I was hanging out, smoking a bowl with a few of my geek friends a while ago, and one of them who chose not to partake made some sort of a comment about getting a contact buzz. This sparked a long discussion about whether or not a contact buzz was actually possible in this situation. I don't remember the exact math we figured out (I was stoned, after all), but we pretty much figured out the quantity of smoke you would have to inhale to get a buzz, compared that to the relative saturation of THC when taking a hit from the pipe, compared to that suspended in the air (most of which is post-exhalation (second-hand)).

We came to the conclusion that you would have to smoke about 1/8 of an ounce in a room no bigger than a mid-size car for a "contact buzz" to be at all possible (and that's for someone with a very low tolerance). Most of the time when people speak of this phenomenon, they are really referring to a psychosomatic response.