Bioassay and Internal Dosimetry

BIOLOGICAL PROPERTIES

DOE-HDBK-1079-94

Tritium Primer

Bioassay and Internal Dosimetry

Exposure to tritium oxide (HTO) is by far the most important type of tritium exposure. The

HTO enters the body by inhalation or skin absorption. When immersed in tritiated water vapor,

the body takes in approximately twice as much tritium through the lungs as through the skin.

Once in the body, it is circulated by the blood stream and finds its way into fluids both inside

and outside the cells.

According to International Commission on Radiological Protection (ICRP) Publication 30, the

derived air concentration (DAC)c for tritium gas (HT) and HTO are 540,000 Ci3/m and

21.6 Ci/m3, respectively. The ratio of these DACs (25,000) is based on a lung exposure from

the gas and a whole body exposure from the oxide. However, as was noted earlier, when a

person is exposed to HT in the air, an additional dose actually results: one to the whole body.

During exposure to HT, a small fraction of the tritium exchanges in the lung and is transferred

by the blood to the gastrointestinal tract where it is oxidized by enzymes. This process results

in a buildup of HTO until the HT is removed by exhalation at the end of the exposure. The

resultant dose from exposure to this HTO is roughly comparable to the lung dose from exposure

to HT. Thus, the total effective dose from an HT exposure is about 10,000 times less than the

total effective dose from an equal exposure to airborne HTO. For both HTO and HT exposures,

a bioassay program that samples body water for HTO is essential for personnel monitoring at

tritium facilities.

Sampling Schedule and Technique

After HTO enters the body, it is quickly distributed throughout the blood system and, within 1

to 2 hours, throughout all water in the body. Once equilibrium is established, the tritium

concentration is found to be the same in samples of blood, sputum, and urine. For bioassay

purposes, urine is normally used for determining tritium concentrations in body water.

Workers who may be or who have been exposed to tritium are normally required to submit urine

samples for bioassay periodically. The sampling period may be daily, biweekly, or longer,

depending on the potential for significant exposure.

Special urine samples are normally required after an incident or a work assignment with a high

potential for exposure. After a possible exposure, the worker should empty the bladder 1 to

2 hours later. A sample taken after the bladder is emptied should be reasonably representative

of the body water concentration. A sample collected before equilibrium is established will not

be representative because of dilution in the bladder, or because of initial high concentration in

the blood. However, any early sample may still be useful as a sign of the potential seriousness

of the exposure.

c. The DAC is defined as that concentration of a gas, which, if a worker were exposed to it for one

working year (2,000 hours), would result in an annual dose of 5 rem.

Tritium

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