It works at a very low concentration-10 partsperbillion-so you can't smell the compound.
这种物质在极低浓度下(10ppb)就可以发挥作用，因此，你是闻不到这种物质的。
2
Wasps can smell chemicals in concentrations as tiny as a few partsperbillion in the air.
黄蜂能够嗅出极小浓度的化学成份，哪怕它们在空气中的含量是十亿分之几个单位。
3
His proxies also argued that, as the current legal ozone limit-75 partsperbillion-was up for review in 2013, it anyway made sense to delay reducing it.