Secondhand Weed Smoke Is Bad for Rats. What About Humans?

Hotboxing your friends might get them high, but are there other effects? While past studies indicate that extreme exposure to secondhand pot smoke causes mild intoxication, a recent study also suggests that it may be worse for you than secondhand cigarette smoke.

The study, published last week by the American Heart Association, found that when rats were exposed to a minute's worth of cannabis smoke, their blood vessels took at least three times longer to function normally again than when they were exposed to tobacco smoke. This resulted in the rats' arteries carrying blood less efficiently for 90 minutes after one minute exposure, as compared to 30 minutes with tobacco.

"Many people avoid being exposed to tobacco smoke but feel that marijuana smoke is benign," UCSF's Dr. Matthew Springer, professor of medicine and senior author of the study, told Motherboard.

"Now with increasing [cannabis] legalization, the opportunities for people to be exposed to secondhand marijuana smoke will increase, and those who smoke marijuana for medicinal purposes may not realize that their families and neighbors could be affected by their smoke."

Repetitive short-term impairment of artery function from tobacco smoke can have long-term effects, he said, so the same might also be true for marijuana smoke. The arteries carrying blood less efficiently limits the amount of oxygen to the heart, which could temporarily increase the chances of heart attack or stroke, especially if someone is vulnerable, explained Springer.

But can a study based on rats adequately be applied to humans?

Springer said yes: "Notably, humans and rats respond similarly to tobacco smoke exposure, and rats respond similarly to tobacco and marijuana smoke exposure (longer duration for marijuana)," he said. "So it is reasonable to suspect that humans and rats will respond similarly to marijuana smoke."

Cardiologist Dr. John Pippin, director of academic affairs for the Physicians Committee for Responsible Medicine, an organization the opposes animal testing, disagrees. It's already known that the inhalation of smoke causes the blood vessels to narrow, said Pippin, no matter whether it's from cannabis, tobacco, campfire, or even incense.

But he says the study's conclusions about rats don't tell us enough about indications for humans since the physiological comparisons between humans and rats are unreliable. He pointed to his research that says 95 percent of the medications that prove successful when tested on rats fail when tested on humans.

"That's why I had to shake my head and say this is some of the most useless research I've ever seen," he said. "The message I take home: Don't light up that bong in a room with your pet rat."

While Pippin points out many people would willingly participate in a study that exposes them to a minute of pot smoke, Springer says that marijuana's legal status makes it difficult to experiment with people.

"Still, ultimately I think we need to have more studies of how humans respond acutely to smoke," Springer said.