The number of potential vape-related lung illnesses across the country — and in Massachusetts, according to the Department of Public Health — is on the rise. Just yesterday the CDC released updated numbers reporting there are 530 cases nationwide. So far 8 people have died. WGBH Morning Edition Host Joe Mathieu spoke with physician and Boston University professor Dr. Michael Siegel to learn more about what's causing the illness. The transcript has been edited for clarity.

Joe Mathieu: You have written that guidance from the government has been vague. It certainly has been confusing for a lot of people. We're dealing with a few different substances here. Are you concerned about both nicotine and marijuana vapes?

Dr. Michael Siegel: Yes, I think that there's been a lot of confusion. The CDC has not been putting out very clear information. About the most specific they've gotten is "don't use e-cigarettes and don't vape." But we knew that at the beginning. From the beginning of this outbreak, we knew that the cases were linked to "vaping." What we need is much more specific information, and the only thing that we've been able to tease out from the data so far is that 80 to 90 percent of these patients have reported the use of THC. In other words, they're not vaping the traditional nicotine e-liquids that we keep hearing about. What they're vaping is illicit THC vape cartridges that are essentially purchased off the street. These are black market products. So we truly have no idea what is in those products. There has been one potential contaminant called Vitamin E acetate oil. This is an oil that is being used as a thickening agent in black market THC cartridges. It may be that is responsible for at least the bulk of these cases because oil is certainly not meant to be inhaled into the lungs and when the lungs get coated with oil, they literally cannot function.

Mathieu: So just the concept of vaping marijuana oil is problematic?

Siegel: Absolutely. And I think that's really fallen under the radar screen because we've been talking for several years now about vaping and Juuling nicotine-containing products. What's going on behind the scenes that youth are not being so upfront about is that they're actually vaping marijuana. There are THC cartridges that are spreading through distribution channels [and] through our schools. And I think what this outbreak is telling us is that the issue is not necessary vaping, the issue is what are you vaping? What is going into that product?

Mathieu: It's confounding in a state that has tried to be a leader in the legalization of marijuana for both medical and recreational purposes, but also this is the state that's a leader in health care research. We're not rationalizing these.

Siegel: Yeah, I think somehow because of the legalization of recreational marijuana, it has created a perception that marijuana is safe. And I think the reason that it was legalized was not that there were no harms associated with it, but because people were being criminalized and that was causing racial justice problems. So I think it's important for people to realize that it's legal, but that doesn't mean that it's safe, and the products that aren't highly regulated are potentially dangerous.

Mathieu: These chemicals that you can go to a dispensary and buy inside a vape cartridge are not being regulated by our own state government. And not by the FDA, certainly?

Siegel: So the cartridges that you purchase from a licensed dispensary are regulated. The most dangerous products by far are the ones that are being bought on the black market because apparently Vitamin E acetate oil is very commonly used as a thickening agent. And there are tens of thousands of those cartridges all over the country.

Mathieu: And most likely to end up in the hands of underage users.

Siegel: Exactly. We know that youth are using these things. The most recent data from the National Youth Tobacco Survey said that 70 percent of kids who are regularly vaping are vaping THC. Now, that doesn't mean exclusively THC; they may also be vaping more traditional products. But 70 percent of kids who regularly vape are using THC, which means by definition pretty much those are products that they're getting off the black market. That has gone completely under the radar screen.

Mathieu: E-mails are going out from licensed [medical and recreational] marijuana dispensaries here in the state reassuring people that their vapes are safe. Should they believe them?

Siegel: Well, I think that for the most part that is a reasonable assumption. These are regulated products and the dispensaries at least know what's in them. The majority of these cases have been from black market cartridges. There's only one that I'm aware of that has been associated with a legal product from a dispensary. So I think the dispensaries are basically correct in that their products are relatively safe.

Mathieu: This whole story can be very confusing for people who were seeking a safer alternative, whether it's cigarettes or smoking actual combustible marijuana flower. The idea was none of this is good for you, but at least this is better than smoking. Do you worry about a backlash where people are going back to buying packs of cigarettes and rolling joints because they're afraid of the vape pens?

Siegel: Absolutely. I think that is the huge negative repercussion that's happening because the CDC isn't being very clear. What they're basically saying is don't vape [and] don't use any cigarette product. That's leading a lot of smokers, who actually had quit smoking, to say, "hey, I'm going to go back to smoking." And that has disastrous public health consequences. So again, 80 to 90 percent of these cases are linked to vaping marijuana, and I think that the CDC should be saying that.

Mathieu: Do you get the sense we're about to learn a lot more about this?

Siegel: We'll definitely learn a lot more about this as time goes on, but I think that we know enough now to be basically sending a clear message to youth do not vape marijuana, period. The evidence is pointing towards illicit THC carts that are made with oils that are that are literally poisoning the lung, and that's what kids need to know about.

Editor's note: This article was updated to clarify that the numbers of potential vape-related lung illnesses in Massachusetts came from the Mass Department of Public Health.