Dr David J Atherton explains how the sun can affect the skin and provides some simple tips on protecting children from the sun's damaging rays.
The sun emits energy in the form of electromagnetic radiation; life on earth is totally dependent on this energy. Electromagnetic radiation arrives at the earth’s surface in a wide range of wavelengths.
The longest waves are known as infrared. Infrared rays comprise about 40% of the solar energy reaching the earth’s surface. They are invisible and warm the planet, protecting it from freezing. The visible wavelengths that we recognise as light arrive as a spectrum of colours and are shorter than infrared rays. Infrared and visible light have relatively little biological effect on skin.
The shortest wavelengths that reach the earth’s surface are those that we call ultraviolet (ultraviolet radiation = UVR). These are also invisible, but have profound effects on living organisms including humans. The shortest ultraviolet wavelengths are very harmful indeed, and we are extremely fortunate that these are mostly absorbed in the ozone layer of the earth’s upper atmosphere.
Justifiable anxiety has been provoked by the discovery in the 1970’s that this protective barrier was being seriously depleted by atmospheric pollution, particularly by freons in refrigeration appliances and aerosol propellants. This should be a matter of huge concern to us all; this effect has already increased rates of lethal melanoma, and may continue to do so.
UVR is traditionally subdivided into UV-A and UV-B. UV-A comprises the longer wavelengths, UV-B the shorter wavelengths. UV-B is more intense but less penetrating. For example, UV-A will pass through glass but UV-B won’t. UV-B is responsible for the most important direct benefit of solar radiation to humans: the production in the skin of vitamin D, to which I will return later.
Effects of ultraviolet radiation on the skin
These effects can conveniently be subdivided into early and late.
Vitamin D production
Strictly speaking, "vitamin" D is not a vitamin since it is synthesised by our own bodies, and it should be more correctly regarded as a hormone, essential for normal handling of calcium by every cell in the body. Deficiency of this hormone is a particularly serious problem in children, in whom it causes rickets, a syndrome which includes impaired bone development, stunting of growth, muscle weakness, immune deficiency and fits. Industrialisation and the arrival of slums in the 18th and 19th centuries led to children being deprived of sunlight and, as a consequence, an epidemic of rickets. Eventually the cause was discovered, also the fact that it could be treated and prevented by administration of cod liver oil – hence the designation as "vitamin" D. It is important to be aware that a recent survey indicated that rickets is still with us, particularly in high-risk communities.
More recently, other benefits of vitamin D have become established. Vitamin D reduces the risk of many types of cancer including breast, ovarian, prostatic, colonic and rectal cancer. Vitamin D also protects against osteoporosis and hip fracture in the elderly. There is now good evidence that Vitamin D also reduces the frequency of respiratory tract virus infections, and this may be one of the reasons that these are so much more common in the winter months. Vitamin D also appears to reduce the risk of several autoimmune diseases, perhaps particularly multiple sclerosis.
It now appears probable that the range of levels of natural skin pigmentation seen in different ethnic groups largely relates to the importance of maintaining adequate vitamin D synthesis in different climatic zones. A dark skin, which protects against many of the harmful effects of UVR, allows adequate vitamin D synthesis in tropical zones but not in more northerly zones, in which lighter skin allowed more efficient vitamin D production in the presence of lower levels of UVR. In fact, lighter skin will produce the same amount of vitamin D in about a fifth of the time in the sun as dark skin.
In northerly climates such as the UK, even with lighter skin, humans appear to be unable to make adequate amounts vitamin D for about 6 months of the year, between October and April. It is also clear that the young are more efficient at making vitamin D than are adults.
The amount of time required to produce adequate amounts of vitamin D depends on many factors, including the latitude, the altitude, the month of the year, the time of day, the amount of cloud cover, and of course, the proportion of skin exposed. However, it is not difficult to produce vitamin D during the months from April to September. For infants with a light skin in the UK, it appears likely that 30 minutes exposure of the whole skin excluding the nappy area to sunlight is sufficient for a week’s vitamin D production. If only the face is exposed, however, a total of 2 hours exposure would be required. It is important to be aware that both a tan and sunscreens will increase the exposure time required.
Sunburn is a reflection of lethal damage to epidermal cells, and to small blood vessels in the underlying dermis. The cardinal sign of sunburn is redness (erythema), but when severe this may be accompanied by blistering, due to the accumulation of fluid below the epidermis. There is generally a delay between UVR exposure and the appearance of sunburn of 3-5 hours, but the reaction may not reach its peak until 12-24 hours. It is principally UV-B that is responsible for sunburn. How much exposure is required depends on many factors including the degree of natural pigmentation of the skin.
If a child is sunburned, it can be useful to use a topical steroid to reduce inflammation. Research is under way to establish the value of antioxidants in green tea, for example, which may also be able to reduce the risk of sun damage leading to development of skin cancer. Children who have sunburn will also have increased loss of water through the damaged skin and require oral fluid replacement, particularly if blistering occurs.
Tanning results from increased production of melanin in the melanocytes located in the epidermis. This melanin is packed into packets called melanosomes. Melanocytes have long dendritic extensions which connect with surrounding epidermal cells. It uses these extensions to inject melanosomes into epidermal cells where they provide protection against the effects of UVR. Tanning is largely a response to UV-B wavelengths. Due to the complexity of these processes, it takes about 72 hours for tanning to appear following UVR exposure.
The skin has a powerful immune system, whose primary function is protection against skin infection by a variety of micro-organisms and against infestation by parasites. UVR has a powerful suppressive effect on this function, which is probably the reason that herpes labialis (cold sores) is not an uncommon complication of sun exposure.
One might think that these are not issues with which we need to be particularly concerned when considering the UVR on children’s skin. However, it has been shown that the majority of our UVR exposure occurs in childhood, and that this is therefore when most of the skin damage occurs that leads both to UV ageing (photoageing) and to UV carcinogenesis (photocarcinogenesis). It is the protective measures we take with our children that will count in their later adult lives; the measures we take ourselves as adults are of much less importance.
Although some ageing takes place in the skin even in the absence of UVR exposure, it is estimated that about 90% of the ageing effects observed in the skin are the direct result of UVR exposure. These effects include wrinkling, dryness, mottled pigmentation and laxity. The wrinkling and laxity appear to result largely from damage to elastic fibres in the dermis.
The commonest skin cancers are basal cell carcinoma and squamous carcinoma. The risk of these cancers correlates well with cumulative lifetime UVR exposure, particularly in those with lighter skin colour.
Malignant melanoma is a more dangerous form of cancer, and one that is steadily becoming more common. While it is clear that UVR exposure is important, its precise relationship has been difficult to determine. It now appears likely though that the main provocative factor in malignant melanoma is intermittent high-level UVR exposure and sunburn in childhood. There is animal evidence that suggests that even a single sunburn experience may be enough to result in later development of malignant melanoma.
Protection of the skin from ultraviolet radiation
The skin provides a range of natural defence mechanisms against damage by UVR. The most important and best known of these is melanin pigment. As we have considered earlier, this pigment is manufactured and packaged into melanosomes in melanocytes, from where it is injected into epidermal cells. In the epidermal cells it protects by absorbing UVR energy. In general, those who start with naturally darker skin will tan faster than those with naturally light skin.
Those with naturally more pigmented skin will be better protected against the harmful effects of UVR, and the lighter-skinned will be helped by getting a tan. However the benefit of natural pigmentation and of tanning is mostly expressed in terms of protection against sunburn. It appears that natural pigmentation or a tan provide less protection against photoageing, and one can clearly see severe photoageing in photographs of elderly blacks and oriental people who have led an outdoor life. Furthermore, the process of getting of a tan by a lighter-skinned individual will involve a risk of sunburn and will certainly involve unavoidable photoageing and photocarcinogenic damage.
The skin possesses numerous other protective features in addition to melanin. The outer layer of the skin, the stratum corneum, itself reduces UVR penetration. There are also substances present in the skin and subcutaneous tissues that help reduce UVR damage, including beta-carotene.
The main aims of sun protective measures are prevention of sunburn reactions and prevention of cumulative long-term skin damage, as we have already discussed. Although the first thing that parents consider in relation to protection of their children is sunscreens, other measures are more effective and therefore more important.
Time of year, time of day and shade
Understanding the factors that determine the level of UVR exposure of the skin is the key to protection of children. Although it used to be said that one need not be concerned about the sun in the UK between November and the end of March, this is not now necessarily always the case, probably mainly because of damage to the upper atmospheric ozone layer in the last 50 years or so. Nevertheless, in general, the months April to October inclusive are the ones that matter, and the hours between 10am and 4pm, when the sun’s rays have to traverse the least amount of atmosphere to reach us. This means that we should schedule children’s outdoor activities outside these hours as far as possible in the months that matter. When this can be achieved, other protective measures will not be needed. It is when this can’t be achieved that we need to consider such measures.
It is important to be aware that UVR intensity is greatly increased at altitude, even during the winter months. Clouds do reduce UVR intensity somewhat but it does penetrate cloud depending on thickness; also clouds scatter UVR so that shade is not as effective. Snow, water and sand reflect UVR, increasing exposure and also reducing the effectiveness of shade.
Simple structural measures should be regarded as a priority for schools, such as the planting trees to provide shade in playgrounds. Parents need to consider such measures for their own gardens. Children need shade for areas of relatively static play, such as sand pits, which can be protected by a canopy.
Clothing and headwear
• Clothing and headwear are other considerations that are in my view more important than sunscreen use. In general, clothing should cover arms and legs in the summer months. Choice of fabric makes a big difference. Dark colours protect better than lighter ones. In general, the tighter the weave the more protective the fabric will be. A good way to test this is to look at the light through the fabric and check how much penetrates. If you can see through clearly, the level of protection is not very good. A thin white T shirt probably only provides the equivalent of SPF2. It is also worth noting that wet clothing is less protective than dry clothing. For added softness for children’s clothing, it is advisable to use non-biological products such as Fairy Non Bio that have a mild fragrance and have been evaluated for skin safety. Fairy Non-Bio skin safety has been reviewed by Allergy UK and awarded their British Allergy Foundation Seal of Approval endorsement. Clothes washed in Fairy Non Bio have been shown by the manufacturer to be as gentle next to skin as clothes washed in water alone.
A suitable fabric softener will minimise the friction of clothing; again it is best to choose products that have been thoroughly evaluated for skin safety like Fairy Fabric Softener.
Sunscreens are skin applications that protect by reflecting and/or absorbing UVR. There is a bafflingly large choice available. There are 2 broad categories of agents in sunscreens: physical and chemical.
Physical agents largely work by forming an opaque white barrier that reflects away UVR. Their main disadvantage is that they are usually visible as a white film that adults regard as unattractive; their whiteness will be more apparent the darker a child’s skin is naturally. Tinted versions are available, which can be more acceptable, particularly in darker skin types. Despite this problem they should be preferred for children because they are equally effective for UV-A and UV-B, because they are very much less likely to irritate the eyes and because their use avoids the child being exposed to chemical agents that are associated with a greater risk of adverse effects.
Chemical agents work by absorbing UVR energy. Each chemical absorbs best at a particular wavelength. This means that even when several chemicals are used in combination, the full spectrum of UV wavelengths will not be dealt with as effectively as by physical agents. Chemical agents are generally not as effective in the UV-A wavelengths.
The effectiveness of sunscreens is most commonly expressed as an SPF (sun protection factor) value. This value is the ratio between the doses of UVR required to produce minimal sunburn on skin with the sunscreen compared to unprotected skin. The SPF value gives a reasonable indication of the degree of protection against UV-B wavelengths but little information in respect of UV-A. No widely accepted measure of UV-A protection is yet available.
It is also very important in practice to know whether a sunscreen preparation will remain effective when a child is playing with water, or swimming, or sweating.
Sunscreens have been associated with a number of unwanted effects. The biggest problem has been allergic reactions to chemical agents, and sometimes to preservatives and other additives that these formulations may contain. However, allergic reactions seem to occur in no more than 2% of the population. Stinging sensations are not uncommon with some preparations, usually due to a content of potentially irritating substances such as alcohol or preservatives in children with particularly sensitive skin, principally those who are prone to eczema.
Getting the balance right
In the early 20th century, increased awareness of the role of sunlight in vitamin D synthesis led to outdoor activities and sun exposure being regarded as healthy. Previously a tan was regarded as a sign of lower social class, indicating outdoor work, but it was Coco Chanel who famously changed perceptions and made the suntan a fashion symbol. However, by the middle of the 20th century it was becoming clear that UVR was a cause of skin cancer, and the pendulum started to swing the other way. Now, with more evidence of a non-skin cancer-protective role for vitamin D, we are becoming concerned that we may have overdone protection against sun exposure. We are looking for a new balance and a consensus has not yet emerged.
However, for the time being it seems sensible to regard all the sun-protective measures I have listed, including sunscreen application to the face, as advisable for children in the UK (who will be in the sun for more than an hour during the peak UVR period of the day between April and October). One should choose a sunscreen having at least SPF15 which states that it has good UVA protection and is water-resistant. If a child is forced to be in the open for more than 2 hours at a high-risk time, the sunscreen should be re-applied reasonably frequently, depending on circumstances. What remains unclear is whether this level of protection will allow fully adequate vitamin D production, but even leaving hands uncovered without applying sunscreen should allow adequate vitamin D production with more or less daily exposure of around an hour. It is probably a good idea to provide an oral vitamin D supplement during the winter months, November to March inclusive.
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