Modes of Entry into the Body - doe-std-1128-98

DOE-STD-1128-98

the ICRP 48 model yielding the lesser value. Factors affecting radiobiological effects

include the mode of entry of plutonium into the body, its distribution in the body, and

its transfer to a fetus.

2.4.1

Modes of Entry into the Body

Radioactive material can enter the body by four different pathways: by

inhalation, through a wound, by ingestion, or by absorption through intact

skin. These pathways may occur singly or in any combination.

-- Inhalation is probably the most prevalent mode for occupational intake

of plutonium. It also provides a generally conservative assumption of

intake for designing bioassay programs.

-- Wounds are potentially the most serious mode of intake because of the

high dose-per-unit uptake of plutonium. Wounds can result from direct

penetration by an object (i.e., a puncture or cut), from abrasion, or from

burning by an acid, caustic, or thermal source.

-- Occupational ingestion of plutonium poses a relatively small risk

because the uptake factor from the GI tract to the blood is quite small

and because most of the alpha energy from transformations within the

GI tract is absorbed by the contents of the GI tract, rather than by the

target tissues of the tract itself.

-- Absorption of plutonium through intact skin is, for practical purposes,

almost nonexistent. However, when removing skin contamination, care

must be taken to ensure that the skin integrity is not damaged by rough

or extensive decontamination procedures. If the skin integrity is

damaged, the result can be considered a wound, regardless of how it

occurred.

2.4.2

Distribution Within the Body

Three commonly encountered biokinetic models have been promulgated by

the ICRP for the internal distribution and retention of plutonium. These

models are identified by the ICRP publications in which they were first

reported: ICRP 30, Part 1 (1979), ICRP 48 (1986), and ICRP 30, Part 4

(1988b). The models are similar with regard to the organs of significance,

but differ with regard to the fraction of uptake deposited in the organ and

its respective retention (or clearance) half-time in the organ. The three

models represent the ones most widely used in dosimetry and in

commercially available computer codes. In all three ICRP models, once

plutonium has reached the bloodstream, it is translocated primarily to the

liver and skeleton. In the skeleton, it is deposited primarily on the endosteal

surfaces of mineral bone, from which it is gradually redistributed

throughout the bone volume by resorption and burial. Because of the

extremely slow nature of this redistribution, plutonium is considered to be

uniformly distributed over bone surfaces at all times following skeleton

deposition. A small fraction of the translocated plutonium reaches the

gonads. Although the gonadal fraction is different for males and females,

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