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NOTE: The organic row listed in this table is for insoluble OBT. In other words the
row is for OBT that is particulate in nature, to include oil mists. See
Section 5.2.5 for a discussion of soluble and insoluble OBT.
Particle size distributions which are "monodisperse" are comprised of particles
which are all the same size. The parameter s g is the geometric standard deviation
of the distribution. When s g=1, no variability in the size distribution exists. When
s g>1, the distribution is called "polydisperse".
Subsequent text in this chapter investigates the dose from tritiated particulates as a
function of different particle size distribution assumptions, from 0.5 to 10 m AMAD.
Dose variability over this range can then be compared to the dose from a 1 m
default size. Sub-micrometer particle size distributions are only produced by hot
processes such as combustion (Cheng 1999b). This assertion is supported by
Newton, 1987, and Dorrian, 1995. These references show that high energy
processes such as oxyacetylene torch, electric arc cut rod, and plasma torch
produce sub-micrometer particle size distributions, while lower energy processes
such as band saw, reciprocating saw, and grinder produce larger particle size
distributions. High energy cutting of tritium contaminated objects will release tritium
from the objects in the form of HTO. Evaluation of particle sizes from 0.5 to 10 m
is therefore expected to be an appropriate evaluation of most respirable particulate
STCs. Dissolution Rate
The dissolution rate for a tritiated material is the rate at which tritium is released
from the material to physiological (lung) fluid as HTO. For a given tritiated
particulate intake, the dissolution rate affects the amount of tritium that remains in
the lung to impart dose there. The effect of a varying dissolution rate assumption
on the dose assigned from a given intake would be the same regardless of the
method used to determine intake. Therefore, uncertainty in the dissolution rate also
cannot be used to choose a preferred data collection method (urine bioassay vs.
fecal bioassay vs. air concentration values). ICRP 71 gives DCFs for tritiated
particulate intakes that vary from 0.3 to 14 times the DCF for HTO intakes,
depending on the Absorption Type, which is primarily a function of the dissolution
rate. Assuming the slowest dissolution rate for an intake would be a conservative
assumption, in lieu of characterizations to the contrary, but would not grossly
overestimate dose for a given intake. This is the basis behind applying ICRP 66
Absorption Type S ("slow", 99.9% dissolving with a halftime of 7000 days) in the
derivation of tritiated particulate DCF in Section 5.2.2. Default Parameters For ITPs
Dose conversion factors for ITPs are dependent on the particle size distribution and
absorption characteristics of the aerosols in question. ICRP 66 provides guidelines
for default assumptions to be used when detailed characterization data are
unavailable. In many cases, the characterization process may require breach of
contaminated systems. The assumptions listed in Table 5-7 should therefore be
used to assess dose from suspected ITP intakes. These assumptions are
conservative and consistent with ICRP 66 recommendations for assessing
occupational exposure.

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