Is Secondhand Marijuana Smoke as Dangerous as Secondhand Cigarette Smoke?

"Many people actively avoid being exposed to tobacco smoke but feel that marijuana smoke is benign, primarily because they have heard for years that they should avoid cigarette smoke but nobody has told them to avoid marijuana smoke," Springer said in an email. In a press release, he explained that exposure to smoke impedes blood flow, and when blood flow is constrained the risks of heart problems increase. It's not the properties of cannabis itself that cause problems, he said; it's the physical act of burning something and then inhaling it. His advice: avoid all smoke.

"At this point, we're saying that inhaling any smoke is detrimental to your health," Springer said. "I think that people should avoid inhaling smoke whether it's from tobacco or marijuana cigarettes, forest fires, barbecues—just avoid smoke."

Some marijuana advocates argue that it might be overly cautious to make recommendations based on a study using rats. "You can't extrapolate pre-clinical animal trials to humans," said Paul Armentano, deputy director for the National Organization for the Reform of Marijuana Laws (NORML). (It's also worth noting that the study was funded by the National Institutes of Health's National Institute on Drug Abuse, an organization that is staunchly against legalizing pot.)

There's certainly some evidence to support Armentano's suspicion. A 2014 review of the literature on animal models acknowledged their limits in predicting the conversion of pre-clinical cancer therapies to clinical trials. They found that "the successful translation from animal models to clinical cancer trials is less than 8 percent."

However, Springer told Broadly, "In this case, the rats are a good model; blood vessels of rats and humans respond similarly to tobacco smoke, and rat blood vessels respond similarly to tobacco and marijuana smoke (although duration is longer for marijuana), so it is reasonable to suspect that human blood vessels will respond similarly to the rat vessels to marijuana smoke."

There are already some studies on how cannabis affects humans—whether via smoking, vaping, or eating. One study suggests that the short term effects Springer found in his study might not amount to anything serious in the longterm. In 2006, researchers analyzed 15 years of data from an ongoing longitudinal study tracking the development of coronary artery disease in young adults in the US, the Coronary Artery Risk Development in Young Adults (CARDIA) study. They found that "marijuana use was not independently associated with cardiovascular risk factors" in people who casually smoked weed. After analyzing the long-term health of 3617men and women, of whom roughly half consumed cannabis, the study reported that while cannabis users certainly ate more calories, there were no effects on the markers of cardiovascular disease: diastolic blood pressure, total cholesterol, or HDL cholesterol. In contrast, tobacco use has been shown to increase these risk factors.

But Springer says he still advises caution. "The repeated short-term impairment of artery function from tobacco smoke can have long-term effects; one might expect that to occur with marijuana smoke also but… the effect has not been detected yet in long-term studies. I think now that we have shown this short-term effect however, it is still reasonable to avoid smoke from any source and to make sure that non-smokers are protected from unwanted smoke exposure," he said.

Armentano suggests some practical alternatives: "Cannabis consumers who are concerned about smoke exposure have several alternative delivery methods at their disposal, including vaporization, tinctures, extracts, and edibles. In particular, vaporization permits users to continue to experience the rapid onset of effect while significantly reducing consumers' exposure to unwanted gasses and potential contaminants."