3/21/18:  Due to recent coverage of our research on NPR, numerous articles have now shown up in the media stating that we showed that "marijuana smoke is 3 times worse than tobacco smoke." We did not actually say this. To clarify, we reported that the two kinds of smoke had comparable effects on blood vessel function, but the effect from marijuana lasted at least three times longer than that from tobacco. It's an oversimplification to say that this means that marijuana smoke is three times worse than tobacco smoke; we are studying only one of the many potential harmful effects of tobacco smoke. It's more accurate to say that they have similar negative effects on blood vessels and that marijuana's effect lasts longer. We have sent requests to as many of the news writers as we have been able to track down, to ask that they change the wording.