Mustering Evidence Against Secondhand Marijuana Smoke

Whether it was marijuana with THC [the drug's active ingredient] or placebo marijuana with no cannabinoids, it was the same. This is a result of exposure to smoke from burning plant material.

Matthew Springer, PhD, a professor in the UCSF Division of Cardiology, isn’t anti-drug or anti-marijuana. After a decade of researching tobacco and more recently studying cannabis, commonly known as marijuana, he is simply anti-smoke.

Just 1 minute of exposure to secondhand marijuana smoke diminishes blood-vessel function in rats to the same extent as 1 minute of exposure to secondhand tobacco smoke. However, the harmful cardiovascular effects of marijuana smoke last considerably longer – more than 90 minutes, compared to less than 30 minutes for tobacco smoke, says Springer, the senior author of a 2016 study on the subject.

“Whether it was marijuana with THC [the drug’s active ingredient] or placebo marijuana with no cannabinoids, it was the same,” Springer says. “This is a result of exposure to smoke from burning plant material.”

UCSF scientists like Springer recognize marijuana’s contradictory status: The drug has significant proven and potential therapeutic uses, but it can also lead to tremendous public health problems. Everyone agrees that a stronger evidence base is key.

Springer, who has published widely on the effects of secondhand smoke, acknowledges that his study didn’t prove that marijuana smoke is harmful for humans. “There’s the distinction between proving something is harmful and showing that it might not be harmless,” he says. “I want to err on the side of safety.”

His current research involves measuring the cardiovascular system’s sensitivity to alternative delivery systems for tobacco and marijuana – such as e-cigarettes, which aerosolize liquid solutions of nicotine or THC, and so-called volcano vaporizers, which heat rather than burn the plant material.

Despite Springer’s initial expectation that the vaporizers would not cause the same kind of vascular problems as tobacco and marijuana smoke, his team reported in 2019 that exposure to vaporizer aerosol causes similar inhibitory effects on blood vessel function.

“Researchers around the world have been finding over the last few years that inhalation of different types of smoke, vaporizer aerosols, and e-cigarette aerosols all interfere with vascular function,” Springer says. “We are trying to learn why, but the public health message is clear: Breathe clean air.”