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Family Doctor Books
Preview of Understanding Skin and Sunlight

What is sunlight?

Sunlight is so-called electro-magnetic radiation energy of many different wavelengths emitted by the sun; it travels through space at the enormous speed of 186,000 miles per second. Such energy provides us with the heat and light we need to live, as well as delivering damaging ultraviolet (UV) rays. The way in which this radiation affects us depends on its wave-length, which determines how it is absorbed by molecules in different tissues. These tissues include those in the eye that are responsible for vision and those 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 the health of our skin.

When these rays penetrate 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 pene-trates to ground level, and is made up of about 5 per cent UV, 40 per cent visible and 55 per cent infrared radiation.

Category Wavelength (nanometres, nm) Relevance to life on earth
Cosmic rays 0.000001 Dangerous and potentially cancer-producing, but penetrate to Earth only in insignificant amounts
Gamma rays 0.0001 Dangerous and potentially cancer-producing, but penetrate to Earth only in insignificant amounts
X-rays 0.01 Dangerous and potentially cancer-producing, but penetrate to Earth only in insignificant amounts; also used artificially in medicine
Ultraviolet (UV) 100-400 Causes short- and long- radiation term damage to exposed living matter, particularly, in humans, sunburn, photoageing and cancer of the skin
Visible light 400-800 Allows us to see; enables plants to create food molecules; drives human biorhythms; lifts human mood
Infrared radiation 800-17,000 Warms our bodies
Radiofrequency 100,000,000 Harmless and of no known radiation significant effect; used artificially for tele- communications

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 other creatures via 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 biological, so-called circadian, rhythms. Our mood and sense of well-being may also be affected by visible light; deprivation of bright light can cause a type of winter depression known as seasonal affective disorder (SAD).

Visible light energy powers photosynthesis, the process by which plants live and grow.

Very small amounts of UV radiation also promote the synthesis of vitamin D in the skin, which strengthens bones and thereby prevents rickets. However, vitamin D also comes in our diet - for example, from fish oils, some meats eggs and dairy products which usually provide all we need. Overall, it therefore seems that the UV radiation part of the spectrum may not be of any value 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, UV radiation is also sometimes used by doctors to treat skin conditions if nothing else is effective, although some damage to the normal skin still occurs during that therapy.

UV radiation

The UV radiation component of sunlight is small but biologically important, consisting of the wave-lengths 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 atmosphere and does not penetrate to ground level, so the solar UV radiation that reaches 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 nevertheless 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 effective than UVA in 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, it is responsible for only about 10 to 20 per cent of the harmful effects of sun exposure. There is clear evidence, however, that regularly exposing your skin to the high-dose UVA from most sunbeds causes damage similar to that resulting from sunlight, although sunbeds often emit a great deal of UVB as well. UVA also plays an important role in the development of a whole host of abnormal skin rashes 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 by many fluorescent and other lamps, and also by arc welding equipment, and may be an important source of exposure for people who work with them. Special UV radiation lamps are also designed for careful use under medical supervision in skin conditions such as psoriasis and eczema. Many people are further exposed in their workplace or at home to very-low-intensity UV radiation from fluorescent lights. As a result of the minimal UV output involved, however, these are not generally believed to cause measurable skin damage. However, tungsten halogen spot lamps are potentially dangerous if used continually, as they can cause sunburn after minutes to an hour or so of exposure and probably have the potential also to cause skin photoageing and perhaps cancer after many years of constant use.

HOW UV radiation behaves
  • UVC (100 to 290 nm) is completely filtered by the ozone layer and does not reach the Earth's surface.
  • UVB (290 to 320 nm) makes up about five per cent of the total solar UV radiation around midday in summer, but is responsible for 80 to 90 per cent of sunburn, photoageing and cancer.
  • UVA (320 to 400 nm) makes up about 95 per cent of the total solar UV radiation around midday in summer, but accounts for just 10 to 20 per cent of UV-related skin damage; however, it plays an important role in the development of abnormal skin reactions to the sun, the most common of which is polymorphic light eruption, commonly known as prickly heat.

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 and latitude, whereas altitude, cloud cover, terrain and the amount of sky visible are also modifying factors of 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. At this time, 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 opportunity to be absorbed or deflected in transit. As a result, about one-third of the total daily UV radiation is received between 12:00 and 14:00, and three-quarters between 10:00 and 16:00.

The higher the sun is in the sky, the shorter the distance sunlight has to travel through the atmosphere and the lower the likelihood of the radiation being absorbed or deflected.

The levels of UVB in particular vary significantly during the day, being much more susceptible to the atmosphere's 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. In practical terms, there-fore, 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 still need to keep skin exposure to a minimum between around 11:00 and 15:00 in the summer as radiation levels are persistently high during this period.

Changes in UVA and UVB levels ON A TYPICAL
CLOUDLESS SUMMER DAY*
Time
UVA (%)
UVB (%)
UVC (%)
Sunrise-
9:00-11:00
11:00-13:00
13:00
13:00-15:00
15:00-17:00
17:00-sunset
60
90
95
100
95
90
60
12.5
20
95
100
95
20
12.5
0
0
0
0
0
0
0
*Relative to intensity of UVA and UVB, respectively, at the solar zenith (13:00).0.0001Dangerous and potentially cancer-producing, but penetrate to Earth only in insignificant amounts

An easy rule of thumb is that, if your shadow is shorter than your height, you shouldn't be exposed to the sun unprotected.

An easy rule of thumb is that, if your shadow is shorter than your height, you shouldn't be exposed to the sun unprotected. Early in the morning and later in the day, however, shadows are longer and there is much less harm from sunlight.

Season

Seasonal variations in UV radiation intensity, particularly of UVB, are most pronounced in temperate climates such as in northern Europe, including the UK. In these regions UVB can vary in strength by up to 25-fold between winter and summer. UVA intensity is, however, more constant, being less susceptible to reflection, scattering and consequent weakening during a longer or shorter passage through the atmosphere.

On the other hand, nearer the equator, UV radiation levels vary much less, being high all year round, because the sun is always relatively high in the sky in the middle of the day, regardless of the time of year.

The further you move from the equator, the greater the seasonal variation in UV radiation intensity. In other words, the shorter the distance that UV radiation has to travel through the atmosphere, the less opportunity it has to be absorbed or scattered in transit.
Summer solstice (Northern Hemisphere): UV radiation has the shortest distance to travel through the atmosphere.
Vernal/autumnal equinox.
Winter solstice (Northern Hemisphere): UV radiation has the greatest distance to travel through the atmosphere

Geographical latitude

The further you are from the tropics, the less UV radiation there is: the average annual exposure of a person living in Hawaii (20 degrees N) is approximately four times that of someone living in northern Europe (50 degrees N). This again is caused by the increased distance the UV radiation has to travel through the Earth's atmosphere at higher latitudes.

UV intensity increases with altitude because of the reduced distance the radiation must travel through the atmosphere.

Altitude

As a general rule, 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 passes a shorter distance through the atmosphere to reach high-altitude regions.

Clouds only moderately reduce the amount of UV radiation reaching the ground - you can still burn on a cloudy summer's day.

Cloud cover

Clouds usually only moderately reduce the amount of UV radiation reaching the ground, having a proportionately much smaller effect than they do on temperature, so you can still burn easily 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 can, on occasion, reduce the likelihood of sunburn by about 50 per cent, and very heavy cloud by as much as 90 per cent.

In other words, it is still possible to burn in summer even when it is cloudy, cool and dull. Pollution has a similar effect to clouds, again reducing the effects of UV radiation just a little.

Wind

Wind, unless very warm, has the falsely reassuring effect of reducing your skin temperature so that you feel cool even though UVB levels are unchanged. You can therefore get as badly sunburned in a breeze as you can without one. This is even more likely on a cloudy day when you may be unaware of the sun's strength and more likely to stay out longer.

Window glass

Most glass used for 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 prevent UVA-induced skin rashes and long-term damage.

Some surfaces reflect UV radiation well, allowing more of it to reach your skin and increasing your risk of damage

Surface reflection

Some surfaces reflect UV radiation well, allowing more of it to reach your skin and increasing your risk of sunburn. Thus, grass reflects only about three per cent of UVB whereas a dry, white, sandy beach reflects up to about 25 per cent. However, although calm open water reflects no UVB when the sun is high, rippling water and rough seas reflect much more, perhaps up to 20 per cent. This means that you can get sunburned much more quickly on a beach, even under a parasol, or sailing, than in your back garden. This sunburn risk may be increased still further by UV radiation scattering from the sky (see below). Fresh snow also reflects large amounts of UVB, up to 85 per cent, which, together with the altitude and 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.

Ozone is a gas that prevents much noxious UV radiation from reaching the Earth's surface. Ozone is produced mainly at tropical and mid-latitudes in the stratosphere.

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 when you can see lots of sky you are still at risk of burning and other skin damage from UVB, even if well protected from direct sunlight by clouds, trees, buildings or a parasol. Up to two-thirds of the UVB arrives in this way and only about a third to a half in a direct line from the sun. Visible light and heat are much less affected by this process.

ENVIRONMENTAL Risk factors FOR SKIN DAMAGE
Several factors influence the intensity of sunlight and its potential to cause skin sunburn, photoageing and cancer:
  • Time of day: risk greatest between the hours of 11:00 and 15:00 in the UK, when the sun is highest in the sky
  • Time of year: risk greatest during the summer months, when the sun rises higher in the sky
  • Geographical latitude: risk greatest near the equator, where the sun always rises high in the sky
  • Cloud cover: risk greatest on a cloudless day, although light cloud only mildly reduces this risk; even heavy cloud removes only 50 to 90 per cent of the radiation
  • Reflection: risk greatest near UV-reflecting surfaces, including sand, snow and rippling water
  • Wind and water: risk not affected by the cooling effect of these
  • Amount of sky visible: risk greatest when lots of sky can be seen; up to two-thirds of UVB radiation arrives indirectly at all angles from the atmosphere (scattering) rather than just direct from the sun, so the risk is reduced by only as little as a third if the sun is directly obscured but wide expanses of sky are still visible

 

Ozone is a gas that prevents much noxious UV radiation from reaching the Earth’s surface. Ozone is produced mainly at tropical and mid-latitudes in the stratosphere.

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 and some UVB, which turns it back to oxygen again. At present, there is a 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. If, on the other hand, all this absorbed radiation did reach us, vast numbers of vulnerable single-celled organ-isms that are part of food chains, such as plankton in the oceans, would very probably die and possibly eventually end all life. While this was threatening, however, we would face increased risks of sunburn, photoageing and cancer, although we could significantly reduce these by taking more care outside.

It is now well known that certain chemicals and gases, predominantly synthetic chlorine and fluorine compounds used as aerosol propellants and coolants in fridges, can alter this ozone balance if they escape into the atmosphere and inactivate the ozone. In 1974, when scientists first saw that this was beginning to happen, they also warned about the resultant potential for an increase in UV radiation intensity at the Earth's surface. Now ozone 'holes', areas of relative depletion, have repeatedly been recorded by scientists from the British Antarctic Survey during the South Polar spring; the problem is more severe in this region because of the extreme cold which intensifies the process of inactiv-ation. For the moment, however, ozone loss elsewhere in the world and at other times of year, when UV radiation intensity is high enough to matter, is much less. Nevertheless, there is considerable concern that the phenomenon may become much more widespread if measures are not rapidly taken to reduce the responsible pollution on a world-wide scale. Fortunately, however, major steps in this direction are indeed now under way.

In summary, despite annual periods of ozone depletion in some parts of the world, particularly the Southern Hemisphere, there has not been a great deal of evidence of any corresponding significant increases in terrestrial UVB levels over the past decades. If the antipollution measures referred to above continue to be adopted, no major increases are now likely; if they are ignored, however, the risk of future problems remains extremely high. It is therefore clear that other factors have more to do with the rise in the incidence of human skin cancer over the last 50 years than any increased UVB levels as a result of ozone depletion. 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 and improved diagnostic techniques are also likely to be significant.
 
KEY POINTS
  • The ozone layer of the atmosphere filters out the solar UV radiation most harmful to living matter
  • This layer is now being depleted by synthetic chemicals, which can diffuse into the atmosphere
  • International agreement is currently reducing the use of these chemicals
  • UV radiation intensity, not yet significantly elevated in populated areas, may be maintained at normal levels by these measures
  • Other factors are responsible for the present increasing prevalence of sun-induced skin damage, most likely lifestyle changes