Why is it that media reports of rain cause most people to grab an umbrella before they leave the house, but a sunny forecast does not make these same folks reach for their sunglasses, even when the Ultraviolet Index (UVI) rises and alerts them to an increasing risk of ocular damage from intense sunlight?

Recent studies sponsored by the United States Environmental Protection Agency reveal both good news and bad news. The good news is that nine out of ten people know the UVI rates the intensity of harmful ultraviolet radiation; they also understand that, as the UVI increases, they need to increase their level of sun protection. The bad news? In spite of knowing how to interpret the UVI information, survey respondents did not always act on their knowledge. Even more worryingly, the study showed that, although women are more likely to protect their eyes with sunglasses than men, over one-half of the women surveyed confessed that they did not protect their eyes from harmful UV light on a regular basis.

Light and the Human Eye

The human eye responds to light produced by the sun in wavelengths from about 190 nanometers (nm) to 430 nm - the higher number indicates red light, while the lower indicates violet light. Light with wavelengths shorter than the eye can see is beyond violet - hence the name ultraviolet (UV). There are three bands of high-energy ultraviolet light which the earth receives from the sun: UVA (315-400 nanometers), UVB (290-315 nm) and UVC (220-290 nm).

Most of the UV light generated by the sun is absorbed by the ozone layer, or is simply reflected back into space. Only a very small amount reaches Earth; this light is received as direct rays or diffuse light. The ozone layer protects living things from much direct light but this layer, which is being jeopardized by environmental dangers caused by human technology or carelessness, can offer only partial protection from UV exposure because of the high level of diffuse UV light. UVB light is of particular interest to the 3 Os because clinical evidence linking solar UV rays to ocular damage has shown that UVB is the most damaging part of the UV spectrum to sensitive eye tissue.

Factors influencing the amount of UVB light exposure

The amount of UVB light at ground level is determined by several factors, including the following: the latitude the season ,the cloudiness of the sky, the current amount of ozone in the atmosphere, the altitude the time of day. UV light exposure is highest at noon in summer on the equator when the sky is clear.

Disorders caused by exposure to UVB rays and people at heightened risk

According to statistics published by the American Optometric Association (AOA), over 99% of harmful UV rays is absorbed by the anterior structures of the eye, although some light does reach the light-sensitive retina. And, even though almost every living thing on Earth needs a degree of natural light to see, UV radiation present in sunlight is not usefuon.

The lens, cornea and retina are all vulnerable to UVB rays. It is probable that the macula in the aphakic eye is likely at risk from UVB rays following cataract surgery unless a UV- absorbing intraocular lens was inserted at the time of surgery; ongoing research is examining the link between exposure to UVB light and age-related macular degeneration.

The AOA suggests that UV plays a part in the development of age-related cataract, cancer of the skin around the eye and corneal degenerative changes. Recent epidemiological studies on the retina suggest that intraocular malignant melanoma and other related retinal disorders and prolonged exposure to sunlight in childhood may be linked.

It has been estimated that just two hours outdoors at noon on ground covered by snow induces photokeratitis (corneal 'sunburn'); it takes about 6-8 hours to reach this state on a beach on the water. People or play outdoors in daylight (especially between the hours- between ten and three o'clock in the summer months) are three times more to develop pterygium (fleshy growths on the corneal and six times-more likely to have climatic droplet keratopathy (deposits of altered proteins) or opacified corneas. People of any age with lightcoloured eyes have less natural pigmentation to block UV light, and are therefore at increased risk.

Children typically spend lots playing outdoors. As damaging UVB light has proved to be cumulative, it makes sense to ensure that children protect their eyes with tinted lenses and wide-brimmed hats while outside. Protection for this age group is even more important because the cornea and lens of a child is extremely transparent, and UV rays are easily absorbed by young eyes. At heightened risk are patients who have recently undergone eye surgery and people who are taking photosensitizing drugs like tetracycline.

About styles, designs and coatings

There are an almost bewildering variety of styles and designs on the market today. Which style is best? The AOA recommends wrap designs to limit entry of peripheral rays which cause glare and squinting. Lenses should absorb the entire spectrum to 400 rim; tints which do not alter natural colours, like gray, brown and green, are often recommended by the 3 Os; blue block tints might be helpful to skiers, boaters and pilots as the amber lenses tend to heighten contrast. Polarized lenses cut glare, which is beneficial to drivers and people engaging in activities on water or snow. Many polarized lenses now contain a UV blocking substance. Mirror tints/metallic coatings reduce the amount of visible light entering the eyes; they do not necessarily provide UV protection.

Gradient lenses are permanently shaded from top to bottom, or from top and bottom towards the centre of the lens. Single gradient lenses are dark on top and lighter on the bottom, which cuts glare from above; double gradient lenses are darker on the top and bottom of the lenses, with a clear area in the middle, which is not appropriate for driving, as the dashboard controls are obscured. Some sunglasses carry labels which guarantee to protect eyes from up to 90 % of harmful infra-red (IR) light.

How important is infra-red protection? Several clinical studies show that there is no close connection between eye disease and exposure to IR light. And, although IR radiation produces heat, natural sun light contains low levels of IR, which the human eye tolerates well. As for trendy translucent sunwear, unless the lenses are made of polycarbonate which has inherent UV protection, experts feel that this type of sunwear does not provide enough UV protection.

Prevention is the key

Nobody is immune to UV-related eye disorders. Any factor in a person's lifestyle which increases exposure to direct sun-light represents an increased risk. Fortunately, preventing ocular damage from harmful UV radiation is simple, safe and quite inexpensive. People who work or play outdoors for long periods each day will definitely benefit from year-round sunglass protection.