Aetiology

Acute sunburn occurs as the result of exposure to ultraviolet (UV) radiation that exceeds the individual's minimal erythema dose. UV radiation consists of UVA (320 to 400 nm), UVB (280 to 320 nm), and UVC (200 to 280 nm). During the summer months, approximately 3% to 5% of the UV radiation at the earth’s surface is within the UVB waveband and >95% is UVA (no UVC reaches the earth’s surface because of atmospheric absorption). UVB is up to 1000 times more erythemogenic than UVA and is the most significant causative factor of sunburn. UVB intensity is highest between 10 a.m. and 4 p.m. and can be 2 to 4 times greater than in the early morning and late afternoon.[12] These peak hours are when exposed individuals are at greatest risk of sunburn and subsequent DNA damage.

Pathophysiology

UVB (ultraviolet B) radiation primarily targets genomic DNA, generating characteristic cyclobutane pyrimidine dimers and (6-4) photoproducts (6-4 PP) that are associated with actinic keratoses and squamous cell cancers of the skin.[13] Although the chromophore of UVA radiation is unclear, animal and epidemiological studies demonstrate that UVA over-exposure is also associated with photo-ageing and skin cancer.[14] Ultraviolet (UV) light induces p53 tumour suppressor protein within keratinocytes, leading to a transient cell cycle arrest during which either DNA is repaired or an apoptotic pathway is initiated. These apoptotic keratinocytes, recognised by their eosinophilic cytoplasm and pyknotic nuclei, are identified as 'sunburn cells', the classic histological finding on skin biopsy of acute sunburn.[15] Mediators, including prostaglandins, other arachidonic acid metabolites, substance P, and various cytokines, have been implicated in the vasodilation and increased vascular permeability of blood vessels in the upper dermis. These mediators can be produced directly by UV radiation or indirectly by keratinocyte apoptosis, which then leads to the clinical manifestations of erythema, oedema, warmth, and pain.[16]

Classification

Fitzpatrick skin type classification[1]

Skin type: baseline skin colour, sunburn and tanning history

  • I - White: always burns, never tans

  • II - White: always burns, tans minimally

  • III - White: burns moderately, tans gradually

  • IV - Olive: minimal burning, tans well

  • V - Brown: rarely burns, tans darkly

  • VI - Dark brown: never burns, tans darkly black.

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