DAVID GREENE, HOST:
And let's stay here in California, which is one of more than 20 states where marijuana is legal in some form, which, of course, means people can be exposed to marijuana on the street. So what about secondhand smoke? Is that a concern? Reporter Marissa Ortega-Welch asked a California scientist who's looking at this closely.
MARISSA ORTEGA-WELCH, BYLINE: It happened a few years ago in the middle of a Paul McCartney concert in San Francisco even before recreational pot was legal in California. Matthew Springer, who directs a research lab at UC San Francisco, had a scientific epiphany.
(SOUNDBITE OF SONG, "LIVE AND LET DIE")
PAUL MCCARTNEY: (Singing) Live and let die...
MATTHEW SPRINGER: About halfway through the concerts, people in front of me started lighting up. And for a few naive split seconds, I was thinking to myself, hey, they can't smoke in the AT&T Park. I'm sure that's not allowed. And then I realized that it was all marijuana.
ORTEGA-WELCH: A visible haze rose over the audience.
SPRINGER: Paul McCartney actually stops between numbers and sniffed the air and said, there's something in the air. It must be San Francisco.
ORTEGA-WELCH: Springer got to thinking. San Franciscans would never tolerate cigarette smoke like that anymore. Why were they OK with pot smoke?
SPRINGER: Well, I guess it's because they assumed that, unlike the tobacco smoke, that the stuff was not harmful.
ORTEGA-WELCH: Springer was already researching the effects of secondhand tobacco smoke. He was running tests on rats using cigarettes. Maybe he could do the same thing with joints.
SPRINGER: By the time I left the concert, I was resolved to at least try to make this happen.
ORTEGA-WELCH: He knew it would be hard. Since pot is still illegal under federal law, Springer had to buy specially approved government cannabis. And he can't test it on humans. So he went back to the rats. In the lab, Springer puts a cigarette or joint in a plexiglass box. Then he lights it, and the chamber fills with smoke.
SPRINGER: And then a sleeping rat - you know, they're asleep. They're anesthetized. So they're inhaling the smoke.
ORTEGA-WELCH: Secondhand tobacco smoke makes it harder for the rats' arteries to expand and allow a healthy flow of blood. This effect last about 30 minutes. But if it happens over and over, the artery walls can become permanently damaged. And that can cause blood clots, heart attack or stroke. Springer discovered that the exact same thing happens with secondhand smoke from pot. And the arteries take an hour longer to recover. Pot can also contain leftover pesticides and solvents, as well as mold, heavy metals and even salmonella.
SPRINGER: People think cannabis is fine because it's natural. I hear this a lot. I don't know what it means. OK. It doesn't have as many chemical additives.
ORTEGA-WELCH: Even if the pot's clean, you're still inhaling smoke. And smoke itself is bad for your lungs. Certainly, living with a smoker is worse for your health than just going to a smoky concert. But Springer says the less you can breathe in any kind of smoke, the better.
SPRINGER: People should think of this not as an anti-THC conclusion but an anti-smoke conclusion.
ORTEGA-WELCH: Even vaping devices release a cloud of chemicals. Springer is studying the health effects of those, too. Some health advocates advise caution. Cynthia Hallett leads Americans for Nonsmokers' Rights in Berkeley. She says people need to recognize secondhand pot smoke can be harmful just like tobacco smoke.
CYNTHIA HALLETT: We have to go back to where we were in the early '70s and '80s. And just remember, it's OK. And it is still polite for you to say, would you mind not smoking around me? It's bothering me.
ORTEGA-WELCH: As more and more states legalize pot, we'll have to figure out the etiquette for how to deal with it. For NPR News, I'm Marissa Ortega-Welch in San Francisco.
(SOUNDBITE OF TRACE BUNDY'S "CLOUD FOREST")
GREENE: Stories you just heard on secondhand pot smoke and postpartum depression are part of a reporting partnership with local member stations and Kaiser Health News.
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