Sunscreens protect skin from UV-A (320–400 nm — photoaging, oxidative damage, melanoma) + UV-B (290–320 nm — sunburn, NMSC, vitamin D synthesis). Two classes by mechanism: (1) Organic ("chemical") filters absorb UV → re-emit as heat. UV-B absorbers (octinoxate / octisalate / homosalate / oxybenzone); UV-A absorbers (avobenzone — broadband UV-A but photolabile; needs photostabilizer like octocrylene). Photo-stability — avobenzone alone degrades in sunlight within ~30 min; modern formulations co-pair stabilizers. Endocrine-disruption concerns: oxybenzone + octinoxate banned in some jurisdictions (Hawaii, Key West) due to coral-bleaching + endocrine disruption signals — limited human data. (2) Inorganic ("mineral") filters reflect + scatter UV: zinc oxide (broad UV-A + UV-B), titanium dioxide (mostly UV-B + UV-A2). Nano vs non-nano debate — nano improves cosmetic feel but inhalation hazard (aerosol formulations). SPF = UV-B protection ratio (SPF 30 blocks ~97%, SPF 50 ~98%); PA (Asian rating) + UVA-PF (Boots star) for UV-A. EU + AU regulators require both UV-A + UV-B labelling; US FDA still considers UV-A optional. Cross-links: epidermal keratinization melanogenesis (melanogenesis as endogenous photoprotection), ferroptosis gpx4 lipid peroxidation (UV-driven membrane lipid peroxidation), vitamin d metabolism (sunscreen vs. vitamin D synthesis trade-off).
Organ Systems
integumentary
Pathway Steps
solar UV-B (290–320 nm) → epidermal DNA damage (CPD + 6-4PP photoproducts) — via sunburn + non-melanoma skin cancer; vitamin D photolysis. UV-B (290-320 nm) is absorbed directly by DNA, generating cyclobutane pyrimidine dimers (CPDs) and 6-4 photoproducts — the mutagenic lesions behind most UV-induced skin cancers. Because UV-B causes direct DNA damage and sunburn, it was historically the main sunscreen target (the basis of the SPF rating).
solar UV-A (320–400 nm) → dermal oxidative damage + photoaging + melanoma risk — via penetrates deeper than UV-B; ROS-mediated injury. UV-A (320-400 nm) penetrates deeper into the dermis and acts largely through reactive oxygen species, driving photoaging (collagen breakdown, wrinkling) and contributing to melanoma. Recognizing UV-A’s role drove the shift to “broad-spectrum” sunscreens, since SPF alone mainly reflects UV-B protection.
organic UV filter + UV photon → absorption → vibrational relaxation → heat — via chromophore absorbs at filter-specific wavelength; re-emission as heat. Organic (chemical) UV filters work by absorption: their conjugated structures absorb UV photons, reach an excited state, and relax by releasing the energy as harmless heat. Each filter covers a specific band, so broad coverage needs several combined — and that absorbed energy is also what can cause photodegradation.
inorganic UV filter (ZnO / TiO₂) → reflection + scattering of UV — via mineral particles physically block; broadband by combining. Inorganic (mineral) filters — zinc oxide and titanium dioxide — act mainly by reflecting and scattering UV (with some absorption), giving broad-spectrum, photostable, low-irritation protection. This physical mechanism is why mineral sunscreens are preferred for sensitive/pediatric skin, at the cost of whitening.
avobenzone + UV-A photon → photo-isomerization + degradation — via requires octocrylene / Tinosorb stabilizer to maintain efficacy. Avobenzone is the main organic UV-A filter but is photo-unstable: UV-A excitation makes it isomerize and degrade, losing protection over time. This is why it must be paired with photostabilizers (e.g. octocrylene) — a key formulation consideration for durable UV-A coverage.
photo-protected skin → reduced CPD + reduced melanoma + photoaging risk — via cumulative dose-response; SPF + reapplication interval drive real-world protection. Effective photoprotection reduces CPD formation and downstream oxidative damage, lowering the risk of skin cancer (including melanoma) and photoaging. Randomized data confirm regular sunscreen use reduces melanoma and squamous-cell carcinoma — the clinical payoff of blocking the upstream DNA/ROS damage.