Macdonald C R, Laverock M J
Atomic Energy of Canada Limited, Environmental Science Branch, Whiteshell Laboratories, Pinawa, MB R0E 1L0 Canada.
Arch Environ Contam Toxicol. 1998 Jul;35(1):109-20. doi: 10.1007/s002449900357.
Protection of the environment from radionuclide releases requires knowledge of the normal background levels of radiation exposure in the exposed biotic community and an estimate of the detriment caused by additional exposure. This study modeled the background exposure and dose to the lungs of small burrowing mammals from 222Rn in artificial burrows in radon-rich soils at a site in southeastern Manitoba. E-PERM chambers used to measure 222Rn in soil showed good reproducibility of measurement, with an average coefficient of variance (CV) of about 10%. Geometric mean (GM) 222Rn concentrations at nine randomly selected sites ranged from 5,490 Bq/m3 (GSD = 1.57, n = 7) to 41,000 Bq/m3 (GSD = 1.02, n = 5). Long-term monitoring of 222Rn concentrations in artificial burrows showed large variation within and between burrows and did not show consistent variation with season, orientation of the burrow opening, or levels of 226Ra in the soil. Annual GM concentrations in individual burrows ranged from 7,480 Bq/m3 (GSD = 1.60) to 18,930 Bq/m3 (GSD = 1.81) in burrows several meters apart. A grand GM of 9,990 Bq/m3 (GSD = 1.81, n = 214) was measured over the site for the year. An exposure model was constructed for five small mammal species based on their respiration rates and the number of hours spent in the burrow, active or hibernating, exposed to soil gas 222Rn, and the time spent out of the burrow exposed to atmospheric 222Rn. A background dose of 0.9 mGy/a from atmospheric 222Rn (40 Bq/m3) was estimated for a large-bodied (80 kg), nonburrowing animal living on the soil surface. The highest exposures (mJ/a) in burrowing mammals occurred in those species with the highest respiration rates. Hibernation accounted for a small fraction of total annual exposure (<5%) because of very low respiration rates during this period. Absorbed dose to lung (mGy/a) was highest in the pocket gopher and decreased in the larger animals because of larger lung mass. Using mean 222Rn concentrations from the field studies and an equilibrium factor (F) of 0.5, doses to lung ranged from 90 mGy/a in the badger to 700 mGy/a in the pocket gopher. These doses closely correspond to those estimated from published dose conversion factors (DCFs) of 1.4 mGy per mJ . h/m3 for whole lung. For the ground squirrel, the DCF approach gives an estimated dose of 300 mGy/a versus 270 using the respiratory flow rate method. Based on these results, doses exceeding 500 mGy/a may be common in mammals and birds (i.e. the burrowing owl) living in radon-rich soils. Published risk coefficients for small mammals suggest that about 17 cancers would occur in 1,000 animals at these exposure rates. Although the potential effects from these exposures were not examined in this study, the study raises questions about how the animals may respond physiologically to this largely natural stress.
保护环境免受放射性核素释放的影响，需要了解受影响生物群落中辐射暴露的正常背景水平，并估计额外暴露所造成的损害。本研究对曼尼托巴省东南部某地点富含氡的土壤中人工洞穴内小穴居哺乳动物肺部因222Rn产生的背景暴露和剂量进行了建模。用于测量土壤中222Rn的E-PERM室测量结果具有良好的重现性，平均变异系数（CV）约为10%。九个随机选择地点的几何平均（GM）222Rn浓度范围为5490 Bq/m3（几何标准差GSD = 1.57，样本量n = 7）至41000 Bq/m3（GSD = 1.02，n = 5）。对人工洞穴内222Rn浓度的长期监测表明，洞穴内部和不同洞穴之间存在很大差异，且未显示出与季节、洞穴开口方向或土壤中226Ra水平的一致变化。相距数米的各个洞穴的年GM浓度范围为7480 Bq/m3（GSD = 1.60）至18930 Bq/m3（GSD = 1.81）。该地点当年的总GM为9990 Bq/m3（GSD = 1.81，n = 214）。基于五种小哺乳动物的呼吸速率、在洞穴内（活跃或冬眠）暴露于土壤气体222Rn的小时数以及在洞穴外暴露于大气中222Rn的时间，构建了一个暴露模型。对于生活在土壤表面的大型（80千克）非穴居动物，估计来自大气中222Rn（40 Bq/m3）的背景剂量为0.9 mGy/年。穴居哺乳动物中暴露量最高的（毫焦/年）是那些呼吸速率最高的物种。由于在此期间呼吸速率极低，冬眠占年度总暴露的比例很小（<5%）。口袋地鼠肺部的吸收剂量（毫戈瑞/年）最高，而在体型较大的动物中则因肺质量较大而降低。使用实地研究中的平均222Rn浓度和平衡因子（F）0.5，肺部剂量范围从獾的90 mGy/年到口袋地鼠的700 mGy/年。这些剂量与根据已发表的每毫焦·小时/立方米1.4毫戈瑞的全肺剂量转换因子（DCF）估计的剂量密切对应。对于地松鼠，DCF方法估计的剂量为300 mGy/年，而使用呼吸流速方法为270 mGy/年。基于这些结果，对于生活在富含氡土壤中的哺乳动物和鸟类（即穴小鸮），超过500 mGy/年的剂量可能很常见。已发表的数据显示，在这些暴露率下，每1000只小型哺乳动物中约有17只可能患癌症。尽管本研究未考察这些暴露的潜在影响，但该研究提出了关于动物如何在生理上应对这种主要为自然应激的问题。
