Cannabis smoke contains many of the same carcinogens and chemicals found in tobacco smoke (Moir, Rickert et al. 2008, Wei, Alwis et al. 2016). Exposure to secondhand cannabis smoke can impair endothelial function, which increases the risk of cardiovascular disease (Wang, Derakhshandeh et al. 2016). However, US data show that the perceived health risks of marijuana use are, in fact, declining among adults (Compton, Han et al. 2016). We measured the concentrations of airborne fine particles (PM2.5) and cannabinoids at an indoor cannabis event where dabbing and vaporizing were the only cannabis emissions. We found average particle concentrations of 200-600 micrograms per m3 and peak concentrations over 1,600 micrograms per m3. Particle concentrations this high are seen in extreme air pollution events like wildfires (Landis, Edgerton et al. 2018, Li, Han et al. 2018) and severe industrial pollution (Nagar, Singh et al. 2017, Li, Han et al. 2018). Exposure at these concentrations can cause cardiovascular and respiratory disease (Zheng, Ding et al. 2015, Li, Fan et al. 2016). We show that dabbing and vaporizing cannabis can create levels of indoor air pollution that are hazardous to human health, in the absence of actual combustion.