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Marijuana Secondhand Smoke Isn’t as Bad as Tobacco Smoke

Secondhand smoke Cannabis Now
Photo Gracie Malley for Cannabis Now


Marijuana Secondhand Smoke Isn’t as Bad as Tobacco Smoke

Results from a legitimately-needed inquiry are being misinterpreted in media. Is this willful ignorance? No, just anti-cannabis bias that’s happened before, and will likely happen again.

Few serious people would bother to argue tobacco is healthier than cannabis. Even for a contrarian, it’s a doomed exercise. Surely few people could honestly make this argument. Certainly they couldn’t declare that secondhand exposure to cannabis smoke is worse than breathing secondhand tobacco smoke.

Yet such a line is being pushed in our discourse, right now — and not for the first time.

“Secondhand smoke more dangerous from marijuana than cigarettes: Study,” reads a headline published in the Washington Times on March 20. Other news outlets were more circumspect. “Are there risks from secondhand marijuana smoke? Early science says yes,” the Philadelphia Inquirer posited on March 26, but the takeaway is the same.

Such source material has been used to ring a “maybe marijuana is as bad as cigarettes, or worse!” bell for much of the past decade. You could work very hard and look very deep and not find a better example of a false equivalency, based on a faulty premise.

Incredibly, the institution responsible for pushing a line that’s leading news outlets and their readers to put both cannabis and tobacco in the same deadly bucket happens to be based in San Francisco, the historic home of medical marijuana and the legalization movement.

Matt Springer, a researcher from the University of California, San Francisco — one of the nation’s leading institutions for data about smoking and lung health — has been investigating the impact of cannabis smoking on lung health for much of the past decade. He opened this line of inquiry after attending a Paul McCartney concert at AT&T Park in 2010, where he smelled lots and lots of marijuana being burned.

Springer and the university published findings suggesting cannabis smoke is just as bad — or even worse! — than tobacco smoke in 2014 and 2016, as well as earlier this year, setting off the same round of alarmist headlines.

Each time, the source material came from the same body of work. Springer and his researcher team exposed lab rats, whose cardiovascular systems aren’t too dissimilar from ours, to second-hand marijuana smoke.

They found the rats’ blood vessels became constricted and took longer to return to normal than when they were exposed to tobacco smoke. The data changed somewhat — in 2014, the researchers found that rats’ blood vessels’ ability to dilate and increase blood flow was 70 percent reduced, a figure they changed to 50 percent in 2016.

Cannabis users likely have experienced the same phenomenon as Springer’s lab rats. Cannabis use leads to a brief temporary spike in blood pressure as the body’s blood vessels constrict. This can lead to a heightened heart rate, and light-headedness or dizziness if a person stands up quickly. It can also lead to the feeling that “I’m freaking out, man” and the accompanying lifetime of declaring “weed freaks me out.”

The thing is (and this is important!) tobacco use involves a host of other deleterious health effects that have not been linked to cannabis use. You might have heard of them. Lung cancer, COPD, heart disease, reduced lung function. To date, the only longitudinal study examining marijuana use found no link between cannabis smoking and these horrible diseases. In fact, initial results suggested they have a mild protective effect. The findings were “against our expectations,” said Donald Tashkin, the University of California, Los Angeles researcher who led the study. (Also notable: Tashkin’s study should be given more weight than other results from Europe and the United Kingdom, where most cannabis users consume the drug mixed with tobacco, a practice less widespread in the U.S.)

This latest recycling of Springer’s research, willfully manipulated by the Times and other outlets to make it appear that cannabis is in the same league as tobacco, is the fault of Kaiser Health News, which published a story whose message was essentially, “cannabis smoke exposure is under-studied.” And yes, this is true. What’s not by any metric true is the theory that the two drugs pose similar risks to human health.

Tobacco smoking is the world’s leading cause of preventable death. More than 480,000 Americans die every year because of smoking cigarettes — including 41,000 people whose deaths are attributable to secondhand smoke exposure. Notably, and famously, cannabis has killed no one, despite many well-funded efforts to prove otherwise.

But the thing is, tobacco users currently enjoy more freedom and rights than cannabis users. Smoking cannabis in public, anywhere at all, is punishable by a citation in California, Colorado, and most everywhere else the drug is legal. A cigarette user in the exact same spot, the patio of a bar, for example, can puff away with impunity. That’s intellectually dishonest by any metric.

TELL US, do think secondhand smoke from tobacco is worse than from cannabis?



  1. Matt Springer

    April 3, 2018 at 11:29 am

    This is Matt Springer at UCSF, whose lab did the research mentioned here. There are a number of inaccuracies in this article that I would like to clarify.

    First, the data were not “changed” from our 2014 conference presentation to our 2016 paper. The conference presentation reported results of our earliest experiments; whereas the 2016 paper reported more depth experiments with different conditions. It’s not unusual for different experiments to report different average effect sizes; the variability between a 50% drop and a 70% drop is one of the reasons that we do multiple experiments.

    Second, the current wave of news coverage is based on a recent news report about secondhand marijuana smoke that included our work. People and news outlets who had not seen the earlier reports saw this one and reported it as they felt was appropriate.

    Third, the example of Donald Tashkin and links to lung cancer are irrelevant; we are studying cardiovascular effects. The last majority of deaths that have been attributed to tobacco-related diseases have been cardiovascular in nature, so it is important to look beyond cancer.

    Similarly, the often-repeated line that tobacco kills many people and marijuana has never killed anyone is a meaningless statement. We have a huge amount of research over several decades linking tobacco smoke to many diseases that kill people; they die of the diseases themselves and our knowledge base enables us to link tobacco to increased risk of those diseases. Research on marijuana, especially secondhand smoke exposure, is very sparse, despite a few large studies with (importantly) conflicting results in which some have indeed shown correlations between marijuana “use” and cardiovascular disease.

    Lastly, it’s important to point out that some of the headlines out there (“marijuana smoke is 3 times worse than tobacco smoke”) have over-generalized and over-interpreted our results. I agree with the author’s assessment of these headlines; we would not have made such generalizations ourselves, and I have attempted to get some of these headlines toned down. What we can say is that the effect on vascular function of secondhand exposure to marijuana smoke at realistic levels lasted at least 3 times longer than the effects of similar levels of secondhand tobacco smoke, in our rat model. To me, the longer duration is not necessarily relevant to potential effects on humans, and this does not prove secondhand smoke from marijuana harms humans. What it does do, however, is to provide evidence that secondhand smoke from marijuana is not necessarily harmless, as some people have assumed when mistaking “lack of evidence of harm” for “evidence of lack of harm.”

    • Will

      April 3, 2018 at 1:06 pm

      I have a few questions about the methodology used for the study. I found this description in article from NPR about your study;

      “In the lab, Springer puts a cigarette or a joint in a plexiglass box, lights it and lets the chamber fill with smoke. Then he vents out most of the smoke to the point that it is hardly visible, to simulate being around a smoker. Then an anesthetized rat is exposed to the smoke for one minute.”

      – If the above description is correct, how ‘real world’ is an anesthetized rat in a plexiglass box relative to various spaces human beings occupy?
      – What is the volume of the plexiglass box relative to the body mass of the rat?
      – To the above question, is the volume of the box meant to simulate a human being in a car (more akin to a plexiglass box or chamber I would think)? A small or large room in a house? A concert/sports venue?
      – Does CO2 accumulate in the box as the rat breathes? Would this matter/not matter?
      – Does the rat being anesthetized have anything to do with arterial anomalies noted? Would an un-anesthetized rat in the same environment yield the same results?

      I don’t expect you to answer these questions here and I’m not asking these questions in an attempt to debunk any conclusions your study derived. But it’s understandable to question specifically designed research environments (and methods) when it comes to reported outcomes.

      • Matt Springer

        April 4, 2018 at 2:10 am

        Will, thanks for your comment; your questions are all quite valid. It seems this website does not e-mail us when new comments are posted (or perhaps it got blocked by UCSF’s spam filter), so I didn’t see your note initially. I won’t be able to keep checking back here since I’m being pulled in many directions, but I want to address the points you raised.

        First a general comment about the model and its real-world relevance. Much of the effort that goes into a study like this is focused on creating as relevant a model as we can, and addressing or controlling for any differences between what that model and the real world to the greatest extent possible. I urge anyone interested to check out the original paper in the Journal of the American Heart Association, which was written in a way that the most important parts should be understandable to people without a medical/physiology background, freely accessible without subscription at An article like the one at NPR can’t possible provide all of the details necessary for someone to make informed evaluations of the methods, so I’m happy to fill in the details.

        With that said, here are the answers to your specific questions:

        “…how ‘real world’ is an anesthetized rat in a plexiglass box relative to various spaces human beings occupy?“ and subsequent questions about volume and what it simulates: I’ll discuss anesthesia more below. The ~20 liter Plexiglas box is a reasonable model for spaces that humans occupy because the important factors under study are the concentration of smoke, how even the exposure is over time, and the duration of exposure. Within that box, we attempt to expose rats to realistic levels and we have a particle monitor to measure the level in real time. I forget if the article mentioned it but our exposure time for our more recent marijuana experiments was only one minute. The exposure is constant, rather than pulsatile, because we are actually mimicking being around smokers at a party or rock concert, etc. (the article’s “being around a smoker” was close but not completely accurate). Determining real world levels takes some guesswork based on tobacco, since we don’t know nearly as much about ambient levels of marijuana secondhand smoke as we do for tobacco, but note that the smoke was invisible in our clear exposure chamber (a photo of the chamber containing smoke is in the journal article).

        “Does CO2 accumulate in the box as the rat breathes? Would this matter/not matter?”: Especially with a 1-minute exposure time, it would be minimal, but even in our initial smoke studies that kept the rat breathing the air in the chamber for 30 minutes, we did arterial blood gas measurements and determined that there was no significant change in oxygen and C02 in the blood. Moreover, it’s not a closed system; there are small holes in the box to allow sampling of the air and these remain open. Remember also that the impairment of vascular function occurs when the chamber contains smoke, but not when the same chamber contains clean air.

        “Does the rat being anesthetized have anything to do with arterial anomalies noted? Would an un-anesthetized rat in the same environment yield the same results?”: The anesthesia is equivalent in the groups exposed to various kinds of smoke and in the control group exposed to clean air. If you are suggesting that the difference between air and smoke would not be observed in un-anesthetized rats, I’m not aware of a basis for that difference; however, also consider that anesthetized rats and conscious humans have the same vascular response to tobacco smoke.

        One last general comment about the CannabisNow article is that it (and subsequent comments) refers to arteries constricting; but actually the property being measured is the arteries’ ability to vasodilate when they need to. Healthy arteries vasodilate in response to increased flow; that dilation is impaired by smoke so the arteries don’t dillate as much as they normally would. None of this actually involves constriction, despite what is in most of the news coverage about our work.

        As mentioned above, I won’t be able to keep checking in at this specific site and it does not notify, but will try to circle back at some point.

  2. SmarterThanTheAverageBear

    April 3, 2018 at 9:51 am

    What you failed to cover here is that research shows it is the continued constricting, over a long time period, that leads to arteriosclerosis – hardening of the arteries – as the arteries lose their ability to relax over time . One of the takeaways from their research is that pot smoke keeps arteries constricted approximately 3 times longer than cigarette smoke does, as such it is not unreasonable to consider, that in the case of artery hardening, cannabis smoke may in fact be more dangerous than tobacco smoke. You don’t do your cause any good by being as guilty of bias as those you rightly accuse of it.

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