Worldwide there has been an alarming increase in the incidence of skin cancer occurring over the past two decades, particularly in the populated Sunbelt regions. This near epidemic of skin cancer has resulted in many organizations, including the National Skin Cancer Foundation, heightening public awareness of the problem of solar radiation, and encouraging preventative measures. One of the most notable preventative changes that has occurred in the world has been the development of the sun protection factor (SPF) which provides rating for sun protection given to various sun screen products. The SPF system for sunscreens is based on units of time required for a given skin type to reach erythema condition under selected radiation exposure and ranges from 0 to 50 with increasing protection.
Other than the skin, the only other human organ directly exposed to sunlight is the eye. Although other parts of our body, including the immune system, may be adversely affected by sunlight, damage to the eye from certain wavelengths of sunlight is now well documented. Examples of such damage include cataract, pterygium, keratitis (snow blindness) and possibly macular degeneration. Since virtually all traditional sunscreens are toxic to the cornea and would interfere with vision, the typical method of sun protection for the eye, beyond normal anatomical and physiological protection, has been the use of sunglasses.
The spectrum of solar radiation incident on the earth's surface extends from 290 nm to 23 um. Wavelengths between approximately 400 and 1400 nm are transmitted by the ocular lens to the retina. The ocular lens of the human eye absorbs wavelengths below 400 nm. Consequently, the lens provides the primary protection for the retina from the hazardous effects of short-wavelength radiation.
The damaging effect of radiant energy upon the eye is dependent upon the wavelength or energy content of the photons. Long wavelength radiation, in the near infrared, is relatively harmless, whereas shorter wavelength radiation, in the near ultraviolet, is very damaging. For example, there is approximately 3000 times more energy required at 1064 nm than at 350 nm to produce a retinal lesion of equal severity.
Within the visible range (400-700 nm), the damaging, or toxic effects, of radiation increase progressively as photon energy rises, but not in a simple, linear manner. However, there is a sudden rise in the amount of damage produced in the retina when the photon energy reaches a wavelength of approximately 510 nm. This is followed by a precipitous increase in the severity of deleterious effects through the remainder of the visible part of the spectrum and continuing into the ultraviolet. The high-energy segment of the visible region (400 to 500 nm) is more hazardous to the retina than the low-energy portion (500 to 700 nm). Moreover, because this increased toxicity occurs at the border between the perceived colors of green and blue, the phenomenon is referred to as the blue-light hazard.
Ultraviolet (UV) radiation comprises invisible high-energy rays from the sun that lie just beyond the violet/blue end of the visible spectrum. Although more than 99% of UV radiation is absorbed by the lens of the eye, a portion reaches the light-sensitive retina. The UV radiation present in sunlight is not useful for vision. There are good scientific studies that support that UV absorption by the eye contributes to age-related changes in the eye and a number of serious eye diseases.
Ultraviolet radiation in sunlight is commonly divided into three components: UV-A (315 to 400 nm) radiation that causes tanning but is also thought to contribute to aging of the skin and skin cancer; UV-B (280 to 315 nm) radiation that can cause sunburn and predispose to skin cancer;; and, UV-C (110 to 280 nm) radiation that is nearly completely absorbed by the ozone layer before reaching the Earth's surface. UV radiation plays a role in the development of various ocular disorders including age-related cataract, pterygium, cancer of the skin around the eye, photokeratitis and corneal degenerative changes, and may contribute to age-related macular degeneration.
Clinical experience, evidence from accidental exposures, and other experimental studies show that UV-B is more damaging to the eye, presumably because it has higher energy than UV-A. The cornea and lens of the eye absorb most of the UV-B; therefore it can cause damage to these tissues but will not normally damage the retina. However, the retina, if exposed to UV-B radiation, can be damaged. UV-A radiation has lower energy than UV-B and penetrates much deeper into the eye to cause injury to the retina and lens. Neither UV-B nor UV-A has been shown to be beneficial to the eye.
Cataracts are a major cause of visual impairment and blindness worldwide. Cataracts are a cloudiness of the lens inside the eye that occurs over a period of many years. Laboratory studies have implicated UV radiation as a causal factor for cataract. Furthermore, epidemiological studies have shown that certain types of cataracts are associated with a history of increased UV radiation exposure.
Age-related macular degeneration is the major cause of reduced vision in the United States for people over age 55. Exposure to UV and intense violet/blue visible radiation is damaging to retinal tissue in laboratory experiments; thus scientists have speculated that chronic UV or intense violet/blue light exposure may contribute to the aging processes in the retina.
Pterygium is a growth of tissue on the conjunctiva of the eye that may extend onto the clear cornea where it can block vision. It is seen most commonly in people who work outdoors in the sun and wind, and its prevalence is related to the amount of UV exposure. It can be removed surgically, but often recurs, and can cause cosmetic concerns and visual loss if untreated.
Photokerititis is essentially reversible sunburn of the cornea resulting from excessive UV-B exposure. It occurs when someone spends long hours on the beach or in the snow without eye protection. It can be extremely painful for 1-2 days and can result in temporary loss of vision. There is some indication that long-term exposure to UV-B can result in corneal degenerative changes.
Children are not immune to the risk of ocular damage from UV radiation. They typically spend more time outdoors in the sunlight than adults do. Also, in young children, transmittance to the retina is greater because much less light is absorbed by the lens. Solar radiation damage to the eye appears to be cumulative and increases the risk of developing an ocular disorder later in life. Therefore, it is prudent to protect the eyes of children against UV radiation by wearing a brimmed hat or cap and sunglasses.
The use of glasses with absorptive properties when the eyes are exposed to intense sunlight represents a simple, safe, practical, inexpensive, and prudent measure designed to prevent unnecessary radiation damage to ocular tissues. Reducing damage to the outer layers of the center of the retina should slow the rate of deterioration and retard the beginning of macular degeneration. A delay in the onset of age-related macular degeneration by even a few years would significantly lower the prevalence of blindness by allowing many more individuals to complete their lifespan prior to the transition from macular senescence to one or more degenerative diseases.
In the past, sunglasses have customarily been selected and worn primarily for comfort and/or fashion as opposed to use as a medically protective device. As with sunscreens, there is now increasing public awareness of the use of sunglasses to also protect against the harmful effects of sunlight. Unlike the SPF rating system for sunscreens, there has not yet been developed an easy to use, scientifically acceptable, and understandable rating system for sunglasses.
The only generally accepted rating of sunglasses was established by the American National Standards Institute (ANSI) in 1983 which classified sunglasses into one of three broad categories; cosmetic, general purpose and special purpose, according to the filtration/absorption properties of the lens. Labeling of these three categories, however, is voluntary, and unfortunately, provides the consumer with little to no information about the protective value of the product.
There is presently no uniform, or standardized, labeling of sunglasses that provides adequate information to the consumer. Thus, there is a great need for a uniform testing and labeling system that will provide consumers with information about the solar radiation protective properties of sunglasses and other optical lenses.