Life on earth depends on the sun’s warmth and light. Our spirits rise in the sun, and
sunny days evoke memories of long holidays (as well as revision for exams). But the sun
has the potential to harm—there can be ‘too much of a good thing’. Sunburn, skin cancer,
and most photosensitivity disorders are caused by UVR.
How does ultraviolet radiation affect the skin?
The sun emits a broad and continuous spectrum of electromagnetic energy (the solar
spectrum). UVR is divided into short (UVC), medium (UVB), and long (UVA, ‘black
light’) wavelength emissions. Ninety-eight percent of the UVR that reaches the earth’s
surface is UVA. All UVC and a little UVB are absorbed in the atmosphere by the ozone
layer. UVB is absorbed by nucleic acids and proteins in the epidermis and stimulates the
synthesis of vitamin D3 (cholecalciferol). Some UVA is absorbed in the epidermis, but
UVA penetrates into the dermis.
The damage caused by UVR depends on the duration of exposure, as well as the skin
phototype.
• UVB causes immediate sunburn with painful erythema and, if severe, blistering. The
UVB reaction peaks at 16–24h, then the skin desquamates over 2–3 days but becomes
tanned and may develop solar lentigines (sunburn freckles).
• UVR, particularly UVB, damages DNA, and cumulative damage causes skin cancer.
• Chronic UV exposure produces many of the signs we associate with ageing
(photodamage)—wrinkling, blotchy pigmentation, telangiectasia, a sallow
appearance—as well as skin cancer.
• UVR also causes local and systemic immunosuppression, which not only leads to
problems, such as recurrent herpes simplex infection, but also contributes to the
pathogenesis of skin cancer.
How is the skin protected from sun damage?
– Nucleotide excision repair enzymes repair photodamaged DNA, but the capacity to repair
declines with age (or may be abnormal in some rare photogenodermatoses. Epidermal
melanin provides some UV protection by absorbing visible light, as well as UVR.
Melanin also quenches oxygen free radicals.
– Darkly pigmented skin has the same number of melanocytes as lightly pigmented skin,
but more and slightly differently packaged melanin (melanosomes) within keratinocytes
provides greater resistance to solar damage. However, even dark skin burns in the sun, if
the dose of UVR is high enough.
– UVA causes immediate darkening of the skin (oxidation of pre-existing melanin) that
lasts a few hours and is not photoprotective. UVB is the main stimulus for the delayed
tanning detectable 3–5 days after exposure (more melanocytes, synthesis of new melanin,
redistribution of melanosomes). This provides some photoprotection and fades slowly.
Sun exposure also induces the injured epidermis to proliferate. Thickening of the horny
layer (stratum corneum) supplies a little more protection from photodamage.
The solar spectrum
The solar spectrum ranges from cosmic rays, γ rays, and X-rays, through UVR
(100–400nm), visible light (400–800nm), and infrared radiation (800–17 000nm), to
radio waves. UVB and UVA contribute to tanning; UVB causes sunburn.