Skip to content
Home » Secondhand marijuana smoke – it may be unhealthier than cigarettes

Secondhand marijuana smoke – it may be unhealthier than cigarettes

I’ve always been amused by marijuana advocates – they vastly overstate the benefits and understate the risks, sort of the opposite of the anti-vaccine religion. Current research on cannabis shows that there is little robust evidence supporting most of its claimed medical benefits – for example, it does not cure cancer, despite what you see on the internet. On the other hand, there has been only a small amount of research examining the risks of marijuana smoking. But a 2016 article in a major journal examined the effects of secondhand marijuana smoke, and the results should cause us to examine laws to regulate public smoking of marijuana in the same way we do cigarettes.

Here in California, we would be calling the local swat teams to round up all the cigarette smokers in a public space, if the air has even a hint of cigarette smoke. Ironically, no one seems to care about cannabis smoke wafting over us and our children. Maybe some of us just assume that secondhand marijuana smoke was inherently safer than secondhand cigarette smoke. What does the scientific evidence say?

Researching secondhand marijuana smoke

Matthew Springer, Professor of Medicine in the Division of Cardiology at the University of California, San Francisco, has spent a significant portion of his research life examining the effects of secondhand cigarette smoke on individuals. California was one of the first states that began regulating, and eventually banning, public cigarette smoking because of the well-known effects of secondhand smoke on nonsmokers. I have always been virulently anti-smoking, and I remember going to restaurants and having some inconsiderate jerk lighting up his cancer stick while I was trying to enjoy a nice dinner.

Dr. Springer tells the story that he went to a Paul McCartney concert in San Francisco a few years ago (see Note 1) and noticed the haze of smoke. At first, he thought that cigarette smoking was allowed during concerts, then noticed it was hundreds of people lighting up their joints. At the time, recreational marijuana wasn’t legal in the state, but anyone who has gone to a rock concert (even apparently when McCartney is the act) knows that marijuana smoking is not exactly rare. Of course, this led him to wonder if secondhand marijuana smoke was as much a danger to bystanders as cigarette smoke.

It’s socially unacceptable to smoke cigarettes in public any more. According to the CDC, just under 16% of Americans smoke cigarettes in 2016 (frankly, a number so low, I was rather surprised when I looked it up). On the other hand, researchers have determined that the number of Americans smoking marijuana regularly has jumped from 3.9 million to 8.4 million from 2002 to 2014. In a few years, regular marijuana smokers may outnumber regular cigarette smokers in the USA.

I suppose that someone would ask for evidence of harm from secondhand marijuana smoke if we were to regulate public smoking of it in areas that allow its recreational use. So Dr. Springer examined the possibility of harm, and the results were published in the Journal of the American Heart Association.

Because marijuana is illegal in the USA (according to the Federal government, state laws be damned), it is very difficult to do clinical research with the drug (see Note 2). So Dr. Springer’s first step was to examine the effect of secondhand marijuana smoke in rats.

It is well known that secondhand cigarette smoke has a lot of cardiovascular effects on a bystander, including a constriction of arteries which lead to decreased blood flow and higher blood pressure. This could lead to cardiovascular events like stroke or angina. So, Dr. Springer put rats in an enclosed box which he filled with a concentration of cigarette or marijuana smoke that would mimic secondhand smoke.

With tobacco smoke, the constricted arteries lasted for about 30 minutes after the smoke was ended, then the arteries recovered their normal function. Over time, this constriction can lead to permanent damage to the arterial walls, which causes the various cardiovascular events like stroke, clotting, and heart attacks.

On the other hand, arteries took 90 minutes (or triple that of cigarette smoke) to recover from secondhand marijuana smoke. That’s a serious risk to bystanders (and frankly, to marijuana smokers themselves).

As you know, I’m a proponent of biological plausibility, that is, before you can claim a causal link between an event and a biological action, you have to make a reasonable scientific argument that it is plausible. And in this case, there is a strong case for plausibility. A statewide sampling of marijuana products in California have shown that cannabis products may contain dangerous bacteria or mold, or residues from pesticides and solvents. That would be unacceptable in the more highly regulated cigarette market.

Also, lung cells are very sensitive to environmental pollutants, like any type of smoke, because the cells are open to the air. I think it is naive to believe that somehow marijuana smoke is blessed by unicorns because it is “natural.” I think that’s a ridiculous reliance on the appeal to nature logical fallacy – the marijuana plant is no more or less “natural” than the tobacco plant. And it may be just as dangerous.

Now, California’s new marijuana laws do begin to regulate what is in cannabis starting in 2019, so the quality of marijuana should improve, but that itself is not going to suddenly make firsthand or secondhand marijuana smoke safe.


The authors of the study concluded that one minute of exposure to second-hand marijuana smoke,

…substantially impairs endothelial function in rats for at least 90 minutes, considerably longer than comparable impairment by tobacco secondhand smoke. Impairment of FMD (flow-mediated dilation of vessels) does not require cannabinoids, nicotine, or rolling paper smoke. Our findings in rats suggest that secondhand smoke can exert similar adverse cardiovascular effects regardless of whether it is from tobacco or marijuana.

I think it’s important to remind the reader of the issues with this type of study. I appreciate that studies with rats or mice are not exactly systematic reviews of massive clinical studies, but this is the quality of evidence that makes me invoke the precautionary principle – we should avoid the harm unless substantial evidence shows us that the harm is small or nonexistent.

Furthermore, there is strong evidence that secondhand marijuana smoke leaves significant amounts of cannabinoids in the bystander. This is a significant issue for those who have might be taking medications that may be affected by cannabinoids, which include medications for depression or bipolar disorder.

The impact of secondhand marijuana smoke is of substantial concern to children and the developing fetus, something that seems to be ignored by both the marijuana smokers and those who wade through billowing marijuana smoke. And I also remain concerned that there is enough preliminary evidence of a causal link between marijuana smoke and lung cancer that I think that secondhand marijuana smoke should be as serious an issue as cigarette smoke.

I don’t want the reader to think that Dr. Springer (or I, in reviewing his article) is anti-marijuana. I always have to remind the reader that just because I think that marijuana doesn’t have the medical benefits that advocates like to claim or the risks that those same advocates refuse to acknowledge, it doesn’t mean I’m opposed to recreational marijuana. I am anti-smoke.

Putting it very simply, I don’t like smoke of any type getting into my lungs. I don’t like vaping secondhand smoke. I don’t like cigarette secondhand smoke. And I don’t like marijuana secondhand smoke. I’ve spent my life not smoking, trying to protect my lungs so that I can live a life without lung disease, so I like clean air without any kind of pollutants.

But the most important thing is that we have developing evidence that secondhand marijuana smoke may be very harmful. Those who choose to take that risk have that right. However, we should have regulations similar to cigarettes to reduce or prevent secondhand marijuana smoke from harming those of us who do not choose to partake.


  1. We should not judge Dr. Springer’s credibility based on his taste in music. At least, it was not a Nickelback concert.
  2. The Federal rules for researching marijuana for medical uses is extraordinarily complex. It’s fairly straightforward to get approval for using marijuana or any of its active ingredients in animal studies, although it requires a lot of controls in the laboratory so that it is not misused. When I was a grad student we had a huge bottle of cocaine, that we used to speed up the heart rate of rats (not my focus, but it was for other grad students and fellows). The cocaine was kept in a locked cabinet, and every microgram taken out of the bottle had to be accounted for. Of course, we all had keys to the cabinet. Regardless, human clinical trials for specific indications (treating cancer or mental health disorders) using marijuana or its active ingredients is possible, although there are significant regulatory hoops that must be addressed before starting the research study.



Michael Simpson

Don’t miss each new article!

We don’t spam! Read our privacy policy for more info.

Liked it? Take a second to support Michael Simpson on Patreon!
Become a patron at Patreon!