This is called natural background radiation.
这称为自然环境辐射。
In several countries, per capita exposure to medical radiation now exceeded natural background radiation.
在几个国家，人均接受的医疗辐照现在超过了天然本底辐射。
The committee said the detected levels of radioactive iodine-131 were below one-hundred-thousandth of natural background radiation and not considered harmful.
委员会称此次检测到的碘-131辐射量小于天然本底辐射剂量的十万分之一，不会对公众造成伤害。
For most people more than half of their natural background radiation dose comes from radon, a radioactive gas that can accumulate in homes, schools and workplaces.
对大多数人来说，他们自然环境辐射剂量的一半以上来自氡&一种可聚集在家庭、学校和工作场所的放射性气体。
On the basis of analyzing the data for the natural background radiation level in China, the typical values for gamma radiation in indoor and at outdoor, and effective dose equivalent from radon and thoron daughters are recommended.
在分析我国天然本底辐射水平数据的基础上，推荐了室内外γ辐射剂量率、氡和子体平衡当量浓度的典型值。
