A brief technical article by Sushma Panwar, founder of iees the new on-line sunglasses company
We all associate sunlight with warmth, vitality and wellbeing but as sunlight is also a source of UV radiation, it does also present a danger. Most of us are now aware of how UV rays can affect our skin and the ageing process but what we are less conscious of is how UV can harm our eyes and it may come as a surprise to discover the extent of the change and damage it can cause us.
Direct sunlight from reflecting and polished surface presents the greatest risk particularly sunlight reflected from snow, water, car windscreens and windows. It is therefore essential to reduce the affect of UV and to protect our eyes from it with the right sunglasses.
The diagram on the right shows which UV band reaches ground level. The ozone layer absorbs some but not all types of ultraviolet radiation.
Diagram used by kind permission of Environment Canada.
Short wavelength has the greatest effect on our body tissue. A photon (light) releases an electron from its orbit. This produces particles with a positive charge called free radicals. The free radicals can either modify or damage cell membranes. Fortunately this wavelength is absorbed by the ozone layer.
Diagram of the Electromagnetic Spectrum used by kind permission of Weizman Institute of Science
Radiation from the sun is absorbed by the ozone layer, this absorbs most of the UV particularly wavelengths of between 240-290nm (UVC). However, the ozone layer is expected to reduce more so until almost depletion in 2050 and this is likely to increase the cases of skin cancer and the affect on our eyes. Ozone holes are expected to increase ground level (earth's surface) radiation especially the 290-330nm band (UVA).
UV radiation wavelength is from 100nm to 400nm and these are divided into three bands: UVA 380 to 320nm, UVB 320 to 290nm, UVC 290 to 200nm. Although the earth's atmosphere does absorb UV; UVA and UVB still reach ground level and now, because of the ozone holes, UVC (short wavelength) is reaching us. This is a particular issue in the southern hemisphere.
The diagram on the right shows which UV rays reaches the eye.
Diagram used by kind permission of 20/20 Vision Center.
Eyelids - Basal cell and squamous cell carcinomas are benign skin cancers normally found around the eyelids particularly the lower lid and this is a result of sun exposure which the causes photochemical damage. Surgery is usually recommended and if discovered early, the prognosis is good.
Cornea - Absorbs UVB and UVC which can cause damage to the top layer of the cornea - the epithelium. This is called keratitis and it also reduces corneal sensitivity.
Iris - UVB can cause the iris to inflame which can be painful. This is known as uveitis.
Lens - Absorbs UVB, thought to be a factor in the development of cataracts, although formation may be slow and gradual.
Retina - Although less than 1% of radiation reaches the retina, it is still at risk. Retinal photoreceptors do not detect UV so there is no signal before it reaches it. Photoreceptors around the macula are most affected as light hits that area directly. The macula is the centre of the retina, responsible for colour and central vision, so absorption can affect vision and is thought to be one of the causes of macula degeneration.
The main symptoms of UV exposure are watery eyes and a tendency to screw up your eyes. The conjunctiva (the membrane that covers the white of the eye) also becomes red.
The best protection from the effects of UV is to wear good quality sunglasses. There are many choices, particularly for fashion and sports, and wraparound sunglasses that protect eyes at all angles are increasingly popular.
Your sunglasses should offer you 100% protection from UV and they will also improve vision; sunglasses with polarising lenses also cut out glare.
Glare can be visually uncomfortable and disabling and even cause headaches for some. Cutting out glare enhances contrast and gives you better quality vision. People with cataracts often find glare and light scatter a particular problem and will particularly appreciate this benefit.
Choosing the right sunglasses will help you maintain the health of your eyes.
Illustrations used by kind permission, as acknowledged in the text above.