Fabian Castillo was suffering from crippling anxiety when his uncle handed him a marijuana vape pen one day last December.

"It will help calm you down," Castillo said his uncle told him.

Over the next several months, the vape worked as advertised. Castillo, who had just graduated high school in southern California, found himself more mellow and more productive.

He had no idea at the time that he was using, what he now believes, was a bootleg vape pen filled with a toxic mix of chemicals.

But on Aug. 2, Castillo’s breathing grew labored. His mother brought him to the emergency room where, she says, an X-ray revealed severe damage to his lungs.

"I couldn’t breathe. I couldn’t talk. I literally couldn’t even move my hands," Castillo, 19, said.

He spent the next nine days in a medically induced coma. Eight weeks later, he still struggles to breathe deeply.

Fabian Castillo lies in a hospital ICU last month when he was suffering from severe lung damage linked to his use of a bootleg marijuana vape. salazar383412 / Instagram

The soaring popularity of vaping products — both legal and knock-offs — is fueling a public health crisis that has vexed the medical community.

Some 12 people have died from mysterious lung illnesses linked to vape pens, and 805 others have been hospitalized in 46 states, according to federal health officials.

"We are dealing with a new epidemic," said Dr. Melodi Pirzada, a pediatric pulmonologist at NYU Winthrop Hospital in Mineola, New York, on Long Island.

The Centers for Disease Control and Prevention says most of the patients reported using vapes containing THC, the psychoactive ingredient in marijuana. Some state health officials have indicated that Vitamin E acetate, a solvent used to "cut" cannabis for use in vape pens, may be responsible for the outbreak.

But no single substance or product has yet been linked to all of the cases of vaping-related lung disease, the CDC says, leaving the medical community grappling with an exploding health crisis with an unknown cause.

At the same time, the FDA is struggling to police what has rapidly grown into a billion dollar industry with a booming black market, experts say.

The result: Americans have access to an astonishing assortment of THC vape pens without having any way of knowing what’s actually in them.

Seeking answers, NBC News commissioned one of the nation's leading cannabis testing facilities to test a sampling of THC cartridges — 18 in all — obtained from legal dispensaries and unlicensed dealers.

The findings were deeply troubling.

Of the three purchased from legal dispensaries in California, the CannaSafe testing company found no heavy metals, pesticides or residual solvents like Vitamin E.

But 13 out of the other 15 samples from black market THC cartridges were found to contain Vitamin E.

CannaSafe also tested 10 of the unregulated cartridges for pesticides. All 10 tested positive.

The products all contained myclobutanil, a fungicide that can transform into hydrogen cyanide when burned.

"You certainly don’t want to be smoking cyanide," said Antonio Frazier, the vice president of operations at CannaSafe. "I don’t think anyone would buy a cart that was labeled hydrogen cyanide on it."

Pirzada described the existence of myclobutanil as "very disturbing," adding that "it’s going to cause a very toxic effect on the lungs."

The New York pulmonologist also expressed alarm about the presence of Vitamin E, which is also known to cause significant lung damage when inhaled, in the THC mixtures. "It should not be inhaled into your lungs," she said.

Pirzada has treated four patients, all teenagers, suffering from vaping-related lung damage. She said testing conducted on the same vaping mixture used by one of her patients detected the presence of Vitamin E.

The 18-year-old boy arrived at the hospital with pneumonia-like symptoms. But within 48 hours, Pirzada said, his condition deteriorated rapidly and he was placed on a ventilator.

He spent five days on life support before he was able to breathe on his own and recover with the help of steroids. "He required very high levels of support to keep him alive," Pirzada said.

Fabian Castillo was hospitalized with severe lung damage linked to his use of a marijuana vape. NBC News

Such patients have been pouring into hospitals around the country. Over the summer, an 18-year-old girl arrived at UCLA Health with a bad cough, fevers, nausea and labored breathing. Within 48 hours, her lung function deteriorated to the point that doctors sent her to the ICU and hooked her up to a respirator.

The teenager, who reported having vaped tobacco and pot products every day for the past two years, ultimately improved and was released from the hospital.

"She got very sick, very fast," said Dr. Kathryn Melamed, the pulmonologist who saved the teenage girl’s life.

Less than 15 miles from UCLA Health lies a 12-block area of downtown Los Angeles filled with vape shops. The stores sell empty cartridges and packaging, making it easy for anyone with access to THC and a solvent like Vitamin E to produce their own bootleg vape products.

Downtown Los Angeles features a 12-block area filled with shops selling vape supplies that can be used in bootleg products. NBC News

California legalized recreational marijuana for adults over the age of 21 in 2016.

David Downs, the California bureau chief for Leafly, an online publication devoted to the cannabis industry, said downtown Los Angeles acts as the final destination for a supply chain that originates in China.

"This all starts in China where you can get the empty cartridges both for the THC market and the nicotine market, as well as the additives, flavorings, and thickeners that are being put into these cartridges alongside the THC oil," Downs said.

"It’s a very deep, mature, and advanced industry that starts in China and ends in our own backyard."

Downs said anyone who buys bootleg vapes is putting themselves at risk.

"I’ve been saying, ‘Look, if you buy a fake Gucci purse, it’s not going to give you a lung injury, but if you buy a fake vape cartridge, it just might.’" Downs said.

Acting FDA Commissioner Ned Sharpless, speaking to a congressional committee on Wednesday, said investigators are working to identify the toxic products and "follow the supply chain to the source."

"FDA is not pursuing any actions associated with personal use of any vaping products, our interest is in the suppliers," Sharpless told the House Energy and Commerce subcommittee, which has oversight authority over the FDA.

"But to be clear, if we determine that someone is manufacturing or distributing illicit, adulterated vaping products that caused illness and death for personal profit, we would consider that to be a criminal act."

The American Vaping Association insists the outbreak is linked to THC oils and knock-offs.

Laboratory tests found no pesticides or residual solvents in these marijuana cartridges. NBC News

One of the shops visited by NBC News in Los Angeles sold packaging for Dank Vapes Gorilla Glue, the same brand of vape that Castillo was using before he landed in the hospital ICU. The product is not in any way associated with Gorilla Glue, the company that produces super glue and other adhesives.

An aspiring singer, Castillo hasn’t yet returned to full health. Any time he tries to take a deep breath, he feels like he just ran down a flight of stairs. Castillo is also dealing with an odd sensation that leaves him feeling like he’s falling and results in a body twitch.

He said he’s speaking out to deter others from putting their lives at risk by using vapes.

"Everything was put on pause because I decided to vape," Castillo said. "I thought it was safe."