Biokinetic Model for Soluble OBT

DOE-HDBK-1184-2004

calculate ACVo. Accordingly, the most conservative ACVo for ITPs (based on the

CEDE DCFo determined in Section 5.2.4.1 for ITPs [4.3 x 10-10 Sv/Bqo]) is 4.8 104

Bqo/m3 (1.2 Cio/m3, 1.3 10-6 Cio/cm3).

5.2.5

Biokinetic Model for Soluble OBT

For soluble forms of OBT, particularly solvents and the small molecule component

of OBT oil, the assumption can be made that all activity deposited is

instantaneously absorbed into the body (ICRP 78, 1997). Soluble OBT is,

therefore, amenable to available urine bioassay. However, the biokinetic model for

soluble OBT is different from that for HTO. Table 5-15 shows the difference in

fractional uptake and biological halftime for soluble OBTs and HTO (ICRP 78,

1997).

Table 5-15. Fractional Uptake and Biological Halftime for Soluble OBT and HTO (ICRP 78, 1997)

Fractional uptake with 10 day

Fractional uptake with 40 day

biological T1/2

HTO

97%

OBT

50%

5.2.5.1 Intake and Dose Assessment for Soluble OBT

Soluble OBT solvents or oil components do not cause self-absorption of tritium beta

activity as do solid STCs. A DCF and ACV can therefore be expressed in terms of

actual activity. The CEDE DCF that should be used for a soluble OBT intake is

4.1 10-11 Sv/Bq (152 rem/Ci) (ICRP 78), unless a more appropriate DCF is

derived and documented for the type of soluble OBT encountered in the workplace.

To calculate the ACV for vapors of soluble OBT, there are two pathways that must

be considered (which are similar to HTO pathways), inhalation and skin absorption.

If there is no evidence that a soluble OBT is absorbed through the skin, then uptake

via skin absorption is zero and the ACV can be calculated using Eq 5-12. Using

the DCF for OBTs from ICRP 78 of 4.1 10-11 Sv/Bq, an ACV of 5.0 x 105 Bq/m3 is

calculated.

If there is evidence that an OBT is absorbed via the skin the following approach

should be used to calculate the ACV. ICRP 30 states that for exposure to airborne

HTO, two-thirds (2/3) of the intake is from inhalation and one-third (1/3) is from skin

absorption. Therefore, the total intake of airborne HTO is 1.5 times the inhaled

intake. To determine an ACV, the ICRP 30 assumptions for uptake of airborne

HTO are assumed here to be valid for soluble OBT, i.e., 2/3 of the intake is due to

inhalation and 1/3 is due to skin absorption. Therefore, equation 5-12 should be

divided by a factor of 1.5 to account for the OBT taken into the body from skin

absorption. Accordingly, dividing the result of the calculation from the previous

paragraph by 1.5, the calculated ACV for soluble OBT, that accounts for skin

absorption, is 3.3 105 Bq/m3 (9.1 Ci/m3 or 9.1 10-6 Ci/cm3).