UV-A, UV-B and UV-C: the impact of each UV band on humans

Skin is the body's largest and most exposed organ, which makes it the primary site of ultraviolet damage. Whether you spend time on the beach or work outdoors for long hours, it is worth knowing which part of the ultraviolet spectrum does what, because the three bands behave very differently.

Ultraviolet is the region of the electromagnetic spectrum just beyond visible violet, and it is conventionally divided into three bands by wavelength. Here is each in turn.

UV-A rays

UV-A spans roughly 320–400 nm and makes up about 95% of the ultraviolet that reaches the Earth's surface. (Worth stating precisely: that is 95% of the UV, not of all solar radiation. Ultraviolet is only a small fraction of the total energy arriving from the sun.) Its photons carry less energy than UV-B, but its longer wavelength means it penetrates further, into the dermis. There it does its damage largely indirectly, generating reactive oxygen species that degrade collagen and elastin and damage DNA at one remove.

The consequences are photo-ageing (wrinkles, laxity, coarse texture and mottled pigmentation) plus a contribution to skin cancer risk. Ozone does not absorb UV-A appreciably, its intensity varies far less by season and time of day than UV-B, and it passes readily through ordinary window glass. It is also responsible for immediate pigment darkening, a greyish-brown colour that appears within minutes of exposure and fades within hours; persistent pigment darkening lasts longer.

UV-B rays

UV-B spans roughly 290–320 nm. Most of it is absorbed by stratospheric ozone before it reaches the ground, which is why ozone depletion is a genuine public-health concern. What gets through carries substantially more energy per photon than UV-A, and it is absorbed in the epidermis, where it damages DNA directly by forming cyclobutane pyrimidine dimers between adjacent bases.

That direct mechanism makes UV-B the main driver of sunburn and of the common keratinocyte cancers, basal cell and squamous cell carcinoma. It is also the band SPF is measured against, and the band that produces vitamin D in skin. Unlike UV-A, it varies strongly with time of day and latitude and is largely blocked by window glass, so a UV-B dose is much more variable than a UV-A one. Cloud is not a difference between them: it attenuates both bands to a similar degree.

UV-C rays

UV-C is the shortest-wavelength band, 100–280 nm by the CIE definition, and the most energetic and biologically damaging of the three. The reassuring part is that none of it reaches the ground: atmospheric ozone and molecular oxygen absorb it completely. Solar UV-C is not a skin-care concern.

Artificial sources are another matter. Germicidal UV-C lamps, welding arcs and mercury vapour lamps all emit it, and unshielded exposure causes painful photokeratitis (welder's flash) and skin burns within minutes. UV-C germicidal fixtures became far more widespread after 2020, so proper shielding and interlocks matter more than they used to.

Protecting yourself from harmful UV exposure

Because UV-A and UV-B damage skin by different routes, a product that covers only one leaves a real gap. Use a broad-spectrum sunscreen (the claim that certifies UV-A coverage, not just UV-B) at SPF 30 or higher, and reapply roughly every two hours.

The formulator's side of this is choosing filters whose combined absorbance stays flat across 290–400 nm and holds up under irradiation. Photostable triazines such as Sunbest-BMT and Sunbest-HEB and mineral filters such as Sunbest-ZnO are used together for exactly that reason. For how the bands are covered in practice, see UV-A vs UV-B filters.