generally well tolerated by sensitive skin. Their drawback is cosmetic: they leave a visible white cast, especially on darker skin tones, which newer micronised and coated versions reduce but do not eliminate entirely.
Modern organic filters like Tinosorb S, Tinosorb M, Uvinul A Plus and Mexoryl XL offer excellent UVA and UVB coverage in elegant, transparent formulations. These newer-generation filters are photostable, meaning they do not degrade significantly under UV exposure — a problem that affected earlier organic filters like avobenzone when used without stabilisers.
Reapplication: When and Why
Sunscreen does not stop working after a fixed period. It degrades through a combination of UV-induced photodegradation, physical removal by sweating and rubbing, and dilution by sebum. The standard recommendation to reapply every two hours is a practical guideline, not a scientifically precise interval.
If you are sitting indoors near a window, your sunscreen is unlikely to degrade meaningfully over a full working day. If you are swimming, sweating heavily or towelling off at the beach, you may need to reapply more frequently than every two hours. The goal is to maintain adequate coverage on the skin, and the appropriate interval depends on what you are doing, not on a countdown timer.
Daily Sunscreen: What the Evidence Says
A landmark 2013 study published in the Annals of Internal Medicine followed 903 adults in Queensland, Australia over four and a half years. Participants randomised to daily sunscreen application showed 24 per cent less skin ageing than those who used sunscreen at their own discretion. This is the strongest randomised evidence that daily sunscreen use slows photoageing, and it has shaped public health advice ever since.
For people who spend most of their day indoors, the case for daily sunscreen is still supported but more nuanced. Glass blocks most UVB but transmits UVA, so window-adjacent office workers receive meaningful UVA exposure. The clinical significance of this incidental exposure over decades is plausible but not quantified as precisely as outdoor exposure. The consensus among dermatologists remains that daily application is the simplest protective habit available.