ULTRAVIOLET RADIATION AND THE EYE - Environmental UV Radiation: Impact on Ecosystems and Human Health and Predictive Models - page 258

International Commission for Non-Ionizing Radiation Protection 17,41-46 and the
American Conference of Governmental Hygienists (ACGIH) 23 are by far the widest
known. Both groups have recommended essentially the same limit based in large part
on ocular injury data from animal studies and human accidental injury studies. The
guideline to protect the skin, lens and cornea is an S UV (Ȝ) weighted daily (8-hour)
exposure H eff of 3 mJ/cm 2 or 30 J/m 2 normalized at 270 nm. This corresponds to a limit
of 27 J/cm at 365 nm. This limit is just below the level that produces a barely detectable
increase in corneal light scatter and substantially below levels that produce clinically
significant photokeratitis at 270 nm. The daily exposure limit is also about 1/3 to 1/4 of
a minimal erythemal dose and less than 1/2 the exposure necessary for clinically
reported keratitis. Annex A presents the ACGIH/ICNIRP human exposure limits based
upon S UV (Ȝ) for wavelengths greater than 250 nm.
A number of field survey measuring instruments have been developed which
employ detectors that match the S UV (Ȝ) action spectrum as shown in Figure 4 (See
Annex Table for listed values). However, the geometry of the measurement is also of
enormous importance when assessing risk of UV exposure to the eye. Outdoor safety
assessments and epidemiologic studies can arrive at erroneous conclusions if measurements
ignore geometrical factors and epidemiological assignments of exposure are seriously in
error. A number of “reasonable” assumptions previously made by some epidemiologists
regarding relative exposures have been shown to be false 6 .
13. The challenge of measuring actinic UVR in sunlight
Both the quantity (irradiance) and quality (spectrum) of terrestrial ultraviolet
radiation varies with the solar zenith angle (Z) , i.e., the angular position of the sun below
the zenith (where Z = 90 o - elevation angle above the horizon). The sun's position varies
with time-of-day, the day of the year, and latitude. This variation is particularly striking in
the UV-B spectral region (280-315 nm) because of the greater atmospheric attenuation
along the direct atmospheric path. Stratospheric ozone absorption and molecular
(Rayleigh) scattering by atmospheric N 2 and O 2 combine to attenuate the global UVR in
this spectral region. However, in the troposphere, further absorption by air pollutants such
as the oxides of nitrogen, sulfur-dioxide and ozone, and Mie scattering by water vapor in
clouds and particulates can significantly add to the attenuation. Clouds and haze reduce
the global UV (ground, horizontal) irradiance. However, haze and clouds redistribute the
UVR, so that the UVR (i.e., the sky “brightness,” or more correctly, radiance) of the
horizon-sky can actually increase in comparison to a clear, blue-sky day. Since water
vapor in clouds greatly attenuates infrared radiation, but does not significantly attenuate
ground-level UVR, the warning sensation of heat to reduce the risk of sunburn can be
absent on an overcast day. A light overcast, or light clouds scattered over a blue sky, do
little to attenuate the UV global irradiance (unless a dense cloud lies directly over the sun),
and many severe sunburns occur at the beach under such conditions. A light cloud cover
may reduce the terrestrial global UVR (measured on a horizontal surface) to about half of
that from a clear sky, although the horizon-sky UVR radiance does not decrease—and can
even increase. Even under a heavy cloud-cover the scattered ultraviolet component of
sunlight (termed the diffuse component,” or “skylight”) is seldom less than 10% of that
under clear sky 1 . Only very heavy storm clouds can virtually eliminate terrestrial UV