Radiation emitted from the sun includes visible light, heat and ultraviolet (UV) radiation. The schematic below shows this range of the electromagnetic spectrum with the very small visible light range exploded to show the colours that we are able to see.
Even the colours of the rainbow sometimes hurt!
Visible light waves emitted by the sun are the only electromagnetic waves that we can see. We see these as the colours of the rainbow. Each colour has a different wavelength. Red has the longest wavelength and violet has the shortest wavelength. When all the waves are seen together, they make white light. When this radiant energy as light, at normal intensity levels, enters our eyes, and strikes the sensitive pigment cells on the retina – the light sensitive layer of cells at the back of our eyes, it causes a photochemical reaction in the specialised retinal cells known as photoreceptors, which manifests to us as the sensation of light or sight. Clearly a beneficial effect of the visible range of radiant energy to us is sight. However this visible range can also be damaging depending on its intensity. Too intense and it would result in potentially permanent damage of the retina’s delicate photosensitive cells. Indeed this is exactly what happens when the sun is inadvertently gazed at directly, or an eclipse of the sun viewed directly without appropriate protection, causing a solar burn of the retina and often permanent damage to sight.
So what exactly is Ultraviolet (UV)?
Ultra Violet means “beyond violet” since violet is the shortest wavelength of visible light of the rainbow colours and UV light is the next shortest. The UV region of the electromagnetic spectrum covers the wavelength range 100-400 nanometres (nm)(1 nm = 1×10-9 meter), and is divided into three bands: – UVA (315-400 nm), UVB (280-315 nm) and UVC (100-280 nm). Everyone is exposed to UV radiation from the sun. This radiation can affect our skin, eyes, and immune system, regardless of our skin colour. A small amount of UV however, is essential for the production of vitamin D, yet overexposure may result in detrimental acute and chronic health effects.
As sunlight passes through the atmosphere, all UVC and approximately 90% of UVB radiation is absorbed by the ozone gas which is found mostly in a region located in the stratosphere several miles above the surface of the Earth. Absorption of most UVB occurs due to atmospheric water vapour, oxygen and carbon dioxide. UVA radiation however is less affected by the atmosphere. Therefore, the UV radiation reaching the Earth’s surface is largely composed of UVA with a small UVB component.
Various factors in the environment affect levels of ambient UV light. Firstly, the higher the sun in the sky, the higher the ambient UV radiation level. Thus ambient UV radiation varies with time of day and time of year, with maximum levels occurring when the sun is at its maximum elevation, at around midday (solar noon) during the summer months. Secondly, the closer you are to the equator, the higher the ambient UV radiation levels. UV radiation levels are highest under cloudless skies. Even with cloud cover, ambient UV radiation levels can be high due to the scattering of UV radiation by water molecules and fine particles in the atmosphere. Thirdly, at higher altitudes, a thinner atmosphere filters less UV radiation. With every 1000 metres increase in altitude, ambient UV levels increase by about 10%. Although ozone absorbs some of the UV radiation that would otherwise reach the Earth’s surface, ozone levels do vary over the year and even across the day and thus may potentially affect levels of ambient UV. Finally, UV radiation is also reflected or scattered to varying extents by different surfaces, e.g. snow can reflect as much as 80% of UV radiation, dry beach sand about 15%, and sea foam about 25%.
Is there a simple measure of UV?
A gauge of ambient UV levels is given by the UV Index, which was established by the World Health Organization (WHO). The UV Index is a concise measure that factors in time of day and year, geographic location, weather conditions, and pollution. The UV Index is reported to the nearest integer, and varies from 1 (low) to greater than 11 (extreme).
Many weather forecasts on radio, television and on weather websites provide this information as a matter of course for various locations around the world.
Some basic eye anatomy!
To better understand the effects of UV on the eye and the surrounding tissue it is important to take note of some basic anatomy of the eye.
The penetration and absorption of UV by biological tissues varies with wavelength and is dependent on the tissue involved. The longer the wavelength, the deeper the radiation will penetrate through the skin or the structures of the eye. For the eye, most UVB is absorbed by the cornea and and the remaining by the lens in the eye. Most UVA is absorbed by the crytalline lens in the eye and a little is transmitted on to the retina.
Short-term (acute) effects of UV
If your eyes are exposed to excessive amounts of UV radiation over a short period of time (acute exposure), you are likely to experience an effect called photokeratitis. Like a “sunburn of the eye”, photokeratitis may be painful and includes symptoms such as red eyes, a sensation of grit in your eyes, extreme sensitivity to light and excessive tearing. This effect is also known as “snow blindness” or “arc-eye” depending on the source of the damaging UV. Fortunately, the effects are usually temporary and rarely cause permanent damage to the eyes.
Longterm (chronic) effects of UV
However, wavelengths shorter than 400 nm are capable of breaking molecular bonds of proteins. Therefore longterm (chronic) UV exposure can change and damage cell structures and their content. Breaking bonds of protein molecules causes denaturation, resulting in opacification in the crystalline lens of the eye (i.e. causing cataract – a clouding of the eye’s lens that can blur vision and will eventually require surgery to remove).
Most of the life-long UV exposure is accumulated in childhood when the risk of sunburn is greatest. Therefore protecting children against UV radiation is particularly important. Children and teenagers’ eyes are particularly susceptible to the sun’s damaging UV rays because the crystalline lenses of their eyes are more transparent than those of adults. Their more transparent crystalline lenses allow more UV radiation to reach the retina of the eye causing potential damage. The effects of UV radiation are cumulative, so it’s important to develop good UV protection habits early in life, such as wearing sunglasses with UV protection. Follow the simple recommendations about being safe in the sun detailed above, and choose UV blocking sunglasses that fit the child’s face and lifestyle that are large enough to shield the eyes from most angles. Make sure that the sunglasses continue to fit well, are not damaged, and not out of shape and thus ill-fitting with use, and regularly adjusted to fit properly. A wide-brimmed hat for them to wear with the sunglasses would be a good idea for greater protection.
Finally visit your eye care practitioner regularly for comprehensive eye examinations and for advice on all your eye care needs.