Wear the Damn Sunscreen. But Which One?

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1. The Number on the Bottle Is Only Half the Story

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SPF (Sun Protection Factor) measures protection against UVB radiation. UVB is the wavelength band responsible for sunburn and the direct DNA damage that drives most skin cancers in the epidermis, the outermost layer of skin.

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SPF does not measure UVA. However, UVA drives most of the slow, cumulative work behind what we call aging: wrinkles, pigmentation drift, loss of elasticity. UVA accounts for roughly 95% of the UV radiation that actually reaches your skin at ground level. It penetrates deeper, into the dermis, the layer where collagen and elastin build the skin's structural scaffolding.

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UVA drives the majority of photoaging, and SPF does not tell you how much of it is being blocked.

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What to look for as well. A sunscreen labelled "broad spectrum" with a UVA protection rating. The PA system (Protection Grade of UVA, a Japanese rating from PA+ to PA++++) or the PPD value (Persistent Pigment Darkening, the measurement method behind the PA grades) quantify UVA protection. A PPD of 16 or higher (equivalent to PA++++) is the benchmark worth checking.

Newer filters also extend coverage further into the UVA spectrum. Standard filters traditionally protected up to roughly 380 nm. Mexoryl 400 (sold under the brand name UVMune 400) protects up to 400 nm and closes what was previously known as the UV gap, the deepest UVA wavelengths that most filters did not cover.

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2. UVA Does Not Stop at the Beach

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Three facts that change how you should think about daily exposure.

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UVA passes through car and office windows. Standard glass blocks almost all UVB and roughly half of UVA. A long commute, a window desk, or a sunny café seat add up to measurable UVA exposure over a career. A classic dermatology case (Singer, New England Journal of Medicine, 2013) documented a truck driver whose left cheek, the side exposed to the driver's window over decades, showed dramatically more photoaging than his right. The image is in most dermatology textbooks now because it is almost too neat to be true, and it is.

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UVA is present year-round. UVA intensity varies far less by season than UVB. Winter sun in Zurich still delivers meaningful UVA, especially at altitude. For skin, there is no "off-season."

UVA gets stronger faster at altitude. UV intensity increases by roughly 10 to 12% for every 1,000 metres of elevation. At 2,500 metres on a Swiss ski day, your skin is seeing 25 to 30% more UV than at lake level, plus reflection from snow.

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Daily broad-spectrum protection does its heaviest work when you are nowhere near a beach. It applies indoors, on the commute, at the desk, and throughout a Swiss winter.

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3. Filter Types: Chemical vs. Mineral

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Sunscreen filters fall into two main categories.

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Organic (chemical) filters such as avobenzone, octisalate, and the newer generation of Tinosorb-type molecules primarily absorb UV radiation and convert it into a very small amount of heat. Modern chemical filters are photostable, well-tolerated, and deliver the strongest UVA coverage on the market.

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A note on how chemical filters work. They sit on and within the outermost layer of skin and do not need to be absorbed into deeper tissue to function. UV radiation is absorbed by the filter molecules at the surface and converted into a very small amount of heat. Trace amounts of some filters can be detected in the bloodstream after heavy use (Matta 2020). Detectable does not mean harmful. The FDA continues to evaluate these molecules and the European Commission authorizes them only when they meet safety thresholds. For most people, the documented protection against skin cancer and photoaging outweighs the theoretical concerns.

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Inorganic (mineral) filters such as zinc oxide and titanium dioxide also work primarily through absorption, with a small scattering component. They are often the first choice for reactive or sensitive skin and for use on children.

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Mineral filters tend to sit on the surface and can leave a white cast, particularly on darker skin tones. Chemical filters blend invisibly but may cause sensitivity in very reactive skin. Hybrid formulations combine both and often deliver the best balance: broad coverage with a wearable finish.

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A note on what is being phased out. Oxybenzone and octinoxate are restricted in certain jurisdictions (Hawaii, parts of the Caribbean, Palau) over reef-impact concerns, and some consumers prefer to avoid them for endocrine reasons. The evidence on systemic effects at realistic use levels is limited (Matta 2020), but modern filter options are now so strong that the question is increasingly moot: there are better filters available.

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4. The Modern Filters Worth Looking For

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Not all broad-spectrum claims are equal. The UVA filters in the formulation decide how comprehensive the protection actually is.

Modern filters worth looking for on the label:

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• Tinosorb S and Tinosorb M. Excellent photostability, broad UVA and UVB coverage.
• Uvinul A Plus. Strong UVA-I protection (the deepest-penetrating UVA band).
• Mexoryl SX (ecamsule) and Mexoryl XL (drometrizole trisiloxane). Strong UVA coverage with robust clinical data.
• Mexoryl 400 (UVMune 400). The newest of the Mexoryl family. It blocks ultra-long UVA wavelengths up to 400 nm, the band that older filters miss.
• Zinc oxide. The most broad-spectrum mineral filter.

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A simple framework: look for products that list at least one modern UVA filter, carry a PPD ≥ 16 or PA++++ rating, and feel comfortable enough on your skin that you will actually use them every day. The best sunscreen is the one you wear consistently.

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5. Application: The Blind Spot

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Here is where theory and practice diverge.

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SPF values are tested at an application density of 2 mg of product per square centimetre of skin. Real-world studies measure application densities between 0.5 and 1.0 mg/cm², a quarter to half of the tested amount (Petersen & Wulf, 2014). Which means the labelled SPF is almost never the effective SPF.

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A practical guide. The two-finger rule: two strips of sunscreen along your index and middle finger approximate the right amount for your face. For face and neck together, three fingers. For face, neck, and décolleté, four.

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The areas most often missed: the ears, the hairline, the back of the neck, the tops of the hands. These are also among the areas with the highest rates of UV-induced damage, precisely because they are forgotten. Skin Longevity extends beyond the face. A sunscreen that stops at the jawline is doing half its job.

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Re-application. Under real-world conditions, sunscreen loses a meaningful share of its protection within two hours of activity, sweating, or friction (Diffey 2009). For a desk-and-commute day, a morning application plus a mid-afternoon touch-up on exposed areas is the realistic target. For a ski day, a lake day, or a long summer afternoon, every two hours is not optional.

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6. SPF 30 vs. 50 vs. 100 – The Math

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The percentages mislead people. SPF 30 blocks approximately 97% of UVB. SPF 50 blocks approximately 98%. Stated that way, the difference looks trivial.

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Now flip the maths. SPF 30 lets through 3% of UVB. SPF 50 lets through 2%. That is a 50% reduction in what actually reaches your skin, not a 1% one. Over a lifetime of daily use, that cumulative difference is substantial.

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Because almost no one applies the full 2 mg/cm², a higher SPF provides a buffer for the under-application reality. When you under-apply an SPF 50, the effective protection still tends to land closer to what a well-applied SPF 30 would deliver at tested density. For everyday use, SPF 50+ broad spectrum gives the most tolerance for real-world application habits.

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Our recommendation at everskin. SPF 50+ broad spectrum, every morning, regardless of weather. On cloudy days, up to 80% of UV still reaches your skin. In the mountains, plan for the altitude multiplier and re-apply on the snow.

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The Bottom Line

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Sunscreen is one of the most thoroughly researched interventions in dermatology. In the largest randomised controlled trial of its kind, daily broad-spectrum sunscreen users showed 24% less visible skin aging over 4.5 years than discretionary users (Hughes et al., 2013). No over-the-counter intervention in skincare has a better evidence-to-cost ratio.

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Skinspan, the duration your skin remains in optimal health, has a short list of highest-ROI levers. Daily SPF sits at the top of that list, and everything downstream (treatments, topicals, in-clinic work) compounds better when the UV foundation is in place.

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The key takeaways:

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• Look beyond SPF alone. Check both UVB protection (SPF 50+) and UVA protection (PA++++ or PPD 16+).
• Look for modern filters on the label. Tinosorb S, Tinosorb M, Uvinul A Plus, and the newer Mexoryl 400 / UVMune 400 deliver the fullest UVA coverage.
• Apply generously: two finger-lengths for the face, three for face and neck.
• Higher SPF provides a real-world buffer for the under-application gap.
• UVA continues through windows and at altitude. There is no indoor exemption.
• Extend your SPF to the neck, chest, and hands. See next week's post for why those zones ask for their own attention.

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‍Book your doctor’s consultation here.

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Sources:‍

1. Diffey BL. Sunscreens as a preventative measure in melanoma. Br J Dermatol. 2009;161(s3):25–27.
2. Hughes MCB et al. Sunscreen and Prevention of Skin Aging: A Randomized Trial. Ann Intern Med. 2013;158:781–790.
3. Matta MK et al. Effect of Sunscreen Application on Plasma Concentration of Sunscreen Active Ingredients. JAMA. 2020;323(3):256–267.
4. Green AC et al. Reduced Melanoma After Regular Sunscreen Use. J Clin Oncol. 2011;29:257–263.
5. Petersen B, Wulf HC. Application of sunscreen: theory and reality. Photodermatol Photoimmunol Photomed. 2014;30(2–3):96–101.
6. Osterwalder U, Herzog B. Sun protection factors: worldwide confusion. Br J Dermatol. 2009;161(s3):13–24.
7. Passeron T et al. Sunscreen photoprotection and vitamin D status. Br J Dermatol. 2019;181(5):916–931.
8. Latha MS et al. Sunscreening agents: a review. J Clin Aesthet Dermatol. 2013;6(1):16–26.
9. Ziglar J et al. Sunscreens: Updates on Filters and Formulations. Photodermatol Photoimmunol Photomed. 2025;41:e70026.
10. Singer S. Unilateral Dermatoheliosis. N Engl J Med. 2013;366(16):e25.
11. Cole C, Shyr T, Lim HW. Metal oxide sunscreens protect skin by absorption, not by reflection or scattering. Photodermatol Photoimmunol Photomed. 2016;32(1):5–10.