This is actually one of the things we studied in forensic toxicology - the case of Ross Rebagliati, a Canadian snowboarder whose Olympic gold medal was in jeopardy after testing positive for marijuana. His defense was that he did not partake in smoking, but was in the vicinity as others smoked.

The conclusion in our class is that passive inhalation is usually not enough to give significant blood THC concentrations - one study reporting blood THC levels of 1 - 6 ng/mL right after exposure (to give context: with normal inhalation, blood THC rises to above 100 ng/mL in the minutes immediately after, and settles to about 30 ng/mL in about 20 minutes, in a typical user). And even obtaining that level required extreme exposure - high concentration of smoke and limited volume ("hot-boxing"). Subjects complained of such severe eye irritation that they requested sealed goggles.

So my toxicology professor is adamant passive inhalation does next to nothing, physiologically.