So no, I am not suggesting anyone try this, because it would need study and in any case I'm probably mistaken. I'm suggesting ... if I'm not mistaken... it is worth someone studying....
So two possible reasons low dosage viral exposure are not contagious come to mind, first, there are no cells infected by the exposure, so no clinical effect. The second, the exposure does infect such a small number of cells the body's defense mechanism can easily fight off the infection. Not sure if the answer to that question is known or not.
If it is the latter,
Might that explain why the general population is becoming full of people with antibodies, who where never symptomatic? In NY a study showed 20% of asymptomatic people had antibodies as of May 1. (see worldometer among others for details)
And if that is indeed the effect, could not micro exposure (proximity, not inoculation) have a vaccination effect? Meaning having healthy volunteers knowingly briefly pass through air with controlled micro contamination from known disease victims, in controlled conditions known not to produce disease, but also known to produce an antibody reaction? (Did I say controlled often enough?)