The SPF Myth That Needs Correcting
The "physical vs chemical" terminology is outdated and scientifically inaccurate. The correct terms are inorganic (mineral) and organic (chemical) UV filters. Both work by absorbing UV radiation — the mineral filters additionally reflect some UV, but their primary protection mechanism is absorption, not reflection (a myth perpetuated by the cosmetic industry for decades).
Inorganic (Mineral) Filters
| Filter | UV Coverage | Characteristics |
|---|---|---|
| Zinc Oxide (ZnO) | UVA + UVB (broadest of any single filter) | Photostable; white cast in non-micronised form; reef-safe concern debated |
| Titanium Dioxide (TiO₂) | UVB + UVA1 (incomplete UVA coverage) | More cosmetically elegant than ZnO; often combined with ZnO for full UVA |
Mineral filters sit on the skin surface and do not penetrate the stratum corneum (particularly in non-nano formulations). They are the preferred choice for sensitive, reactive skin, rosacea, and post-procedure skin.
Organic (Chemical) Filters
| Filter | UV Coverage | Notes |
|---|---|---|
| Avobenzone | UVA1 | Unstable without photostabiliser (octocrylene, Tinosorb) |
| Octinoxate / Octisalate | UVB | Systemic absorption detected in FDA studies; EU restricted |
| Tinosorb M / S | Broad spectrum UVA + UVB | EU-approved; not yet FDA-approved; most photostable organic filters available |
| Mexoryl SX / XL | UVA + UVB | L'Oréal patents; excellent photostability; available outside US |
HEV Blue Light and Iron Oxides
Standard SPF — both mineral and chemical — does not protect against HEV blue light (400–500nm). Only formulations containing iron oxides (the pigments that create tinted/toned sunscreens) provide meaningful HEV protection. If hyperpigmentation is a concern — particularly in Fitzpatrick types III–VI — a tinted mineral SPF with iron oxides is strongly recommended. See the Digital Exposome guide.
The most important skincare rule: A broad-spectrum SPF 30+ applied daily is the single most evidence-backed anti-ageing intervention available without a prescription. It prevents UV-triggered collagen degradation, melanogenesis, and epigenetic ageing simultaneously. The best SPF is the one you will actually wear every day.
Application Guidelines
- Apply as the final step in AM routine (after moisturiser, before makeup if applicable)
- 2mg/cm² = the tested amount — approximately ¼ teaspoon for face, neck, and ears
- Reapply every 2 hours under direct UV exposure; daily reapplication for indoor/office use
- SPF does not stack: SPF 30 + SPF 30 ≠ SPF 60
For the epigenetic case for SPF, see Epigenetics of Skin Aging.