Van B. Nakagawara, O.D.
There has been increasing concern about the detrimental effects of ultraviolet (UV) radiation on the eyes and vision. Recently, a joint position statement on the hazards of exposure to UV radiation in sunlight was adopted by the National Society to Prevent Blindness (now known as Prevent Blindness America), the American Academy of Ophthalmology, and the American Optometric Association, which demonstrates how seriously vision specialists view this situation.
Additionally, a new UV Index Program, scheduled to be initiated this summer (sponsored by the U.S. Environmental Protection Agency and the National Weather Service), will identify daily UV radiation levels throughout the country and increase individual awareness of UV hazards. Furthermore, Bausch & Lomb Incorporated has developed a UV radiation educational program that eye doctors can share with their patients. As part of that program, a Ray-Ban UV Indexsm will supply information on daily UV radiation levels to local weather and news sources.
In this article, we will answer the questions: What is UV radiation, how does it affect vision and the eyes, and are pilots at increased risk for UV exposure?
UV radiation, which is not useful for vision, is composed of invisible high energy rays from the sun that lie just beyond the violet/blue end of the visible spectrum (Note: Figure 1). UV radiation is commonly divided into three components: UV-C (100 to 280 nanometers), UV-B (280 to 315 nanometers), and UV-A (315 to 400 nanometers). UV-C is absorbed by the earth's ozone layer and has little clinical effect in its natural state. Clinical evidence has shown that UV-B is more damaging, particularly to the crystalline lens of the eye, presumably because of its higher energy. Laboratory studies have implicated UV radiation as a casual factor for cataracts, and epidemiological studies have shown that certain types of cataract are associated with higher exposure to UV, especially UV-B radiation. UV-B radiation has been shown to contribute to the development of other ocular disorders, including pterygium, cancer of the skin around the eye, photokeratitis, corneal degenerative changes, and age-related macular degeneration. UV-A radiation has been shown to damage the retina of both aphakic and pseudophakic (aphakia corrected with an intraocular lens) eyes.
Any factor that increases sunlight exposure of the eyes will increase the risk of ocular damage from UV radiation. Individuals whose work or recreation involves lengthy exposure to sunlight are at greatest risk. UV can be reflected from surfaces such as snow, water, and white sand. The risk is greatest during the midday hours, from 1000 to 1500 hours, and during the summer months. UV radiation levels increase nearer the equator, so residents in the southern United States are at greater risk. UV levels are also greater at high altitudes. With the depletion of the ozone, more UV radiation will reach the earth, and the risk of ocular damage is expected to increase. Since the human lens absorbs UV radiation, individuals who have had cataract surgery are at increased risk of retinal injury from sunlight. Individuals with retinal dystrophy or other chronic retinal diseases may also be at greater risk, since their retinas may be less resilient to normal exposure levels. Solar radiation damage to the eye may be cumulative and exposure earlier in life may increase the risk of developing an ocular disorder later in life.
Pilots are potentially more at risk for UV radiation exposure. Since by definition pilots are at higher altitudes, they may be at risk for higher exposure, especially in open cockpits and in hot air balloons. It is estimated that there is a 4% increase in UV radiation with each 1000 feet of elevation.
Most flight activities are under visual flight rules, which expose the pilot to those times of the day when UV radiation is most intense. Flight patterns are normally away from cities and residential areas, often over undisturbed snow, water, and even sand that can reflect higher levels of UV radiation. Aircraft windscreens may protect against some forms of UV radiation, but not all, nor to the extent recommended by vision specialists. All of the ocular effects associated with UV exposure can reduce visual acuity and affect the pilot's ability to pass aeromedical certification standards.
Dr. Van B. Nakagawara manages the Civil Aeromedical Institute's Vision Research Section, Aviation Physiology Laboratory.