At least 15 US states have identified more than 120 cases of lung disease or injury that could be linked to vaping, a survey of health departments has found.

American states with the most cases include Wisconsin, with 15 confirmed cases and 15 more under investigation. Illinois has 10 confirmed cases, while 12 more are under investigation.

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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.

Study needed

"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 on Friday.

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 hospitalised.

In an email Friday, the US Centers for Disease Control and Prevention 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.

Chevron Right Icon 'There are still many unanswered questions.'

"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.

"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."

The Minnesota Department of Health reported this week that some patients were hospitalised 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.

Hard to track

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 the 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 analysing data from emergency departments.

Chevron Right Icon 'There is no diagnostic code.'

"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 but 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."

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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.

Chevron Right Icon 'It's hard to tell what to expect in these cases.'

"It's hard to tell what to expect in these cases because it's something new," Choi said.

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 hospitalised.

Many causes

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.

In Wisconsin, "all patients reported vaping prior to their hospitalisation, 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 of 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.