Vertberate eyes are subject to various environmental stresses. For example, a relentless, year after year bombardment of photons can alter the delicate tissues of the eye. Ultraviolet light (UV), in particular, can be harmful. It triggers photochemical reactions, altering molecules that absorb such light. The result is a cascade of harmful oxidative reactions. If you've ever had a sunburn, you've experienced one form of UV's ability to damage tissue. Fortunately, most people are able to repair such damage, though with extreme sunburns this can take a while.
In some people, the dangers of UV are greatly amplified by a genetic condition that subverts tissue's normal capacity to repair UV damage to DNA. In such people, the vulnerability of the skin and eyes are dramatically increased. As an example, take the genetic disorder xeroderma pigmentosum ("XP"). An autosomal recessive condition, xeroderma pigmentosum's name means "dry skin with discolorations." This name captures very well the chronic condition of many victims' skin. In the U. S. A., XP affects about one person in every 250,000; in Japan, about one person in every 40,000 is affected. XP is caused by defect in any one of seven genes that normally provide the cell with instructions on how to make a protein that is important in repairing UV-induced DNA damage. The severity of the condition varies with which gene(s) are affected. This differentail explains why the typical case of XP in the U.S is more severe than the typical case in Japan. People with XP must avoid light, sunlight as well as strong indoor light, which can also contain UV energy. They are about 1,000 times more likely to develop skin cancers and various eye diseases than are people without XP --all because of UV exposure. In XP victims, even the most potent sunblock lotions cannot prevent damage by normal daylight.
In all people, UV exposure is deleterious to the health of the eye. The UV band of the spectrum is usually divided into three sections: UV A, UV B and UV C. UV radiation with wavelength between 314 and 400 nm is UV A. The UV B runs from 290 to 315 nm; the UV C band is below 290 nm. By the way, UV C is the radiation that is used medically, in germicidal and sterilization applications, as in germicidal lamps. The shorter wavelength bands (B and C) in sunlight tend to be more effectively absorbed by ozone in the atmosphere, which reduces their opportunity to do biological damage.
To reinforce how damaging UV can be, UV A radiation at 325 nm is about 1,000 more effective in damaging tissue than is light that is near the middle of the visible spectrum, say 550 nm. Think about that the next time you don't use UV absorbing sunglasses!
Together with changes from other sources, such as genetic accidents, exposing the eye to light --particularly strong light over many years-- can undermine visual function. The effect on the lens is particularly striking.
As a result of the orderly, geometrical arrangement of its protein fibers, the lens of the human eye is normally astonishly transparent. Light passes through relatively unimpeded on its way to the retina. The photomontage below shows how clear lenses can be --and unclear they can become. The images show lenses extracted from eyes of different ages (images come from Sidney Lerman's book Radiant Energy and the Eye, MacMillan Publishers, 1980).
The upper left panel of the photomontage shows a lens taken from a very young eye (its owner was just six months old) . That lens is so transparent that it is virtually invisible against the light gray background.Note how, over years, the lenses tend to darken --become less transparent. This means an older lens will tend to transmit less of the incident light than will a younger lens. And because most visual functions, including acuity, depend upon the amount of light reaching the retina, this darkening means most visual functions will degrade with age.
Don't be fooled by the photomontage: not all lenses from, say, a 19 year old eye, will have exactly the same degree of transparency. The loss of transparency depends upon genetics and light exposure, as well as age. In fact, with age, lenses can lose transparency at different rates. Normally, the eye has several ways to defend itself --and, as a result of genetics and age-- these defenses are differentially effective in various individuals. As John S. Werner (University of California at Davis) notes, the eye contains several antioxidant molecules, including melanin, ascorbic acid and superoxide dismutase. These antioxidants have some capacity for neutralizing phototoxic reactions. Another means of defense is provided by cells' ability to repair themselves, by replacing damaged parts. But there are limits to the effectiveness of these defenses --and the ability to repair damage usually diminishes with age.
Incidentally, an age-related loss of lens transparency can actually be helpful, despite the fact that it helps to undermine some visual functions. The older, less transparent lens is a more effective absorber of short wavelength light (including UV A) than is a young, crystal clear lens. Because it absorbs short wavelength light, the older lens keeps some harmful radiation from reaching the retina. This affords the retina's delicate cells some protection against photodamage.
So the yellowing of the lens that typically accompanies aging, may not be entirely bad. Without it, vision might be compromised even more.