The poison was something that could be detected if one focused on the rods. After all, it was releasing the poison itself. It was detectable, but it wasn't curable.
It was one of the lethal poisons Han Li had ever created. And only these huge rods were the ones that could contain it. Once the poison entered the body, it would immediately suffocate the victim without anything to do about it. Not just that, it would also affect the internal organs, making them hurt like hell.
As it was incurable, the victim could only watch one's self dying.
That's what happened Old White and the monk. There was nothing to fight back a poison which has entered their body. As for Old Black? He was still alive. However, he realized the poison a tad late, thus he inhaled a bit of it before protecting himself.