Sunlight comprises infrared, visible and ultraviolet (UV) rays. This guidance focuses on the balance of risks and benefits from the UV rays, specifically the UVA and UVB rays that reach the earth's surface. Exposure to artificial UV light (such as from sunbeds) is beyond the remit of this guideline.
Sunlight exposure offers people a number of health benefits, but excessive exposure can also cause health problems.
Vitamin D is essential for skeletal growth and bone health. The major natural source of vitamin D is from skin synthesis following exposure to sunlight. It is also obtained from dietary sources (see NICE's guideline on vitamin D: supplement use in specific population groups).
The immediate risk from overexposure to the UV rays from sunlight (both UVA and UVB) is damage to the skin's DNA and inflammation associated with sunburn. The main long‑term risk of DNA damage (for which sunburn is a proxy) is skin cancer, either built up gradually over a lifetime or due to short bursts of high exposure.
Communicating the risks and benefits of sunlight exposure is challenging. On the one hand, people have been advised to protect their skin from the sun to avoid skin cancer. On the other hand, they have been advised to expose themselves to sunlight to ensure that they get enough vitamin D.
Unless carefully interpreted, the evidence on the role of sunlight in preventing low vitamin D status can conflict with sun protection messages. In the UK, 25 nmol/litre of serum 25-hydroxyvitamin D concentration is currently used as the lower threshold for vitamin D adequacy. Below this level there is an increased risk of rickets and osteomalacia and people are considered to have vitamin D deficiency. However, the SACN is currently reviewing this threshold. (see review 2 Synthesis of effectiveness and cost effectiveness evidence from NICE's guideline on skin cancer prevention).
Overexposure to sunlight can result from spending long periods in the sun on a habitual basis. This is known as chronic exposure and can occur, for example, among people who work outdoors. Overexposure can also occur among people who, for example, spend little time outdoors and then experience short, intense bursts (intermittent exposure). This could occur during occasional holidays, weekends away or even during a sunny lunchbreak.
Chronic exposure is linked more to squamous cell carcinoma. Intermittent exposure is linked to sunburn, basal cell carcinoma and melanoma. Sunlight exposure is also responsible for some common eye diseases, for example cataracts (Yam and Kwok, 2013). Overexposure can also age the skin leading, for example, to premature wrinkling.
Studies have shown that most people are aware of the risks of overexposure to the sun but need to be frequently reminded to protect themselves (Cancer Research UK Trends in awareness and behaviour relating to UV and sun protection: 2003 to 2013). Generally, a significant disparity exists between knowledge and behaviour (Hiom 2006). This may reflect the fact that:
the sun can have a positive effect on psychological wellbeing
many people like to have a sun tan
there is a time lag between exposure and the development of skin cancer and features of photoageing, including wrinkling.
In addition, people may not understand the specific risks they face. For example, they may incorrectly assume that they only need to protect their skin while abroad. Or they may incorrectly assume that skin cancer can easily be treated.
Between October and March in the UK, sunlight contains very little of the ultraviolet B (UVB) wavelength the skin needs to make vitamin D. So people rely on body stores from sunlight exposure in the summer and dietary sources to maintain vitamin D levels (SACN update on vitamin D 2007).
Dietary sources of vitamin D are natural foods, fortified foods and supplements. Natural food sources are very limited and are mostly of animal origin (such as oily fish, red meat and egg yolk). Fortified foods include: formula milks for infants and toddlers, some breakfast cereals and fat spreads (margarines).
The National Diet and Nutrition Survey found that many adults in Britain have a low vitamin D status (23% aged 19 to 64 years and 21% aged 65 years and over). It also found that 14% of children aged 4 to 10 years, and 22% of children aged 11 to 18 years, had a low vitamin D status.
Low vitamin D status has been associated with musculoskeletal conditions – rickets, osteomalacia, falls and lack of muscle strength and function (SACN update on vitamin D – 2007). There have been reports that rickets, caused by lack of vitamin D, is re‑emerging among children in the UK (Pearce and Cheetham, 2010).
Low vitamin D status may also be associated with other non‑musculoskeletal conditions such as diabetes and some cancers. However, there is limited evidence on this and the findings are inconsistent (SACN draft report on vitamin D 2015).
Some people may not be exposed to enough sunlight because of cultural reasons, or because they are housebound or otherwise confined indoors for long periods. Some may be overzealous in the way they protect their skin (World Health Organization meeting report on Solar ultraviolet radiation: global burden of disease from solar ultraviolet radiation; Misra et al. 2008).
Excessive exposure to UV rays is an important and avoidable cause of skin cancer.
Skin cancer incidence rates (melanoma and non‑melanoma) have increased rapidly in England in the past 30 years. There are likely to be a range of reasons for this, but it may include the desire for a tan or increased travel to sunnier countries (Him, 2006).
In 2012, 11,281 newly diagnosed cases of melanoma were registered in England (Office for National Statistics Cancer registration statistics England 2012). In the same year, 102,628 cases of non‑melanoma skin cancer were registered in the UK, although the actual number is estimated at over 250,000 (Cancer Research UK Skin cancer statistics).
In 2012, 1,920 people died from melanoma in England and Wales (Office for National Statistics Mortality statistics: deaths registered in England and Wales (Series DR) 2012). Over 600 (638) died from non‑melanoma in the UK ('Skin cancer statistics').
Melanoma is the second most common cancer in those aged 15 to 34 in the UK. But the risk of all skin cancers increases with age, with people aged 65 and older most commonly diagnosed with late‑stage melanoma.
In 2008/09, it cost the NHS in England an estimated £105.2 million to treat skin cancer (Vallejo-Torres et al. 2013). This is predicted to rise to more than £180 million in 2020.
Primary care spending on treatments for low vitamin D status rose from £28 million in 2004 to £76 million in 2011 (GP online Treating vitamin D deficiency to cost £100m a year by 2013, Health and Social Care Information Centre Prescription cost analysis England 2011).