Category: signaling
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).