I would say that's not completely accurate either at a biological level. For example, if a neurotoxin merely disables sodium channels temporarily, that's differ

I would say that's not completely accurate either at a biological level. For example, if a neurotoxin merely disables sodium channels temporarily, that's different from a neurotoxin that somehow permanently disrupts or damages neurons (I believe some heavy metals work that way?). In the case of permanent damage, there is essentially almost no "safe dose", although I guess very small amounts will be negligible compared to background cell death. But you essentially want as little as possible, which is different from water, that goes from therapeutic to toxic at extreme amounts. (Not sure how TCE toxicity works)