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

鈥淧eople in front of me started lighting up and then other people started lighting up,鈥� said聽, a biologist and professor in the division of cardiology at the University of California-San Francisco. 鈥淎nd for a few naive split seconds I was thinking to myself, 鈥楬ey, they can鈥檛 smoke in AT&T Park! I鈥檓 sure that鈥檚 not allowed.鈥� And then I realized that it was all marijuana.鈥�

Recreational pot was not legal yet in the state, but that stopped no one. 鈥淧aul McCartney actually stopped between numbers and sniffed the air and said, 鈥楾here鈥檚 something in the air 鈥� must be San Francisco!鈥欌�� Springer recalled.

As the visible cloud of pot smoke took shape, so did Springer鈥檚 idea to study the effects of secondhand marijuana smoke.

He started thinking: San Franciscans would聽苍别惫别谤听tolerate those levels of cigarette smoke in a public place anymore. So why were they OK with pot smoke? Did people just assume that cannabis smoke isn鈥檛 harmful the way tobacco smoke is?

Springer was already researching the聽聽on rats at聽聽at UCSF. He decided to run the same tests using joints.

鈥淏y the time I left the concert, I was resolved to at least try to make this happen,鈥� he said.

He knew it would be difficult. Marijuana is still an聽聽under federal law, and Springer鈥檚 research uses federal funds; so he has to purchase聽聽government cannabis for study. He also can鈥檛 test it on humans; hence, the rats.

In the lab, Springer puts a cigarette or a joint in a plexiglass box, lights it and lets the chamber fill with smoke. Then he vents out most of the smoke to the point that it is hardly visible, to simulate being around a smoker.聽 Then an anesthetized rat is exposed to the smoke for one minute.

So far, Springer and his colleagues have published聽聽demonstrating that聽just this one minute of exposure聽to secondhand smoke makes it harder for the rats鈥� 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, 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 secondhand smoke from marijuana. And, the arteries take 90 minutes to recover compared with the 30 minutes with cigarette smoke.

Springer鈥檚 discovery about the effect on blood vessels describes just one harmful impact for nonsmokers who are exposed to marijuana. Statewide sampling surveys of cannabis products sold in marijuana dispensaries have shown that the items may contain聽, or residue from聽.

California law requires testing for these contaminants, and those regulations are being initiated in聽聽over the course of 2018. Because much of the marijuana being sold now was harvested in 2017, consumers will have to wait until early 2019 before they can purchase products that have been fully tested according to state standards.

鈥淧eople think cannabis is fine because it鈥檚 鈥榥atural,鈥欌�� Springer said. 鈥淚 hear this a lot. I don鈥檛 know what it means.鈥� He concedes that tightly regulated marijuana, which has been fully tested, would not have as many chemical additives as cigarettes.

But even if the cannabis tests clean, Springer said, 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 said, the less you inhale any kind of smoke, the better.

鈥淧eople should think of this not as an anti-THC conclusion,鈥� he said, referencing聽, 鈥渂ut an anti-smoke conclusion.鈥�

So is the solution simply to avoid smoke from combustion? In other words, is it safer to eat cannabis-infused products, or use 鈥渟mokeless鈥� e-cigarettes or vaping devices?

Springer still urges caution on that score because vaping, for example, can have its own health effects. Vaping devices don鈥檛 produce smoke from combustion, but they do release a cloud of aerosolized chemicals. Springer is studying the health effects of those chemicals, too.

All this research takes time. Meanwhile, Springer worries that people might come to the wrong conclusion 鈥� that the absence of research means the secondhand smoke is OK.

鈥淲e in the public health community have been telling them for decades to avoid inhaling secondhand smoke from tobacco,鈥� Springer said. 鈥淲e have not been telling them to avoid inhaling secondhand smoke from marijuana, and that鈥檚 not because it鈥檚 not bad for you 鈥� it鈥檚 because we just haven鈥檛 known. The experiments haven鈥檛 been done.鈥�

Antismoking campaigners say we can鈥檛 afford to wait until the research is complete. Recreational pot is already聽.

聽is the president of Americans for Nonsmokers鈥� Rights, based in Berkeley, Calif. The organization was established in 1976, before there was a lot known about the health effects of secondhand smoke from tobacco.

Now that cannabis is becoming more common across the country 鈥� more than 20 cities or states have legalized it in some form 鈥� her organization is taking on the issue of secondhand marijuana smoke, too.

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

鈥淚鈥檓 seeing a parallel between this argument that, 鈥楪ee, we just don鈥檛 have a lot of science and so, therefore, let鈥檚 wait and see,鈥欌�� Hallett said. 鈥淭he tobacco companies used to say the same thing about tobacco cigarettes.鈥�

In California, smoking cannabis is prohibited anywhere tobacco smoking is prohibited 鈥� 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 said, and then, over time, there鈥檚 鈥渁 chipping away at strong policies.鈥�

When it comes to marijuana, Hallett said, 鈥渋t is聽still聽polite for you to say: 鈥榃ould you mind not smoking around me?鈥� 鈥�

础迟听, a cannabis dispensary in Oakland, Calif., pot smokers talk about what responsibilities 鈥� if any 鈥� they should have when it comes to nearby nonsmokers.

鈥淭his is the first time that I have heard secondhand smoke in reference to cannabis,鈥� said Lee Crow, a patient-services clerk at Magnolia. 鈥淚鈥檝e tried to be courteous 鈥� just common courtesy, like with anything.鈥�

The dispensary鈥檚 director of clinical services,聽, admits she thinks a lot about secondhand smoke from cigarettes, but not pot.

鈥淏oth of my parents died of lung cancer!鈥� she said. 鈥淚 will stop a stranger and say, 鈥榊ou shouldn鈥檛 be smoking. My dad died of that!鈥� 鈥�

颁补濒颈蹿辞谤苍颈补鈥檚听, approved by state voters in 2016, requires that some of the state tax revenue from the sale of marijuana be distributed to cannabis researchers. In addition, the state鈥檚 Occupational Safety and Health Standards Board is examining workplace hazards that are specific to the cannabis industry.

This story is part of a reporting partnership with NPR, local member stations and聽.