Earlier quoted context omitted.
If by short you mean multiple weeks? It is known they have a significant effect when consumed together with a weak stimulant, like caffeine or nicotine. Tobacco smoke is another significant source in the human diet.
Where do you get multiple weeks? From what I can find, harman and norharman both have elimination half-lives shorter than half an hour. If it was weeks, they would accumulate to very toxic levels in anyone who smokes or has a few cups of coffee a day.
Your second point is spot on.