The sudden onset of these mysterious illnesses and the patients’ severe and distinctive symptoms have led investigators to focus on contaminants, rather than standard vaping products that have been in wide use for many years.

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The investigation is “starting to point to what solvents are being used, and that can vary a lot” — especially in counterfeit or black-market products, including potentially mislabeled solvents that consumers buy themselves to make do-it-yourself “home brews,” said an official at the Centers for Disease Control and Prevention, which is heading the investigation.

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The CDC and Food and Drug Administration issued a statement Friday morning warning that “anyone who does use e-cigarette products should not buy these products off the street (e.g., e-cigarette products with THC or other cannabinoids) and should not modify e-cigarette products or add any substances to these products that are not intended by the manufacturer.”

Vaping refers to the increasingly popular practice of inhaling vapor from an e-cigarette device, which frequently involves heating a liquid that can contain nicotine, marijuana or other drugs. Those active ingredients are delivered in solvents.

The national health alert about the illnesses — a sign of growing urgency about a health threat that has claimed one life, while hospitalizing scores of others — cautions that officials are a long way from understanding what exactly is making so many people sick. The victims report vaping different products, purchased in different places and used for different lengths of time. Many of those who have gotten ill say they have vaped products containing marijuana, but some also used traditional nicotine e-cigarettes. Some report using both.

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The last week’s dramatic increase in reported cases is probably the result of increased awareness, said the CDC official. The official, who was not authorized to speak on the record, predicted that state officials will eliminate some of the unconfirmed cases.

In the meantime, federal and state officials are urging the millions of Americans who use e-cigarettes to stop vaping until the cause of the illnesses is identified. They have suggested that those using e-cigarettes quit smoking conventional cigarettes and contact their doctors about alternatives, such as patches or nicotine gums.

Some of those warnings have drawn criticism from both industry and health groups for being overly alarmist and broad about which products might be dangerous.

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“People need to stop using these illicit THC products now — and it’s the CDC’s responsibility to say what we do know,” said Michael Siegel, a professor of community health services at Boston University, who believes that e-cigarettes can be helpful to those trying to quit smoking.

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“Yes, there are unknowns. Yes, there may be other products implicated as well. But a large number [of confirmed cases] seem to involve THC oils purchased from ‘pop-up shops,’ ” he said, referring to unlicensed shops that move frequently, advertise by word of mouth and often fail to follow safety regulations.

The complex investigation has also been hobbled by the patchwork of state and federal regulatory authorities involved and the sheer number and kinds of businesses, legal and illicit, selling vaping products. Nicotine vaping products are lightly regulated in the United States, although tougher rules are due to kick in over the next year. Yet e-cigarette use has surged, especially among teens and young adults who have embraced the sleek, high-nicotine model of Juul, the leading brand.

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A growing number of Americans are also vaping marijuana, which is legal in some states but prohibited under federal law.

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Some marijuana vaping devices, such as vape pens, are a fairly recent phenomena and among the most popular devices for vaping cannabis. The vape pens are a potential target for black-market sellers because they’re cheap and easy to fill with counterfeit substances, according to officials and industry sources.

Former FDA commissioner Scott Gottlieb, a harsh critic of San Francisco-based Juul, said he believes it’s unlikely that Juul — or other large national e-cigarette companies — are associated with the recent cases because the illnesses appeared to have developed suddenly and do not appear to be spread evenly across the nation.

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Traditional vaping products could cause chronic problems, Gottlieb said, but these patients have acute illnesses.

“What’s likely causing the harm is something that they are putting in to make it easy or cheap to mix,” he added.

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Some state health officials and clinicians are increasingly focused on contaminants in THC products.

In a conference call Thursday, Utah health officials told physicians the state department is focusing on THC products, said one doctor who took part in the call. The physician spoke on the condition of anonymity because he was not authorized to speak for the health department. Utah has reported at least 21 possible cases in recent weeks. A health department official said they are looking at both THC and nicotine products.

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Pennsylvania’s UPMC health system, which includes 40 hospitals, including the University of Pittsburgh Medical Center, has treated at least 14 individuals with acute lung injuries related to vaping in recent weeks, doctors said. Several patients told clinicians they bought THC products online from a popular brand called Dank Vapes. Industry sources say marketers often fill an empty cartridge with potentially unsafe ingredients and sell to unsuspecting customers.

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“We suspect adulterated or contaminated products, because these [marijuana] products have been out there for some time, and we’ve not seen these cases until this summer,” said Phillip Lamberty, a pulmonologist and critical care specialist at the University of Pittsburgh Medical Center, who cared for three patients in recent weeks. Two of them used products containing THC, he said. He wasn’t sure what product the third was vaping.

One patient purchased his product from an illicit drug dealer. Another patient told Lamberty he had previously smoked marijuana but bought an online product and vaped THC for the first time about a week before falling ill.

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In California’s Kings County, all seven patients who had acute respiratory distress syndrome reported buying marijuana vape cartridges from “pop-up shops,” said Nancy Gerking, the county’s assistant director of public health.

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“The patients had switched from regular retailers to the pop-up shops,” Gerking said. The patients “found a difference between the potency of the products,” she said. “They had to use twice as much, so they were taking twice as much of the product into their lungs.”

State health departments are sending samples to the FDA’s forensic laboratory in Cincinnati, where an initial targeted analysis is testing for THC, CBD and nicotine, officials said. The FDA and other labs are also doing more broad-based testing for solvents and possible contaminants to look for any pattern.

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Friday’s statement said that “the FDA has received about 80 samples and continues to receive requests from states to send more samples for the FDA to analyze. The samples represent a variety of different types of products and substances — a number of which contained incomplete information about the product. The FDA is analyzing those samples for their contents, whether they contain nicotine, substances such as THC or other cannabinoids, or other chemicals and ingredients.”

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Among the possible culprits are oils and “cutting agents” being added to the liquid in vaping devices, especially those containing THC. State officials and marijuana industry sources said black-market dealers appear to be using new substances to thin out THC oil, which is thick, odorless and colorless. Unknown oils, also colorless and odorless, are then being added back to the mixture to give the appearance of purity, according to Alex Traverso, a spokesman for California’s Bureau of Cannabis Control, which regulates commercial cannabis licenses for medical and adult-use cannabis in the state.

“Yes, that is something that is being considered,” the CDC official said when asked about the new cutting agents. During the aerosolization process, the oil heats up and produces the vapor cloud. But when it cools, it returns to its original state, and tiny aerosolized droplets could be inhaled into the lungs. “That could be responsible for problems we’re seeing in these individuals,” the official said.

“People have also talked about some of it getting into the gut,” the official said.

Some patients have complained about vomiting and diarrhea, and the presence of these substances in the gut “may account for the gastrointestinal problems that some people have reported.”

Unlike infectious disease outbreaks, this investigation has been more difficult because investigators don’t have a specific culprit, an established national system of collecting information or even a consistent definition of the illness. Unlike certain infectious diseases, such as measles, which are required to be reported to federal authorities, this illness does not fall in that category. States are not required to report possible cases of vaping-related illnesses to the CDC.