Author: Dr Dhawshini Ravindran, Core Medicine Trainee, Northamptonshire Healthcare, England. DermNet NZ Editor in Chief, A/Prof Amanda Oakley, Dermatologist, Hamilton, New Zealand. January 2018.
This article was supported by an educational grant from La Roche Posay, distributors of sun protection and skin care products in New Zealand and Australia. Sponsorship does not influence content.
Ultraviolet radiation (UVR) forms part of the electromagnetic spectrum — the energy emitted from the sun. UVR is categorised according to wavelength, which ranges from 100–400 nm.
The UVI is higher, that is, UV radiation is more intense closer to the equator compared to longer latitudes, as the sun is overhead so the distance to reach the earth is shorter. UV levels are also greater:
The NIWA Lauder ozone measurements show seasonal and annual climatological variability. Ozone column amounts are measured in Dobson Units (DU), where 1 DU = 2.69 x 1016 molecules/cm2.
Ozone is a trace gas that forms the ozone layer within the stratosphere, which is part of the atmosphere around the earth.
The ‘Montreal Protocol on Substances that Deplete the Ozone Layer’ was signed in 1987. It is an international environmental agreement to protect the ozone layer by reducing the production and usage of ozone depleting substances (ODS) such as CFCs. Since its initiation, atmospheric levels of ODS have significantly decreased and the stratospheric ozone is expected to fully recover by the year 2050.
How does ultraviolet radiation affect the skin?
Chronically sun exposed skin of bald scalp, face, neck and hands has unique characteristics compared to skin that is not exposed to ultraviolet radiation.
UVR is a major carcinogen. It damages the skin by producing reactive oxygen species (free radicals) that damage proteins, lipids, RNA and DNA. According to the two-hit model, the development of skin cancer depends on an individual’s genetic makeup (particularly MC1R signaling polymorphisms — which determine skin/hair colour and sun sensitivity) plus two factors.
Non-melanoma skin cancer
Most BCCs and SCCs in sun-exposed skin carry ‘UV signature’ mutations (cytosine to tyrosine transitions at cyclobutane pyrimidine dimers) in tumour suppressor genes (PTCH1 and p53 respectively).
UV exposure also increases the risk of:
Eyes are partially protected from sunlight by brow ridges. Before reaching the retina, reflected UVR is filtered by the cornea, melanin in the iris, aqueous humour, and the lens. UVR causes oxidative damage to mitochondrial DNA in the macular part of the neural retina and the retinal pigment epithelium. UVR can also promote inflammatory cytokines and transcription factors within the eye.
Exposure to solar UVR is associated with:
Photons of UVR are absorbed by chromophores in the epidermis and dermis, triggering an immune response.
Photosensitive skin conditions arising from failure of the normal immune response include:
The beneficial effects of UVR include:
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