If "contact highs" were real, then people wouldn't need to smoke multiple puffs of a joint: a single puff would give them at least ten (and maybe a hundred or more) times the "dose" of THC that they would get from a usual "contact high."  Just as with tobacco smoke, the dose from someone smoking a full joint/cigarette in a room with you would only be a hundreth to a thousandth of what the smoker gets.  While in most bar/restaurant situations it's more like a thousandth due to heavy ventilation (10 to 15 air changes per hour in any decent Free-Choice venue) sitting around in a small poorly ventilated room in a house might give you up to 100th to 1/10th of a "joint" in an hour depending on the situation.

Note: the above is based on the assumption that THC levels in sidestream smoke is roughly equivalent to what's in mainstream.  Antismokers (tobacco smoke) like to play a statistical "lying game" sometimes by picking out an element in smoke that gets burned away during the high temperature "puff" and is mainly produced between puffs: that's how they get their ridiculous numbers claiming nonsmoking bartenders are "forced to smoke" multiple cigarettes during a shift. It's unlikely that this is the case with THC though or we'd see a lot more brownies and a lot fewer joints.

Michael J. McFadden,

Author of "Dissecting Antismokers' Brains"