Absorbed doses of lungs from radon retained in airway lumens of mice and rats

マウスとラットの気道内腔に滞留したラドンによる肺線量

迫田 晃弘   ; 石森 有  ; 山岡 聖典*; 片岡 隆浩*; 光延 文裕*

Sakoda, Akihiro; Ishimori, Yuu; Yamaoka, Kiyonori*; Kataoka, Takahiro*; Mitsunobu, Fumihiro*

本研究では、気道内腔の空気中ラドンに起因した肺への吸収線量を計算した。本計算は、ラドン曝露実験でよく使用されてきたマウスとラットを対象に行ったが、参考のため、ヒトに対しても行った。まず、気道内腔と環境中のラドン濃度が等しいと仮定して、気道における子孫核種の生成やクリアランスをモデル化し、子孫核種の放射能を計算した。これより、分泌細胞と基底細胞を標的細胞として考え、3つに分割した肺の各領域又は全肺における吸収線量率を求めた。その結果、いずれの吸収線量率も、ヒト,ラット,マウスの順で高かった。例えば、ヒト,ラット,マウスの全肺の線量率はそれぞれ、59.9, 41.7, 25.4pGy/(Bq/m)/hであった。これら数値は、他の2種類のラドン曝露形態による線量率とも比較した。いずれの生物種においても、環境中のラドン子孫核種を直接吸入した場合で線量率が最も高いことを確認した。本研究で求めた肺の吸収線量率は、子孫核種の直接吸入によるそれと比べて、マウスで550倍、ラットで200倍、ヒトで70倍ほど低かった。本計算結果の数値は比較的小さいが、ラドンや子孫核種の吸入による線量の理解にも貢献すると考えられる。

This paper provides absorbed doses arising from radon gas in air retained in lung airway lumens. Because radongas exposure experiments often use small animals, the calculation was performed for mice and rats. For reference, the corresponding computations were also done for humans. Assuming that radon concentration in airway lumens is the same as that in the environment, its progeny's production in and clearance from airways were simulated. Absorbed dose rates were obtained for three lung regions and the whole lung, considering that secretory and basal cells are sensitive to radiation. The results showed that absorbed dose rates for all lung regions and whole lung increase from mice to rats to humans. For example, the dose rates for the whole lung were 25.4 in mice, 41.7 in rats, and 59.9 pGy/(Bq/m)/h in humans. Furthermore, these values were also compared with lung dose rates from two other types of exposures, i.e., due to inhalation of radon or its progeny, which were already reported. It was confirmed that the direct inhalation of radon progeny in the natural environment, which is known as a cause of lung cancer, results in the highest dose rates for all species. Based on the present calculations, absorbed dose rates of the whole lung from radon gas were lower by a factor of about 550 (mice), 200 (rats) or 70 (humans) than those from radon progeny inhalation. The calculated dose rate values are comparatively small. Nevertheless, the present study is considered to contribute to our understanding of doses from inhalation of radon and its progeny.