The health advisory limits are in the part per quadrillion.

健康咨询限度是以千亿分为单位。

Part per quadrillion is one thousandth of that.

千亿分是这个数字的千分。

The nucleus of an atom is tiny-only one millionth of a billionth of the full volume of the atom-but fantastically dense, since it contains virtually all the atom's mass.

原子核很小——只有原子全部容量的千亿分，但密度极大，它实际上构成了原子的全部物质。