Protecting Workers from Ultraviolet Radiation - icnirp

Protecting Workers from Ultraviolet Radiation
great variation with different sites on the face, posture, behavior, sun position in the sky and any
shading. Representative values at different sites on the head are given in Appendix B. The
exposure of the nose relative to the reference global UV exposure was shown to drop from 59%
to 11% as the head tilts from 0 o to 60 o to the vertical. Wearing a brimmed hat further modifies
the exposure over the face, especially if the hat has a wide brim. A 7 cm hat brim will reduce the
UVR facial exposure by a factor of five (Diffey and Cheeseman 1992).
4.4 Ocular Exposure to Solar UVR
Although UVR-sensitive contact lens dosimeters have been developed, they are impractical.
However, the sensitive cornea itself has served as a true ocular exposure dosimeter when subtle,
transient corneal changes were correlated with ambient UV exposure values (Sliney 1995).
Pertinent data have also come from studies of lid opening conditions combined with directional
field measurements and different environmental conditions. Ground reflection becomes very
significant in these evaluations. Appendix B provides information on the reflectance of
ICNIRP/ACGIH-weighted solar UVR and the measured effective UVR (largely UVB) from the
sky for a 40° cone field-of-view.
The relative effective UV exposure of the eye as a function of time of day does not show the
dramatic variations observed for skin exposure. The variation in lid opening plays a large role.
On an overcast day, the eyelids are more open and although the UVB irradiance is reduced by
cloud cover, the actual UVB dose rate to the eye from the sky scatter may be hardly reduced
(Sliney 1995).
Ocular exposure is far more affected by the geometry of exposure than is skin exposure as shown
in Figure 7. The UVR reaching the eye from the Sun is almost limited entirely to indirect UVR
that has been diffusely scattered by the atmosphere and reflected from the ground. The geometry
of exposures from artificial sources such as lamps, welding arcs or lasers is therefore different
from exposures from the Sun.
At sunset, the filtering of UVR and blue light by the atmosphere allows a direct view of the sun.
When the solar elevation angle exceeds 10 degrees above the horizon, squinting is observed
which effectively shields the cornea and the retina from direct exposure. These factors reduce the
exposure of the cornea to a maximum of 5 % of that falling on the exposed top of the head.
However, if the ground reflectance exceeds 15 %, photokeratitis may be produced following 1-
2 hours of midday summertime exposure. Apart from squinting, the photokeratitis threshold
would be achieved in less than 15 minutes exposure for midday summer sunlight. When the sun
is high in the sky, ocular exposure to sunlight reflected from snow produces snow blindness.
Although the cornea is more sensitive to UVR injury than the skin, acute UVR injury is not often
experienced because of behavioral avoidance of direct sunlight exposure of the eye. Individuals
do not look directly overhead when the sun is very hazardous to view, whereas most people may
stare at the sun when it is comfortable (and safe) to observe near the horizon.
A brimmed hat or other headwear associated or not with dark sunglasses, will modify greatly the
UVR exposure. When wearing sunglasses, the pupil and lids open proportionally to the darkness
of the sunglass and peripheral exposure to the eye in the absence of side shields can be
substantial.
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