ULTRAVIOLET RADIATION AND THE EYE - Environmental UV Radiation: Impact on Ecosystems and Human Health and Predictive Models - page 250

ULTRAVIOLET RADIATION AND THE EYE
DAVID H. SLINEY
US Army Center for Health Promotion and Preventive Medicine
Aberdeen Proving Ground, MD, USA
1. Introduction
The human eye is exquisitely sensitive to light (i.e., visible radiant energy), and
when dark adapted, the retina can detect a few photons of blue-green light 1 . It is
therefore not at all surprising that ocular tissues are also more vulnerable to solar
ultraviolet radiation (UVR) damage than the skin. For this reason, we have evolved with
certain anatomical, physiological and behavioral traits that protect this critical organ
from the ultraviolet (UV) damage that would otherwise be certain from the intense bath
of overhead UVR when we are outdoors during daylight. For example, the UV exposure
threshold dose for photokeratitis (also known as "snow blindness" or "welder's flash") -
if measured by an outdoor, global UVR meter designed to respond as the action
spectrum for photokeratitis - would be reached in less than 10 minutes around midday
in the summer sun 2,3 . There are three critical ocular structures that could be affected by
UV exposure: the cornea, the lens and the retina. Figure 1 shows a simple diagram of
the human eye and points to these three critical and complex ocular tissues. The eye is
about 25 mm in diameter and has an effective focal length in air of about 17 mm. Very
little UV reaches the retina. The cornea transmits radiant energy only at 295 nm and
above 1 . The crystalline lens absorbs almost all incident energy to wavelengths of nearly
400 nm. In youth a very small amount of UV-A reaches the retina, but the lens becomes
more absorbing with age. Thus there are intra-ocular filters that effectively filter
different parts of the UV spectrum and allowing only of the order of 1% or less to
actually reach the retina 4 .
The acute phototoxic effect of UVR on the eye, photokeratitis, has long been
recognized. Less obvious are potential hazards to the eye from chronic exposure.
Certain age-related changes to the cornea, conjunctiva and lens have also been thought
to be related to chronic exposure to solar UVR in certain climates. Determining
environmental ocular exposure can be quite difficult and this exposure to sunlight has
been misjudged in many epidemiological studies 2-8 . There are many occasions where
one views bright light sources such as the sun, arc lamps and welding arcs; but such
viewing is normally only momentary, since the aversion response to bright light and
discomfort glare limits exposure to a fraction of a second. Delayed effects are almost
exclusively considered to result from environmental UVR exposure. Hence, the
increased terrestrial UVR related to ozone depletion has been one cause for health
concern 9 .