An animal study reported as an abstract at the American Heart Association annual meeting found secondhand marijuana smoke to be as harmful as tobacco smoke. The endothelial function of the rats studied decreased 50-70% when exposed to secondhand marijuana smoke. This impairment did not depend on THC in the smoke, and was similar to tobacco smoke. (Reference: Wang X, et al "Brief exposure to marijuana secondhand smoke impairs vascular endothelial function" AHA 2014; Abstract 19538)
PositiveTip: Avoid exposure to secondhand smoke regardless of the source.