By Dr. Colin Michie of AUC Medical School
Dr. Colin Michie has worked as a paediatrician in the United Kingdom, Africa, the Caribbean and the Middle East. He is specialised in nutrition, haematology and infectious diseases. Now the Associate Academic Dean for the American University of the Caribbean Medical School in Sint Maarten, his enthusiasm is training medical students and healthcare teams to ensure they deliver better value health care.
We should all use sunscreen. Our skin is our largest organ; sunscreens are a crucial 20th century medical advance that keeps our skin healthy. This summer, there have been new developments; as it appears, some types of sunscreen may harm marine life, including corals. It cannot be sensible to challenge coral regrowth, particularly after the damage marine life suffered from Hurricane Irma.
But why do we need sunscreen? Where is the hazard from the sun? If we have pigment in our skin, or perhaps a decent tan, surely screening is a waste of time? Definitely not! We all need screening from sunshine. Some physics explain why ultraviolet rays are dangerous to all of us.
Our planet is bathed in the sun’s rays: those we can see in sunshine and many we cannot. Roughly 10% of the radiation coming from the sun is beyond the colour violet, or “ultraviolet.” These rays have shorter wavelengths and higher energy. About two thirds of ultraviolet is filtered out in the atmosphere. There are three types of ultraviolet, A, B and C. The shortest wavelength of ultraviolet is called UVC – very few of these rays reach us, as their higher energy causes reactions in the upper atmosphere. UVC can be used for disinfection – for instance, in the manufacture of foods or purification of water. The short wavelengths of UVC cause molecular damage in bacteria and other pathogens, killing them.
Some 5% of the ultraviolet rays reaching us at the earth’s surface are UVB. These contain less energy than UVC but can break bonds between atoms. For this reason, on meeting our skin, these rays can result in ageing and damage to genetic material. Eyes exposed to UVB can develop keratitis and growths on the cornea. Skin exposed to UVB releases melanin – that is, tanning – as well as making vitamin D, a crucial vitamin. When the skin cannot cope with the radiation, inflammation or sunburn will develop. The longest wavelength type of ultraviolet, UVA, represents 95% of the ultraviolet that reaches us at sea level. UVA is least energetic, but it too can cause skin ageing and cancers to develop.
Levels of ultraviolet light reaching us at ground level vary, depending on where you are living, the season, the time of day and atmospheric conditions. For instance, clouds and haze tend to scatter ultraviolet rays so that one can receive more ultraviolet rays on a partially cloudy day than on a clear one. Ultraviolet wavelengths penetrate sea water reasonably well to several metres so swimming will not provide protection against ultraviolet rays. Standard window glass blocks out all but a small amount of UVA; sunglasses protect the eyes against ultraviolet damage.
The energy of ultraviolet rays can cause some substances to send out a lower energy light or luminescence. This longer wavelength of light is fluorescence. Fluorescence plays many crucial roles in marine life. Although most humans cannot see ultraviolet; animals such as insects, birds and dogs are more sensitive to these shorter wavelengths, and to the fluorescence it produces. One reason dogs might find fluorescence useful, for instance, is that it allows them to identify urine. The same trick is used by hygiene inspectors – a portable ultraviolet light helps show up poorly cleaned toilets! Fluorescence from some molecules is used in products such as paper and detergents so as to make the whites appear whiter or brighter.
The energy from ultraviolet light can be used by doctors to treat some medical conditions. The Nobel Prize for medicine was given to Niels Finsen in 1903 because he showed that ultraviolet light could be used to treat tuberculosis of the skin. UVA is employed to treat vitiligo (when the skin loses its pigment) and some types of eczema. Limited doses of UVB are given to patients with psoriasis, a condition in which areas of skin become inflamed and flaky. The ultraviolet light from sun contributes to bleaching – this is why hanging your white washing in the sunshine works!
So given that life-forms have adapted to ultraviolet light for millions of years, how big is the real risk to us? Worrying statistics show that skin cancers are becoming more common. Globally, about a million cases are diagnosed annually, 10% of which are deadly melanomas. The rates of melanoma in children under 18 rose 84% between 1975 and 2005 in the USA. This pattern is related to an increase in sun exposure of humans across the globe. Both squamous cell cancers and melanoma can develop in all skins, whether black or white, if the skin is exposed to ultraviolet light.
Charities managing patients with skin cancer have created guidelines to ensure we are all better informed and hopefully more careful about our sun exposure. For instance, men have a greater risk of melanoma than women, as they use less protection and delay seeking help about potentially malignant skin changes. Blocking of sunlight from our skins is a critical anti-cancer strategy. You might be worried about the ageing effects of these rays, but avoiding their carcinogenic effects is wise. If you have a mark on your skin that worries you, photograph it then go to visit the doctor. Doctors often ask to see you in a few months, but it is useful to document skin changes with your own pictures. (There is a helpful YouTube video entitled “How the Sun Sees You” that is educational).
Read the continuation in the next edition!
