"They are effectively holding their breath, but not before they have been exposed to the smoke."

他们暴露在烟雾之后，实际上屏住了呼吸。

You can go playing football, but not before you have finished your homework. （你可以去踢足球，但首先得完成作业。）

在这个句子里，“but not before” 用来强调时间顺序或条件。它表示某个动作或状态发生的时间是在另一个动作或状态之后。