We can say with near certainty that they are less harmful, because we know what caused harm when smoking cigarettes, and those elements are not present. We also know what is being inhaled is either inert, or the point of smoking.
Also, whether a substance has a measurable LD50 or not is irrelevant. Most medicens have easily measurable LD50's, Some Vitamins have LD50's.
Well, most medicines could be considered medically toxic. It's just a matter of dosing and whether their desired effect outweighs any risk (whether negligible or non-negligible) of adverse reactions[0].
LD50 is only one of many ways to express toxicity; water can be considered toxic under some circumstances as well. It all depends on the context in which we're discussing the usage.
We don't say that Advil is "harmless", or aspirin, or caffeine, or any other substance, even in normal dosage. We simply say that any risk of harm is sufficiently small as to fall within tolerable limits, given the expected benefit of using that substance. But we can't say that about e-cigarettes - at least not yet.
In short, the medical impacts of drugs are difficult to predict, and oftentimes counter-intuitive. I'm just rather tired of the refrain that "e-cigarettes are harmless", because frankly,
(A) we don't have sufficient evidence-based research either to support or to reject that claim, and
(B) extrapolating based on existing evidence-based research and fundamental medical principles suggests that e-cigarettes are not "harmless".
Whether they are less harmful than cigarettes is a separate question, as is whether they are harmful enough to recommend avoiding usage (or regulation, or what have you) is a separate question.
I don't object to any logical arguments surrounding either of those two questions; I just object to fallacious interpretations of medical research to support illogical arguments.
[0] Unlike the word "toxic", I do not use the word "adverse" in the medical sense, which also has a very precise definition - I use it in the colloquial sense.
I can understand railing against the statement "e-cigarettes are harmless" because the active ingredients are definitely not.
That said, if you could replace every cigarette in the world right now with an e-cig. Would you not do it? I would. They have their own host of problems, but not so much as inhaling burnt plant matter has. Even with what we know now, we know it is the lesser of two evils.
I understand, you don't like people being imprecise, neither do I frankly. It's rather hard to stop though, one of those human nature thingies.
We can say with near certainty that they are less harmful, because we know what caused harm when smoking cigarettes, and those elements are not present. We also know what is being inhaled is either inert, or the point of smoking.
Also, whether a substance has a measurable LD50 or not is irrelevant. Most medicens have easily measurable LD50's, Some Vitamins have LD50's.