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One minute of exposure to marijuana secondhand smoke was associated with substantial transient impairment of vascular endothelial function, according to results of an animal study.

Blood vessels took a full 90 minutes to recover function after 60 second exposures to marijuana smoke, compared with a 30 minute recovery time following 60 seconds of secondhand tobacco smoke exposure in the rat studies, reported by of the University of California San Francisco, and colleagues, in the .

The transient impairment occurred in rats exposed to marijuana smoke with and without tetrahydrocannabinol (THC), suggesting that other components of marijuana smoke cause this detrimental effect on blood vessels, Springer told MedPage Today. Rats exposed to secondhand tobacco smoke without nicotine also showed evidence of short-term impairment of endothelial function.

These findings suggest that some other component of smoke not specific to marijuana or tobacco [smoke] -- either the products of combustion or other common plant chemicals that persist after combustion -- trigger the vascular response, Springer said.

"Other than THC and nicotine, marijuana and tobacco have similar chemical components, but these components are present in different ratios in the two," he said. "The big surprise in our newest studies is that the effect associated with secondhand marijuana smoke exposure lasted three times longer than [vascular impairment associated with] tobacco smoke exposure."

Springer and colleagues noted that while there is widespread public awareness of the dangers of secondhand exposure to tobacco smoke, many people still assume that marijuana secondhand smoke is benign.

"Debates about whether smoke-free laws should include marijuana are becoming increasingly widespread as marijuana is legalized and the cannabis industry grows," they wrote. "Lack of evidence for marijuana secondhand smoke causing acute cardiovascular harm is frequently mistaken for evidence that it is harmless, despite chemical and physical similarity between marijuana and tobacco smoke."

Arterial flow-mediated dilation (FMD) measurement was used in the studies to measure transient endothelial function impairment related to different smoke exposures.

Decreased brachial FMD has been found to correlate with adverse cardiovascular outcomes associated with cigarette smoking. Frequent secondhand tobacco smoke exposure has been shown to be associated with impaired FMD, as has exposure to other combustibles, including diesel exhaust, incense, and candles.

"These observations, along with the similar chemical composition of tobacco and marijuana smoke, led us to hypothesize that marijuana smoke would also impair FMD," the researchers wrote. "We developed an animal model that uses micro-ultrasound and a reversible surgical occlusion of blood flow to the leg to measure FMD in the femoral arteries of living rats, analogous to the measurement of bronchial artery FMD in humans."

FMD in the rats was measured before and after exposure to marijuana secondhand smoke at levels similar to real-world tobacco secondhand smoke conditions.

Impairment of FMD was found to be comparable in rats exposed to 1 minute of secondhand marijuana smoke and 1 minute of secondhand tobacco smoke, but recovery took three times longer following secondhand marijuana smoke exposure.

"Both exposures cut blood vessel function by a little over 50%, but recovery after tobacco exposure was about 30 minutes, compared to 90 minutes following marijuana smoke exposure," Springer said.

Impairments were similar in experiments using marijuana in which the THC had been extracted.

"Since this was a rodent model, specific parameters such as the exposure times and exact duration of impairment may not completely match the corresponding properties of exposures in humans," the researchers wrote. "However, the FMD in rats as we measured it shows great similarity to FMD in humans, as shown by extensive physiological and pharmacological validation as we have described previously."

Springer said based on earlier findings showing that responses to secondhand smoke are similar in rats and humans, there is good reason to believe that the responses to marijuana smoke seen in the rat studies also occur in humans.

The researchers concluded that greater awareness by the public, medical community, and policymakers is needed about the potential risks of secondhand exposure to marijuana smoke.

"Our findings suggest that secondhand smoke should be avoided whether the source is tobacco or marijuana," they wrote.

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