by George Aitch
With the recent spell of glorious weather, I’d like to talk about sunlight. If you’re pasty like me, maybe you think that the lacklustre start we had to summer was a good thing. Can pale people can go about their outside business safe under cover of cloud? Everywhere you look this time of year, there are a hundred examples extolling the virtues and sins of staying in the sun or the shade. What does sunlight do to your body and what’s the ideal range for exposure?
As well as the heat and light to fuel almost every ecosystem on the planet, the sun also bathes us in radiation. These waves are a mix of the visible light which we see and other waves on the electromagnetic spectrum. Fortunately, most of the harmful waves are filtered out by a combination of the Earth’s magnetosphere, generated by our liquid core, and what remains of the ozone layer.
Ultraviolet light comes in several different flavours, depending on its frequency (that is, how high energy the wave is). These subtypes are known as UVA, UVB and UVC, listed in decreasing wavelength order. We cannot see any of these – they are beyond the visible spectrum. UVC is almost completely filtered out by our atmosphere, whereas the majority of UVA makes it to the surface. Although all three of these subtypes are harmful, it is UVB which we need to worry about most – it is not completely filtered out and causes more skin damage than UVA, including cataracts, sunburns and skin cancer. It is also important to realise that UVB can reach the skin indirectly, meaning that shade and cloudy days are not completely protective from the harmful effects of the sun.
Sunburn. Ouch. Most of us have been unlucky enough to have one of these at some point. Perhaps we wanted a tan or we didn’t listen to mum when she told us to top up on sun cream. When UV rays hit your skin, a special defence system kicks in. Special cells in your skin called melanocytes absorb the waves via a pigment called melanin. The melanin converts the radiation into heat. Fair skinned people from northern climates such as Europe have less of this pigment than their counterparts from Asia and Africa. There are benefits and drawbacks to this, as we’ll see later on. When these cells are overwhelmed, the high energy waves are not processed and damage the DNA of exposed skin cells. Mostly, these cells die off and inflame the area and this shows up as red, raised and painful skin – sound familiar? Sometimes, the damaged cells do not die off and their mutated genes remain, which leads to skin cancer.
We can avoid this via covering up when it’s sunny outside and wearing a sensible amount of sun cream. Sun cream is a combination of chemicals which behave in the same way as melanin – absorbing UV waves and reemitting them as heat. Sunscreen’s efficacy is measured via its Sun Protection Factor (SPF) which measures how long it takes for UVB rays to damage the underlying skin. An SPF of 30 means that it will take 30 times longer for someone wearing that level of sunscreen to receive harmful effects from the sun.
While talking about summer sunshine exposure, I could hardly fail to mention tanning beds and salons. They have recently taken over as the leading cause of skin cancer. People going after that desired tanned skin are putting themselves at risk of melanoma – a dangerous and deadly skin cancer. All of this without arguable benefit as most tanning beds emit UVA and not UVB which provides no useful biological function. As such, most dermatologists advise against tanning salons and highlight the harms from using them.
Vitamin D is important (it wouldn’t be called a vitamin if it wasn’t). Among other things it helps our immune function (fighting disease) and calcium metabolism helping keep our bones healthy. It also prevents osteoporosis, throat infections, rickets, colorectal cancer and boosts our mood. None of this would be possible without sunlight, as vitamin D is inactive in until it is exposed to UVB radiation.
Sources of the inactive Vitamin D (precursors) include milk, mushrooms, oily fish and fortified cereals and bread. Other precursors are synthesised from cholesterol through sunlight exposure on the skin. Next, those chemicals are transformed first by the liver and then the kidney whereupon the final product, vitamin D, exerts its effects. This process is wholly dependent on sunlight, without which we see vitamin D deficiency diseases such as rickets. In order to prevent this, research suggests that we need ten minutes of sun three times a week on our heads, hands and limbs. Of course, this varies depending on where you are in the world and how dark your skin is.
Seasonal Affective Disorder
Sunlight also impacts your mental health. It may come as no surprise to many to hear that a gloomy summer spent inside without the sun produces gloomy thoughts and feelings. Your biological rhythm (termed a circadian rhythm) is partially set by light hitting the back of your eye and triggering the pineal gland in your brain to secrete melatonin, which stimulates that all important melanin. You too are solar powered! This extends to deficiency, which may produce something formerly known as Seasonal Affective Disorder (or, appropriately, SAD). SAD is a mood disorder provoked by the darker days of winter. The exact nature of the disease isn’t well understood, though light therapy is often used to combat it and it seems to work. Sunlight affects mental health, so much so that the Swedish government have attempted to combat this winter depression by installing UV lights at bus stops so people can top up while they wait.
In summary, it is difficult to state precisely how much sun exposure you need to achieve maximum benefit. This will depend on factors such as where you live, what the weather is like outside, what you eat, what clothes you wear, how dark your skin is and how much sun protection you have on. While this answer may not be satisfying, at least we can evaluate how and why varying levels of sunlight are important and what effects they have on us. From this, we can make a sensible decision. It is good to get outside in the summer and enjoy outside activities, if not because it will improve our health through sun exposure then merely because those things are fun to do. If we’re sensible and wear enough sun screen then we can enjoy the benefits without the harms and store a wealth of vitamin D throughout the year.
For good advice on living in the sun, visit NHS Choices
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Any opinions above are the author’s alone and may not represent those of his/her affiliations. Any comment is based on the best available evidence at the time of writing. All data is based on externally validated studies unless expressed otherwise. Novel data is representative of the sample surveyed. An online recommendation is no substitute for seeing your own doctor and should not be taken as medical advice. Article proofed and edited for publication by Dr LG Condon.
Sources and Further Reading
- Holick MF. Sunlight and Vitamin D. 2002. J Gen Intern Med. 17 (9) 733-5.
- Cranney A, Horsley T, O’Donnell S. “Effectiveness and safety of vitamin D in relation to bone health“. 2007. Evidence Report/Technology Assessment (158): 1–235.
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