A rash of mysterious, vaping-related lung illnesses has broken out across the U.S., with symptoms like difficulty breathing, chest pain, and vomiting. There have been 530 cases and seven deaths reported as of September 17.

• Federal health officials say they still don't know the exact cause.
• Concerns about e-cigarettes like Juul have largely focused on their use of the addictive chemical nicotine. The lung diseases have shone a new light on black market cannabis vapes, which are thought to be a major contributor to the spike in illnesses.
• In addition to marijuana, there are reports of people vaping substances like caffeine, essential oils, and vitamins, though it's unclear how common they are. 
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In July, a teenager in Long Island went to the emergency room. Once healthy, he was experiencing vomiting, fever, chest pain, and difficulty breathing.

The symptoms got worse. In what became a three-and-a-half week ordeal, the 18-year-old ended up in the intensive care unit, and later on a ventilator — treatment that required a medically-induced coma, his doctor, NYU Langone Health's Dr. Melodi Pirzada, told Business Insider. 

Though the teen initially denied any kind of vaping or smoking, his mom soon found a suspect in his room: a black-market cartridge containing THC oil, for use in an e-cigarette. Dr. Pirzada hadn't even been aware that such a thing existed, she said. 

"That was a very big surprise for us," she said. The teen was sent home from the hospital but is still weak and recovering. Dr. Pirzada has since seen two more cases of mysterious lung disease brought on by this type of cartridge, and one more suspected case. 

Read more: Mysterious vaping illnesses are causing life-threatening lung injuries and death. Here's what we know about the people who have been affected so far.

E-cigarette products like Juul have been getting the most attention because their cartridges contain the addictive chemical nicotine and come in kid-friendly flavors like mango, cream, and cucumber. But as vaping-related lung illnesses with symptoms like Dr. Pirzada's sweep the nation, doctors and scientists have another concern with the devices: black market cannabis vapes, which are even more unregulated than legally sold vapes, and may contain — or be tampered with to add — substances other than marijuana. 

"We don't really know the long-term effects of these being inhaled," Brian King, deputy director of CDC's Office on Smoking and Health told Business Insider during a phone interview in May. Giving flavorings in e-cigarette cartridges as an example, "the gut can handle a lot more than the lungs, and it also has a clear exit strategy." 

Vaping-related illnesses and deaths have swept the nation 

Vaping has spiked among young people, raising concerns about nicotine addiction, and prompting an announcement from President Trump earlier this month that all flavored nicotine e-cigarette products would be banned. 

Lately those concerns have escalated amid a rash of mysterious, vaping-related lung illnesses in the U.S. In all, there have been 530 cases across 38 US states and one territory, and seven deaths from these illnesses so far, according to recent figures from the U.S. Food and Drug Administration and the Centers for Disease Control and Prevention. 

People who have been hospitalized in these cases reported symptoms similar to Dr. Pirzada's patient in Long Island: chest pain, difficulty breathing, nausea, vomiting, and diarrhea. Like the teen, some had such difficulty breathing they were placed in medically-induced comas with ventilators to support their damaged lungs.

Today, federal health officials have said they don't know the exact cause, and are studying vapes and their contents in search of a common link among illnesses. 

As e-cigarettes have taken off, a burgeoning black market has formed

Though vapes sold at smoke shops are legal for the time being, they aren't regulated by the FDA, so they have the potential to contain substances not on the label, similar to the unregulated supplement market. Then there are illegally sold vape products, typically THC-containing ones, that can be even more dangerous because of how little buyers know about what's inside them.

There's a growing market for these illegal vapes and cartridges, which use cannabis distillates in a liquid form, rather than marijuana flower, to get users high. Black market sellers can open and tamper with the cartridges, adding substances like caffeine, alcohol, or other ingredients to make their supplies last longer. 

A September 6 study in The New England Journal of Medicine looked at vaping-related illness cases in Wisconsin and Illinois and found that 24 of the 41 THC-using patients they interviewed had used a product that was branded as "Dank Vapes," an elusive black market brand.

People who sell Dank Vape brand devices on the black market advertise them as top-notch marijuana consumption devices, but the person who is running the business is a mystery, according to Inverse. Others have even started copying it with their own "Dank Vapes," making the name itself even more mysterious and difficult to track.

During a September 19 media briefing, CDC and FDA officials said they are specifically examining THC, other cannabinoids, opioids, cutting agents, pesticides, poisons, and various additives.

Vapes can be tampered with, and substances added

Any substance can be added to vape cartridges, whether bought legally or on the black market, making pinning down the culprit of these lung diseases even more complicated. 

Lee Anne Dodge, a program director of SoPo Unite, which works in the community to prevent youth substance use, saw e-cigarette use take off in 2017 at the South Portland, Maine, high school where her office is based. The school has confiscated devices that contain THC from students, but in other cases the contents have been unclear, she said. 

"What we're hearing from students is you can kind of make your own cartridges," which students may be making themselves or buying from outsiders, Dodge said. She noted that students talk about "not knowing what's in" cartridges. 

And while data shows that marijuana is "far and away the most common substance" being used by young people in e-cigarette devices, it's by no means the only one, the FDA's King told Business Insider earlier this year. "We don't have a good gauge on prevalence of other substances," he said. 

There are also anecdotal reports of kids attempting to use substances as varied as caffeine, essential oils, vitamins, and alcohol, because any liquid can be put into such a device's reservoir, he said. Pods and devices intended for many of those substances are also sold online. 

A plethora of videos on YouTube show viewers how to open up a Juul pod or other e-cigarette to refill it, Business Insider found.

The search for what's driving mysterious lung illnesses continues

Federal health officials say they haven't been able to narrow their search down to a specific e-cigarette, product like liquids, refill pods, cartridges, or substances. 

Some with the mysterious lung illness reported using only nicotine vapes, while others admitted to using illegally obtained THC vape products, or a combination of nicotine and THC. Others who were hospitalized said they vaped cannabis they bought from licensed dispensaries where cannabis is legally sold.

Another problem: People don't want to admit that they're vaping, health officials say, which makes their effects that much more difficult to effectively track. People are especially reluctant to speak up about THC product use in states where they're not legal.