UVA & UVB

UVA and UVB: The Two Ultraviolet Threats

Ultraviolet (UV) radiation from the sun spans a spectrum from approximately 100 to 400 nanometres. Two sub-ranges reach the Earth's surface and penetrate human skin: UVB (280–320nm) and UVA (320–400nm). UVC (100–280nm) is almost entirely absorbed by the atmosphere's ozone layer before reaching ground level. UVA and UVB have different wavelengths, different tissue penetration depths, different biological mechanisms, and different seasonal and atmospheric patterns — which is why understanding both is essential for effective photoprotection.

UVB: The Burning Ray

UVB is the higher-energy, shorter-wavelength band. It is primarily absorbed in the epidermis. UVB causes direct DNA damage: it induces cyclobutane pyrimidine dimers and (6-4) photoproducts in DNA — the molecular lesions that drive most UV-induced skin cancers. It stimulates melanin production (tanning as a protective response). It causes sunburn (erythema). UVB intensity varies significantly with season, time of day, latitude, and altitude — lower in winter, weaker at dawn and dusk, absent or minimal in some climates in winter months.

UVA: The Ageing Ray

UVA has lower energy but penetrates significantly deeper — reaching the dermis. It is present at consistent intensity year-round, all day, regardless of cloud cover (clouds block ~20% of UVA), and penetrates window glass. UVA generates free radicals and ROS that degrade collagen and elastin, cause indirect DNA damage, drive pigmentation irregularities, and contribute substantially to skin cancer risk including melanoma. UVA is the primary driver of photoageing (wrinkles, laxity, pigmentation, sallowness).

Broad-Spectrum SPF

SPF ratings measure only UVB protection. "Broad-spectrum" indicates the product also provides meaningful UVA protection — tested by persistent pigment darkening (PPD) assays in the EU/UK or Critical Wavelength test in the USA. Look for broad-spectrum labelling plus UVA-blocking filters: zinc oxide, avobenzone (stabilised), tinosorb S/M, mexoryl.

See Sun Filters and Photodamage.