How Sunscreen Actually Works: SPF & UV Filters Explained
SPF, “broad spectrum,” and “mineral vs. chemical” are the three specs printed on every bottle. Here’s the actual physics and chemistry behind each one — and why the SPF number alone doesn’t tell you what you think it does.
Sources
U.S. Food & Drug Administration, NCBI Bookshelf — StatPearls, American Academy of Dermatology, and Cosmetics & Toiletries.
What You’ll Understand
- How chemical and mineral filters physically stop UV radiation — two completely different mechanisms
- What the SPF number is actually measuring, and why it isn’t a timer
- Why “broad spectrum” is a separate, real pass/fail test — not just marketing language
- Why some sunscreens lose effectiveness chemically over time, not just by rubbing off
- Why mineral sunscreen leaves a white cast, and how some formulas avoid it
Every sunscreen bottle promises the same three things: an SPF number, “broad spectrum” protection, and a filter type — mineral or chemical. The buying guides on this site compare which bottle does that best. This piece explains what those three specs actually mean at the molecular level, because the label alone doesn’t tell you why an SPF 30 mineral formula behaves so differently on skin than an SPF 30 chemical one, or why “broad spectrum” is a real pass/fail test rather than a vague marketing phrase.
Two Ways to Stop a Photon: Absorption vs. Scattering
Every UV filter on the market falls into one of two chemical categories, and they don’t work the same way. Chemical (organic) filters — avobenzone, octinoxate, homosalate, oxybenzone — are molecules built around an aromatic ring conjugated with a carbonyl group. When a UV photon hits that molecule, the ring absorbs the photon’s energy and jumps to an excited electronic state, then drops back down and releases that same energy as heat, according to StatPearls’ review of photoprotection mechanisms. The UV radiation never reaches skin cells intact — it’s converted into a small amount of warmth before it gets there.
Mineral (inorganic) filters — zinc oxide and titanium dioxide — work through a different physical process: reflection and scattering. These are solid particles suspended in the formula, and how much UV they scatter depends on the particle’s refractive index, its size, and how thick a film it forms on skin. Instead of a chemical reaction converting the photon to heat, the particle simply bounces the UV radiation away from the skin’s surface.
That mechanical difference is why the two filter types feel so different on skin. Chemical filters are dissolved molecules that spread thin and absorb quickly, with little visible residue. Mineral filters are literally particles sitting in a film on top of the skin — which is also the root cause of the white-cast issue covered further down.
Chemical filters absorb a UV photon and re-release the energy as heat; mineral filters bounce the photon away from the skin’s surface before it ever gets absorbed.
What the SPF Number Is Actually Measuring
SPF is not a timer, and it doesn’t measure “how strong” a sunscreen is in any absolute sense. It’s a ratio produced by a specific lab test: researchers find a subject’s Minimal Erythema Dose (MED) — the smallest dose of UV radiation that produces visible redness 24 hours later — first on bare skin, then on skin wearing the sunscreen. SPF is simply MED-with-sunscreen divided by MED-without. An SPF of 30 means it takes 30 times more UV exposure to burn protected skin than unprotected skin, under those lab conditions, with 10 to 20 human test subjects averaged into the final number.
The FDA is explicit that SPF values primarily reflect a sunscreen’s UVB protection — the ray responsible for sunburn — not its total UV protection. That’s also why higher SPF numbers deliver shrinking returns: the AAD notes SPF 30 blocks about 97% of UVB rays, SPF 50 blocks roughly 98%, and SPF 100 blocks about 99%. Going from 30 to 100 buys you roughly two more percentage points of UVB blocking, not “three times the protection” the number might imply.
The practical consequence: reapplication timing matters more than chasing a higher number. Every sunscreen — regardless of SPF — needs reapplying roughly every two hours outdoors, because the MED test measures a fixed ratio under controlled conditions that sweat, water, and time all degrade in the real world.
UVA vs. UVB — Why “Broad Spectrum” Covers What SPF Doesn’t
UV radiation that reaches skin splits into two bands with genuinely different effects. UVB (290–320nm) is the shorter-wavelength ray responsible for sunburn — and the one the SPF number measures. UVA (320–400nm) penetrates deeper into skin, drives long-term photoaging, and contributes to skin cancer risk — but it does not register in the SPF calculation at all, since that test only scores visible redness from UVB exposure.
That gap is exactly why “Broad Spectrum” exists as a separate, testable claim rather than a marketing phrase. To legally print it on a US label, a sunscreen has to pass an in-vitro instrument test measuring its “critical wavelength” — the point at which 90% of its total UV absorbance has occurred — and that critical wavelength has to reach at least 370nm, per FDA labeling guidance. A sunscreen can carry a high SPF number and still fail that test if it doesn’t genuinely cover the UVA range.
The UVA band itself splits further into UVA2 (315–340nm) and UVA1 (340–400nm), and this is where mineral filters diverge from each other in a way most product listings never explain. Titanium dioxide covers UVB and UVA2 well but has limited UVA1 coverage; zinc oxide covers the full UVB-through-UVA1 range in a single ingredient, per StatPearls. That’s a real functional difference hiding behind the words “100% mineral” on a bottle — not every mineral formula covers the same UV range.
| Zinc Oxide | Titanium Dioxide | Avobenzone (chemical) | |
|---|---|---|---|
| UVB (290–320nm) | Covered | Covered | Not primary |
| UVA2 (315–340nm) | Covered | Covered | Covered |
| UVA1 (340–400nm) | Covered | Limited | Covered |
| Photostable on its own | Yes | Yes | No — needs a stabilizer |
The Photostability Problem — Why Reapplying Isn’t Just About Rubbing Off
Reapplication advice usually gets explained as “sunscreen wears off,” which implies a purely mechanical problem — sweat, water, toweling. For chemical filters, there’s a second, chemical reason baked into the mechanism itself. Avobenzone, one of the only chemical filters that covers UVA1, is not stable under repeated UV exposure: it can lose 50% to 90% of its UV-absorbing capacity after about an hour in the sun, because the same molecular reaction that absorbs UV energy gradually breaks the molecule down.
Formulators counter this by pairing avobenzone with a stabilizer — most commonly octocrylene, which works by quenching avobenzone’s excited “triplet state” before it can degrade, according to Cosmetics & Toiletries’ review of avobenzone stabilizers. That’s an active ingredient combination working purely to keep another ingredient from chemically falling apart under the exact conditions it’s designed for.
Mineral filters don’t have this specific failure mode — zinc oxide and titanium dioxide aren’t undergoing a molecular reaction that consumes them, so they don’t degrade the way avobenzone does. They do have a different stability issue: uncoated mineral particles can trigger photocatalytic reactions on their own surface under UV exposure, which is why modern formulas coat the particles in silica and dimethicone. The reapplication clock still applies to every sunscreen type, but the underlying reason differs — one is being chemically consumed, the other is simply being worn or washed away.
The White Cast Isn’t a Formulation Failure — It’s Particle Size
Mineral sunscreen’s signature white cast isn’t a sign of a badly made product — it’s a direct consequence of particle physics. Zinc oxide and titanium dioxide particles scatter light across a range of wavelengths, and if a particle is large enough, it scatters visible light (roughly 400–700nm) along with UV, which is what the eye perceives as a chalky white film.
Micronizing those particles down to a size smaller than visible-light wavelengths lets them keep scattering the shorter UV wavelengths while becoming far less visible to the eye — the actual mechanism behind “invisible zinc” and “tinted mineral” formulas that show up across today’s product listings. It’s a genuine engineering trade-off, not a marketing exaggeration: shrink the particle too far and UV protection can suffer, so formulators calibrate particle size to sit just below the visible-light threshold while staying large enough to scatter UV effectively.
What This Means When You’re Buying
None of this changes which specific bottle is the best pick for a given face, budget, or skin type — that’s still a product-by-product comparison. But the mechanism explains why the label differences matter beyond marketing copy: whether a formula leans mineral or chemical changes how it feels and how long it stays effective before reapplication, and “broad spectrum” is a real pass/fail claim worth checking rather than assuming from the SPF number alone.
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Not the focus of this piece, but if you’re shopping: search sunscreen on Amazon →search sunscreen on Amazon.ca →
Sources
- FDA — Sunscreen: How to Help Protect Your Skin from the Sun
- FDA — Labeling and Effectiveness Testing: Sunscreen Drug Products (Small Entity Compliance Guide)
- NCBI Bookshelf, StatPearls — Sunscreens and Photoprotection
- American Academy of Dermatology — How to Select a Sunscreen
- Cosmetics & Toiletries — The Quest for Avobenzone Stabilizers and Sunscreen Photostability
