The yin and yang of sun exposure: achieving a balance for good health
Human exposure to solar radiation primarily affects the skin and eyes. Solar energy is absorbed by a range of molecules, altering them and leading to downstream effects. While the focus has been on ultraviolet radiation (UVR), we are now beginning to appreciate that there are also effects from exposure to the broader solar spectrum. On the negative side, UVR exposure of the skin to causes DNA damage and immune suppression that together can cause skin cancer. Destruction of dermal elastin causes typical signs of photoageing. Benefits of UVR for the production of vitamin D are well-recognised, but there may be a range of non-vitamin D benefits of sun exposure. Sun protection should seek the balance between benefits and risks of sun exposure.
The title of yin and yang aims to capture the notion of a balance between the opposite pressures of obtaining sufficient sun exposure to maximise benefits, but avoiding levels of sun exposure that will incur risks to health. This is captured in Figure 1, which was first proposed in 2002(1) to weigh the benefit of vitamin D production against the risk of skin cancers and eye disease. We still do not know the shape of this curve – whether it is truly U-shaped, J-shaped, or even reverse J-shaped, and whether optimal sun exposure at point B has a narrow range, or is broader.(2) However, there is increasing evidence that benefits of sun exposure extend beyond vitamin D synthesis, and these now need to be considered in balancing risks against benefits. A major challenge to guidance about safe sun exposure is that the optimal sun exposure, point B, differs according to skin type and a range of other factors. Public health messages about optimal sun exposure need to be able to account for a wide range of individual variability in the balance of risks and benefits. In reality, there is no ‘safe’ level of sun exposure – the end point for “damage” is not sunburn, but DNA damage ...