March 19, 2018

The inspiration arrived in a haze at a Paul McCartney concert a few years ago in San Francisco.

"People in front of me started lighting up and then other people started lighting up," says Matthew Springer, a biologist and professor in the division of cardiology at the University of California, San Francisco. "And for a few naive split seconds I was thinking to myself, 'Hey, they can't smoke in AT&T Park! I'm sure that's not allowed.' And then I realized that it was all marijuana.".

He started thinking: San Franciscans would never tolerate those levels of cigarette smoke in a public place anymore. So why were they OK with smoke from burning pot? Did people just assume that cannabis smoke isn't harmful the way tobacco smoke is?

So far, Springer and his colleagues have published research demonstrating that second-hand smoke makes it harder for the rats' arteries to expand and allow a healthy flow of blood.

With tobacco products, this effect lasts about 30 minutes, and then the arteries recover their normal function. But if it happens over and over — as when a person is smoking cigarette after cigarette, for example — the arterial walls can become permanently damaged, and that damage can cause blood clots, heart attack or stroke.

Springer demonstrated that, at least in rats, the same physiological effect occurs after inhaling second-hand smoke from marijuana. And, the arteries take 90 minutes to recover compared to the 30 minutes with cigarette smoke. Springer's discovery about the effect on blood vessels describes just one harmful impact for non-smokers who are exposed to marijuana. State-wide sampling surveys of cannabis products sold in marijuana dispensaries have shown that cannabis products may contain dangerous bacteria or mould, or residues from pesticides and solvents.

But even if the cannabis tests clean, Springer says, smoke itself is bad for the lungs, heart and blood vessels. Other researchers are exploring the possible relationship between marijuana smoke and long-term cancer risk.

Certainly, living with a smoker is worse for your health than just going to a smoky concert hall. But, Springer says, the less you inhale any kind of smoke, the better.

"We in the public health community have been telling them for decades to avoid inhaling second-hand smoke from tobacco," Springer says. "We have not been telling them to avoid inhaling second-hand smoke from marijuana, and that's not because it's not bad for you — it's because we just haven't known. The experiments haven't been done."

Antismoking campaigners say we can't afford to wait until the research is complete. Recreational pot is already a reality.

Cynthia Hallett is the president of Americans for Non-smokers’ Rights, based in Berkeley, Calif. The organization was established in 1976, before there was a lot known about the health effects of second-hand smoke from tobacco..

Hallett says some of the arguments being made in support of cannabis remind her of the arguments made on behalf of tobacco decades ago.

"I'm seeing a parallel between this argument that, 'Gee, we just don't have a lot of science and so, therefore, let's wait and see,' " Hallett says. "The tobacco companies used to say the same thing about tobacco cigarettes."

In California, smoking cannabis is prohibited anywhere tobacco smoking is prohibited — including schools, airplanes and most workplaces. Hallett is worried that the legalization of pot could be used to erode those rules.

It starts with the premise of decriminalization, she says, and then, over time, there's "a chipping away at strong policies."

"This is the first time that I have heard second-hand smoke in reference to cannabis," admits Lee Crow, a patient-services clerk at Magnolia. "I've tried to be courteous — just common courtesy, like with anything."

The dispensary's director of clinical services, Barbara Blaser, admits she thinks a lot about second-hand smoke from cigarettes, but not pot.

"Both of my parents died of lung cancer!" she says. "I will stop a stranger and say, 'You shouldn't be smoking. My dad died of that!' "

California's Proposition 64, approved by state voters in 2016, requires that some of the state tax revenue from the sale of marijuana to be distributed to cannabis researchers. In addition, the state's Occupational Safety and Health Standards Board is examining workplace hazards that are specific to the cannabis industry.

This story is part of NPR's reporting partnership, local member stations and Kaiser Health News.

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