Annual Report

50. Estimation of Internal Dose by Blood Analysis for Exposure to Tritium in Various Chemical Forms

Hiroshi Takeda, Kiriko Miyamoto, Shoichi Fuma, Kei Yanagisawa, Nobuyoshi Ishii and Noriko Kuroda

Keywords: tritium, dose estimation, bioassay, blood, urine, rat


Tritium is released mainly as tritiated water (HTO) or as gaseous tritium (HT) from nuclear facilities into the environment. Differing from gaseous tritium, tritiated water easily enters plants and animals and is partly incorporated into their organic constituents. Human exposure to environmental tritium arises in both forms of tritiated water and organic tritium through the ingestion of water and food contaminated with tritium. Various tritiated compounds are produced and used for medical and biological studies. The workers engaged in these industrial and research tasks will be accidentally exposed to these tritiated compounds. To estimate radiation dose from the exposure to tritium, urine bioassay has usually been performed. However, it is not expected to accurately estimate internal doses for the exposure to organic tritium.

The purpose of the present study is to develop a method applicable to internal dose estimation after exposure to tritium in various chemical forms. In rats exposed to tritiated water or some tritiated organic compounds by a single ingestion, the concentrations of total tritium and organically bound tritium (OBT) in blood and in various internal organs were determined at various time intervals after ingestion. The concentrations of total tritium in urine were also periodically determined. The results showed that the concentrations of total tritium in blood were a little higher than those in most internal organs for all tritiated compounds examined in this study. On the other hand, the concentrations of total tritium in urine were usually lower than those in the majority of the internal organs for most of the period after the ingestion, except a very early period after ingestion of tritiated water when the concentration was highest in blood.

When the cumulative doses to blood and internal organs for 100 days after ingestion of various tritiated compounds were compared, the doses to blood were almost the same as the highest doses to internal organs (Table 7). This indicated that blood analysis would be useful to estimate the maximum internal doses after exposure to tritium in various chemical forms. It was also found from the analysis of blood that the concentration ratio of OBT to total tritium in blood could be used to deduce the circumstances of the exposure. The concentration ratios after ingestion of tritiated water were very low, 0.05 on the first day and 1.0 on the 20th day, while the ratios after ingestion of tritiated organic compounds were 0.4 to 1.6 on the first day and 1.3 to 3.3 on the 20th day. Thus, the ratios were dependent on the chemical form of tritium at exposure and on the time interval after the exposure. It was, therefore, suggested that blood analyses might be used not only for estimating the maximum internal doses, but also for inferring the chemical form of tritium at exposure and the elapsed time after exposure. Although these results were obtained from animal experiments, it could be expected that a bioassay method applicable to human exposure from various tritiated compounds could be developed by comparative study of the metabolic turnover rate between humans and rats.

Table 7. Radiation doses to blood and internal organs of rats for 100 days after a single ingestion of tritiated water or some tritiated organic compounds.
  Cumulative dose (mGy)a for 100 days after a single ingestion
3H-water 3H-leucine 3H-lysine 3H-glycine 3H-glucose 3H-thymidine
Blood
Internal organs
15
12-31
39
21-39
81
39-80
39
24-32
17
14-17
24
16-24
a) : The cumulative doses were calculated assuming that the same amount of radioactivity (37 kBq) per g of body weight was ingested for all the tritiated compounds.


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