Environmental Engineering Reference
In-Depth Information
value of 0.01 from 310-380 nm; 41 hence, the hazardous integrated radiance for a pure
UV-A source is about 2 kJ/(cm 2 ·sr) for the phakic eye. However, in infancy or during
cataract surgery after the cataractous lens has been removed (aphakia) and before a UV
absorbing intraocular lens implant has been placed in the eye (pseudophakia), the retina
can receive considerable UV exposure. The aphakic hazard spectrum A(Ȝ) can be
applied for a hazard analysis (Figure 4).
12. Guidelines for human exposure
Based upon the extensive research carried out during the last few decades on
both acute and delayed effects resulting from UV exposure of the eye, safety guidelines
for limiting UVR exposure to protect the eye have been developed. The guidelines were
fostered to a large extent by the growing use of lasers and the quickly recognized hazard
posed by viewing laser sources. To assess potential hazards, one must not only consider
the optical and radiometric parameters of the optical source in question, but also the
geometrical exposure factors. This knowledge is required to accurately determine the
irradiances (dose rates) to exposed tissues. Thermal injury is rare unless the UV source
is pulsed or nearly in contact with tissue. Generally, photochemical interaction
mechanisms dominate in the UV spectrum where photon energies are sufficient to alter
key biological molecules. A characteristic of photochemically initiated biological
damage is the reciprocity of exposure dose rate and duration of exposure (the Bunsen-
Roscoe Law), and acute UV effects are therefore most readily observed for lengthy
exposure durations of many minutes or hours. The current guidance for UV exposure at
wavelengths greater than 315 nm (UV-A) is 1 J/cm 2 , and this was based upon
conservative assumptions designed to protect the intact crystalline lens from both
thermal and photochemical stress.
As with any photochemical injury mechanism, one must consider the action
spectrum, which describes the relative effectiveness of different wavelengths in causing
a photobiological effect. 1,4,42-45 The relative action spectra for both UV hazards to the
eye (acute cataract and photokeratitis) are shown in Figure 4. 18-22 The UV safety
function S UV (Ȝ) is also an action spectrum which is an envelope curve for protection of
both eye and skin is shown in Figure 4.
The S UV (Ȝ) curve of Figure 4 is an action spectrum which is used to spectrally
weight the incident UVR to determine an effective irradiance for comparison with the
threshold value or exposure limit. 17 With modern computer spread-sheet programs, one
can readily develop a method for spectrally weighting a lamp's spectrum by a variety of
photochemical action spectra. The computation may be tedious, but straightforward:
E eff = E Ȝ ·S UV (Ȝ)·ǻȜ
(1)
The exposure limit is then expressed as a permissible effective irradiance E eff or
an effective radiant exposure. One then can compare different sources to determine
relative effectiveness of the same irradiance from several lamps for a given action
spectrum.
A number of national and international groups have recommended virtually the
same occupational or public exposure limits for UVR. The guidelines of the
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