As well as emitting visible light, the sun also emits short-wavelength ultraviolet (UV) radiation. The ozone layer absorbs the shorter, more hazardous, UV wavelengths and hence prevents them from reaching the earth's surface. This short-wavelength UV radiation can affect human health by causing skin cancer and affecting the immune system. It also contributes to various types of damage to the eyes (eg photokeratitis or snow blindness, and various opacities on and within the eyes, including pingueculae, pterygia and cataract).
What is happening to the ozone layer?
The ozone layer is being damaged by various man-made chemicals (eg chloro-fluorocarbons, or CFCs), which find their way from the earth's surface up to the stratosphere and break down the ozone molecules. Measurements made from satellites, rockets and the ground all show that at certain times of the year, the ozone layer thins by 50% over the South Pole. Fortunately, losses over Britain and other non-polar regions are very much smaller (about 4% per decade since 1979).
Amounts of potentially damaging solar UV have always been greatest at around local noon in mid-summer. They increase as we move towards the equator or to higher altitudes. Sand and snow tend to reflect substantial amounts of UV and hence increase damage risks. Protection is certainly needed during skiing or other activities on snow at high altitudes (particularly during summer) or on beaches, especially when these are near the equator.
How can I protect my eyes?
Wearing a broad-brimmed hat considerably reduces the amount of light striking the eyes when walking or standing. Ordinary clear plastic or high-index glass lenses provide considerable protection against short-wavelength ultraviolet radiation. Sunglasses provide still better protection, although it is important to check that these are made to meet the British Standard for sunglasses (BS EN 1836:1997 or BS 2724:1987): category 2 (20% transmittance) give good protection. Protection against ambient ultraviolet coming from the side or reflected from below is also desirable, and with some modern small sized frames, care must be taken to check that they also protect from above.
Do I still need protection when the sky is cloudy?
Many types of cloud have only a small effect on the amounts of ultraviolet reaching the ground. lt is, then, prudent to continue to protect the eyes from ambient ultraviolet in high-risk environments even when the sky is cloudy.
Do I need to wear sunglasses in Britain whenever it is sunny?
While under most circumstances this is not necessary from the point of view of ocular health when only relatively brief periods of exposure are involved, those spending many hours outside, particularly in summer in highly reflecting environments such as white sand beaches, should certainly wear them. Sunglasses can improve ocular comfort for everyone in bright sunny conditions.
Congleton Eye Care Centre
5-7 West Street
Tel: 01260 270941