The general rule of thumb to remember is:
"Every thing is Paak, unless you are certain it is Najis."
And the certainty has to be 100%. You have to be completely sure it is
Najis, if you are doubtful, it is considered as Paak.
The flush mechanism causes a few litres of water to push out the waste, and
it is unlikely the mist created from that would be Najis. Unless you are
sure it is Najis, it can be considered as Paak.