New evidence shows that legitimate THC vaping products are also making people sick, meaning that illegal, off-brand vapes aren't the only products to blame for the current outbreak of vaping-related illnesses.

The Massachusetts Department of Public Health reports that at least six patients suspected of having the vaping lung illness purchased THC products at licensed marijuana dispensaries in that state.

Health officials did not reveal which dispensaries or brands bought legally were implicated.

This is not the first time that a case of the lung illness has been linked to a legal product. In September, the Oregon Health Authority announced it was investigating the death of a woman who reported using a vaping device from a cannabis dispensary before becoming ill.

"It's been a real failure of public policy that we have this message that if it's bought in a store, it's going to be safe," said Kevin Sabet, president of Smart Approaches to Marijuana, a nonprofit organization that opposes the legalization of marijuana.

Still, bootleg vapes do appear to be behind a majority of vaping-related cases nationwide. On Friday, the Centers for Disease Control and Prevention reported that 80 percent of hospitalized patients with EVALI, or e-cigarette, or vaping, product use associated lung injury, had used a THC product. By far, the most common product was a counterfeit brand called Dank Vapes, used by 56 percent of patients.

Some patients said they only vaped nicotine, not THC, so the CDC recommends avoiding all vape pens and products until more is known.

Since the EVALI outbreak began, a variety of investigations into the illnesses have pointed to multiple problematic ingredients, such as vitamin E acetate used as a carrier oil, or toxic metals leached from the devices into the e-liquids.

Each finding appears to be just one part of the larger explanation of why vaping can be dangerous, experts who study e-cigarettes say.

In addition to oils and metals, "there may be other contaminants that we don't know about yet because the products are so new," said Thomas Eissenberg, a co-director of the Center for the Study of Tobacco Products at Virginia Commonwealth University.

What's more, ingredients in e-liquid can degrade or change in potentially harmful ways once they're aerosolized. When heated, "those liquids turn into different things that were never in the liquid to begin with," Eissenberg said.

Left unregulated, the vape pen market has exploded in recent years, leading to a proliferation of products easily accessible online.

Many, such as Puff Bars and Posh Vapes, have designed their products to look like Juul devices. Juul is not a THC product; it's sold with nicotine only.

"We did a simple Google search for 'disposable Juul' and got 31 different brands of products from the first results page," said Rebecca Williams, a research associate at the University of North Carolina Lineberger Comprehensive Cancer Center.

Williams is the author of a new report that analyzed online marketing of disposable e-cigarette vendors, published Friday in the journal Tobacco Control.

Many of the products Williams uncovered had nicotine concentrations of up to 7 percent, higher than Juul, and had a largely ineffective age verification process for purchasing. Many simply asked, "are you over 21?"

"They're available cheaply and easily accessible by minors," Williams said. Indeed, the use of e-cigarettes among teens has skyrocketed in recent years, doubling since 2017.

Major health groups, including the American Lung Association and the American Heart Association, have urged consumers to avoid all vape products.

"There are definite ways in which using an e-cigarette can harm your lungs and can harm your cardiovascular system," Eissenberg said. "Anybody who's concerned about those things — and I think everybody should be concerned about those things — should not be vaping."

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