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(CNN) —  

At least 15 states have identified more than 120 cases of lung disease or injury that could be linked to vaping, a CNN survey of state health departments has found. The US Centers for Disease Control and Prevention said on Saturday it’s investigating severe pulmonary disease among people who use e-cigarettes in some states.

States with the most cases include Wisconsin, with 15 confirmed cases and 15 more under investigation, according to CNN’s survey. Illinois has 10 confirmed cases, while 12 more are under investigation. California is looking into 19 such cases. The New York State Department of Health said Friday it was “actively investigating” 11 cases. Indiana and New Jersey both reported nine cases, of which Indiana has confirmed six.

Health officials in Connecticut, Florida, Iowa, Michigan, Minnesota, North Carolina, Pennsylvania, Texas and Utah also said they were aware of confirmed or potential cases. A total of 42 states and Washington, DC, provided CNN with a response.

“These latest reports of pulmonary disease in people using vaping products in New York and other states are proof that more study is needed on the long-term health effects of these products,” Dr. Howard Zucker, health commissioner for New York State, said in a statement Friday.

The CDC said Saturday it had counted 94 possible cases of severe lung illness associated with vaping in 14 states from June 28 to August 15.

Health officials in multiple states said it is still unclear whether there’s a connection between the cases or whether vaping definitively caused these illnesses – which led to multiple people being hospitalized.

In an email Friday, the CDC urged doctors to collect information and samples of what similar patients might have been vaping. The agency said it was working with some of these states to share information and facilitate testing.

“There are still many unanswered questions, but the health harms emerging from the current epidemic of youth vaping in Minnesota continue to increase,” Dr. Ruth Lynfield, the Minnesota Department of Health’s medical director and state epidemiologist, said in a statement Tuesday. “We are encouraging providers and parents to be on the look-out for vaping as a cause for unexplained breathing problems and lung injury and disease.”

A tough condition to track

The Minnesota Department of Health reported this week that some patients were hospitalized for “multiple weeks,” in some cases ending up in the intensive care unit. They came in with symptoms including shortness of breath, fever, cough, vomiting, diarrhea, headache, dizziness and chest pain.

Dr. Emily Chapman, chief medical officer at Children’s Minnesota, which reported four cases, said in a statement that these illnesses are tricky to diagnose because they can start off looking like a common infection before leading to more serious complications.

They are also tricky to track, experts say, because vaping-related lung disease is not a condition that’s mandatory to report. Some health departments said they don’t track this data.

Still, states are putting out notices in hopes that doctors will notice the clues and ask the right questions. In Colorado, Georgia and Kansas, health officials have tried to look for cases by analyzing data from emergency departments.

“There is no diagnostic code … for lung diseases related to vaping. So it’ll be hard to follow and track,” said Dr. Humberto Choi, a pulmonologist and critical care specialist at the Cleveland Clinic.

Choi said he has seen three cases himself over the last several months – and he suspects there were others who didn’t think of vaping as a potential cause.

“People had the impression that vaping was something safe,” Choi said. “They don’t connect new symptoms with vaping.”

Choi’s state of Ohio, however, is not one of the states where health departments are currently reporting cases, according to the state’s health department.

“It’s hard to tell what to expect in these cases because it’s something new,” Choi said.

No clear culprit

Thomas Haupt, a respiratory disease epidemiologist with Wisconsin’s Department of Health Services, told CNN earlier this month that the cases in his state were young people who were “otherwise normally healthy, and they were coming in with severe respiratory illnesses, and in some cases, they actually had to go to the intensive care unit and were placed on ventilators.”

The lung disease initially looked like it was caused by an infection, “but every test has come back completely negative,” he added.

Wisconsin’s cases were largely in the southeastern part of the state, Haupt said. This borders the northeastern part of Illinois, where that state’s initial patients were hospitalized.

While officials are still trying to determine which products patients used, some states – including Wisconsin, Minnesota and New York – said the use of both nicotine and marijuana products had been reported.

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In Wisconsin, “all patients reported vaping prior to their hospitalization, but we don’t know all the products they used at this time,” Andrea Palm, the state’s Department of Health Services secretary-designee, said in a statement last week. “The products used could include a number of substances, including nicotine, THC, synthetic cannabinoids, or a combination of these.”

Health experts have pointed to a variety substances in e-liquids they worry may harm cells or contain “dangerous chemicals,” but the full extent of e-cigarettes’ short- and long-term risks are yet unclear. A number of counterfeit and adulterated products have also hit the market, which may have other additives or ingredients. It is unclear whether that plays a role across these cases.

CNN’s Nadia Kounang, Minali Nigam, Jen Christensen, Amanda Watts and Sheena Jones contributed to this report.