Solar radiation
WHAT IS SUNLIGHT?
Sunlight is so-called electromagnetic radiation energy of many different wavelengths emitted by the sun at the enormous speed of 186,000 miles per second. This energy provides us with the essential heat and light we need to live, as well as delivering damaging ultraviolet (UV) rays. These effects depend on the wavelength, which determines how radiation is absorbed by molecules in different tissues. These tissues are in particular those in the eye responsible for vision and in the skin, which are both susceptible to UV injury. In addition, there are a host of other solar rays, such as cosmic rays, gamma rays, X-rays and radio-frequency radiation, but these are present in too small quantities at the surface of the Earth or of too low an energy to affect our skin significantly.
When any of these rays enter the Earth’s atmosphere, they are modified in various ways. For example, visible light is scattered by atmospheric oxygen and nitrogen molecules in such a way that it makes the sky look blue; in addition, some of the overall radiation energy is absorbed and some reflected back into space by these molecules as well as by atmospheric water vapour, dust particles and other constituents. The result is that only about two-thirds of the solar energy arriving at the surface of the atmosphere penetrates to ground level, where it is made up of about 5 per cent UV, 40 per cent visible and 55 per cent infrared radiation.
WHY SUNLIGHT IS IMPORTANT
The energy from sunlight has been essential for the evolution of life on Earth. It has provided visible light for photosynthesis, the process by which plants use such energy to grow and eventually provide food for all other creatures through the food chain. In addition, its infrared rays have given us the warmth we need to live, while visible light is the part of the spectrum that our eyes need to see, and the part that drives our life’s daily rhythms. Our mood and sense of well-being may also be affected by visible light; deprivation of bright light can for instance cause a type of winter depression known as seasonal affective disorder (SAD).
Small amounts of UV radiation also promote the synthesis of vitamin D in the skin, which strengthens bones to prevent rickets and osteomalacia. However, vitamin D also comes in our diet – for example, from fish oils, butter, egg yolks and liver which can provide all we need. Overall, it therefore seems that the UV radiation part of the spectrum may not be of any essential use to us at all, but instead is just responsible for most of the harmful effects associated with sun exposure, such as skin sunburn, photoageing and cancer. However, in spite of this, UV radiation may need sometimes to be used by doctors to treat certain skin conditions if nothing else is effective, although some skin damage also occurs during this process.
UV RADIATION
The UV radiation component of sunlight is small but biologically important, consisting of the wavelengths between 100 and 400 nanometres (nm). These are then further subdivided into three categories:
• UVC: 100–290 nm
• UVB: 290–320 nm
• UVA: 320–400 nm.
UVC is completely absorbed by ozone in the Earth’s atmosphere, so the solar UV radiation reaching us consists only of UVB (up to about five per cent) and UVA (95 per cent or more); these percentages are, however, approximate and the relative amounts vary considerably with the time of day and year, latitude and other factors. Although UVB accounts for only a small proportion of the total solar UV radiation, it is extremely important because these are the wavelengths that are mainly responsible for causing sunburn, photoageing and cancer of the skin. This is because they are many times more efficient than UVA at causing harmful changes to the genetic material of living cells, namely DNA. As a result, even though UVA comprises about 95 per cent of the total solar UV radiation around midday in summer and in the tropics, it is responsible for only about 10 to 20 per cent of the harmful effects of exposure at these times. There is clear evidence, however, that regularly exposing your skin to high-dose forms of UVA from some sunbeds causes damage similar to that resulting from sunlight, although sunbeds now emit a great deal of UVB as well. UVA also plays an important role in the development of a whole host of abnormal skin disorders caused by the sun.
OTHER SOURCES OF UV RADIATION
By far the most important source of UV radiation on Earth is the sun, although the radiation is also emitted artificially and apparently harmlessly in very small amounts by many fluorescent and other household lamps, as well as by similar lamps used medically to treat skin disease and by arc welding equipment, such that it may be an important source of exposure for people who work with these latter devices. Some household tungsten halogen spot lamps are also potentially dangerous if used regularly, as they can cause sunburn after minutes to an hour or so of exposure and probably have the potential also to cause skin photo-ageing and perhaps cancer as a result of frequent use over many years.
HOW UV RADIATION LEVELS VARY
The factor that mainly influences the intensity of terrestrial UV radiation is the height of the sun in the sky, which depends on the time of day, season of the year and terrestrial latitude, whereas altitude, amount of cloud cover, type of terrain and the amount of sky visible are modifying factors of rather less importance.
Time of day
The highest levels of UV radiation in the UK are received in summer within the four hours encompassing the solar zenith (when the sun is at its highest point in the sky), namely between 11:00 and 15:00. During this period, the angle of the sun relative to the Earth’s surface is such that sunlight has the shortest distance to travel through the atmosphere and the least chance of being absorbed or deflected in transit. As a result, about one-third of the total daily solar UV rays is received between 12:00 and 14:00, and three-quarters between 10:00 and 16:00.
The levels of UVB in particular vary significantly during the day, being much more susceptible to atmospheric effects than those of UVA and visible light; thus, UVB intensity increases and then decreases by many times between the hours of 10:00 and 16:00 in summer and in the tropics. In practical terms, therefore, this means that the risk of sunburn is greatest around 13:00 in this country, namely when the sun is at its highest, although you should minimise exposure throughout the period, when between 11:00 and 15:00 in summer radiation levels are persistently high.
An easy rule of thumb is that, if your shadow is shorter than your height, you shouldn’t spend time exposed to the sun unprotected. Early in the morning and later in the day, when shadows are longer, much less harm occurs.
Season
Seasonal variations, particularly UVB, in UV radiation intensity, are most pronounced in temperate climates such as those of northern Europe, including the UK, where changes of up to 25-fold occur between winter and summer. UVA intensity is, however, more constant, being less susceptible to attenuation by reflection, and scattering during the passage through the atmosphere.
On the other hand, UV radiation levels vary much less in the tropics, being high all year round, because the sun is always high in the middle of the day.
Geographical latitude
The further you are from the equator, the less UV radiation there is at any given time, the average annual exposure in Hawaii (20 degrees N) being about four times that in northern Europe (50 degrees N). This again is caused by the increased distance that UV radiation has to travel through the atmosphere at higher latitudes.
Altitude
For every 300 metres (around 1,000 feet) of increase in altitude, the ability of UV radiation to cause sunburn increases by about four per cent; this is because it has a shorter transit through the atmosphere at altitude.
Cloud cover
Clouds usually only moderately reduce the UV radiation intensity reaching the ground, having a much smaller effect than they do on temperature, so you can still burn on a cloudy summer’s day, even if it feels cool. This is because the water in clouds absorbs heat much better than UV rays.
Thus, scattered clouds in a blue sky make only a small difference to the levels of UVB, although complete light cloud cover may reduce the sunburn risk by up to 50 per cent, and very heavy cloud by up to 90 per cent.
In other words, it is possible to burn, in summer and in the tropics, in cloudy, cool, dull weather. Pollution has a similar effect, again tending to reduce the effects of UV radiation just a little.
Wind
Wind, unless warm, has the falsely reassuring effect of reducing skin temperature so that you feel cool even though UV levels are unchanged. You can therefore get as badly sunburned in a breeze as without one, which is even more likely on a cloudy day and which may cool you down even more.
Window glass
Most glass used in windows and car windscreens blocks UVB but not UVA nor, of course, visible light. This means that, although glass markedly reduces the risk of sunburn, it does not protect against UVA-induced skin rashes and long-term damage.
Surface reflection
Some surfaces reflect UV radiation, allowing more of it to reach your skin and increasing your risk of sunburn. Thus, although grass reflects only about three per cent of incident UVB, a dry, white, sandy beach reflects up to 25 per cent. Further, although calm open water reflects no UVB when the sun is high, rippling water and rough seas may reflect much more, perhaps up to 20 per cent. This means that you will sunburn much more easily on a beach, even under a parasol, or sailing, than in your back garden, a risk that may be increased still further by UV scattering from the sky (see over).
Fresh snow also reflects profuse UVB, up to 85 per cent, which, together with the altitude and the misleading cooling effects of wind and weather, accounts for the often severe sunburn experienced by unwary skiers, even in winter.
Temperature
The ambient air temperature (for example, 10°C versus 30°C) or the temperature of any water in which you may be swimming, unless you are on a dive at least several feet below the surface, has little influence on UVB radiation intensity.
Scattering from the sky
UV radiation does not pass smoothly through the Earth’s atmosphere, but undergoes many collisions with air molecules on the way, much as snooker balls collide. As a result the rays reach the ground at all angles from the sky. So if you can see lots of sky you are at risk of UVB skin damage, even if protected from direct sunlight by clouds, trees, buildings or a parasol. Up to two-thirds of the UVB arrives in this way, about a third to a half in a direct line from the sun. Visible light and heat are much less affected.
OZONE DEPLETION AND SKIN CANCER
Ozone is a gas created from oxygen in the upper atmosphere by solar UVC radiation; the ozone then absorbs more UVC as well as some UVB, which turns it back to oxygen again. In the past, there has been a delicate balance between the production and destruction of ozone, the absorption of all UVC and some UVB in the process preventing much noxious radiation from reaching the Earth. However, there is now irrefutable evidence that this balance is gradually changing because of the depletion of ozone by synthetic substances such as from aerosol sprays, although, fortunately, mostly just at times and places where the UV intensity is weak and unimportant, such as spring in the Antarctic. This has steadily worsened although international measures are attempting to prevent it; if all solar UV radiation did reach us, many vulnerable single-celled and other small organisms that are part of food chains, such as plankton in the oceans, might well be compromised and eventually change or end all life on Earth. While this was threatening, we ourselves would face steadily increasing risks of sunburn, photo-ageing and cancer, although we could significantly reduce these by taking more care when outside.
It is therefore clear that factors other than increased UVB levels as a result of ozone depletion are responsible for the rise in the incidence of human skin cancer over the last 50 years. Of these, probably the most important is that we now spend much more of our increasing leisure time in the sun, although the greater age of our population, giving cancers time to develop, and improved diagnostic techniques, enabling us to confirm that cancers are actually present, are also likely to be significant.
