这个世界，放射性物质无所不在。我们是成功生活在其中的一个物种，我们已经进化出对抗自然放射影响的保护机制。
We are a successful species inhabiting a radioactive world and must have evolved protective mechanisms to deal with the effects of natural radiation.
平均每个美国人每年接收360毫雷姆的射线，其中300毫雷姆来自自然放射源，60毫雷姆来自人类活动(雷姆是射线强度单位)。
The average American receives 360millirems of radiation each year, 300 from natural sources and 60 from man-made activities. (a rem is a unit of radiation exposure.)
当氦衰变成为氦-3相对于我们在太阳里看到的那它就可用了，因为地球上多数的氦-4是通过元素铀和钍自然放射性衰变而造成的。
What helium is available is depleted in helium-3 relative to what we see in the sun, because most of Earth's helium-4 is freshly made via natural radioactive decay of the elements uranium and thorium.
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