US health officials are investigating around 100 cases of mysterious lung illnesses believed to be linked to vaping and e-cigarette use in 14 states.

Many of those who have fallen ill are teenagers and young adults. A large number have been hospitalised, with some in intensive care and on ventilators.

Medical authorities said it is unclear whether patients will fully recover.

Clinicians and the public have been told to stay alert for a severe and potentially dangerous lung injury.

Symptoms include difficulty breathing, shortness of breath or chest pain before hospitalisation. Health officials said patients have also reported fever, cough, vomiting and diarrhoea.

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The Centres for Disease Control and Prevention (CDC) said officials are working with health departments in at least five states with confirmed cases – California, Illinois, Indiana, Minnesota and Wisconsin – to determine the cause of the condition after “a cluster of pulmonary illnesses linked to e-cigarette use” was reported among adolescents and young adults in recent weeks.

There are at least 31 cases confirmed by state officials. The CDC said it is probing 94 possible cases in 14 states.

To date, there is no consistent evidence that an infectious disease is the culprit, CDC spokeswoman Kathy Harben said.

While some of the cases appear similar, officials said they do not know whether the illnesses are associated with the e-cigarette devices themselves or with specific ingredients or contaminants inhaled through them.

Health officials have said patients have described vaping a variety of substances, including nicotine, marijuana-based products and do-it-yourself “home brews”.

Underscoring the growing level of concern, CDC officials say they are notifying health-care systems and clinicians across the country about the illnesses and what to watch for. State health departments have also issued warnings.

E-cigarettes have grown in popularity over the past decade despite little research on their long-term effects.

In recent years, health authorities have warned of an epidemic of vaping by underage teenagers.

The leading brand, Juul, said it is monitoring the reports of illnesses and has “robust safety monitoring systems in place”.

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Gregory Conley, president of the American Vaping Association which advocates for vaping products, said that about 10 million adults vape nicotine without major issues each month.

“It appears much more likely that the products causing lung damage are amateur-made street vapes containing THC or illegal drugs, not nicotine,” he said.

But health authorities worry there has not been enough time to establish its safety.

“We haven’t had that kind of history with vaping to be able to assure anyone – teens included – that this is a safe practice,” said Emily Chapman, chief medical officer at Children’s Minnesota, which has cared for four teenagers with the illness.

In the past month, the teenagers presented symptoms that appeared manageable and consistent with viral-type infections or bacterial pneumonia, such as shortness of breath, coughing, fever and abdominal discomfort, Ms Chapman said.

They continued to deteriorate despite appropriate treatment with antibiotics and oxygen support. Some suffered respiratory failure and had to be put on ventilators, she said.

“These cases are extremely complex to diagnose, as symptoms can mimic a common infection yet can lead to severe complications and extended hospitalisation,” Ms Chapman said.

“Medical attention is essential. Respiratory conditions can continue to decline without proper treatment.”

E-cigarettes are a diverse group of products containing a heating element that produces an aerosol from a liquid that users can inhale via a mouthpiece.

Millions of Americans use e-cigarettes, especially young adults. In 2018, over 3 million school students said they had used e-cigarettes in the past 30 days, according to the CDC.

A National Academies of Sciences, Engineering and Medicine report in January found that while e-cigarettes are less harmful than conventional cigarettes, which produce a raft of toxic substances when burned, they still pose health risks.

Among non-smoking adolescents and young adults, there was ”moderate evidence for increased cough and wheeze” and increased incidence of asthma.

But many medical authorities believe there is not sufficient data to know their full effects, especially on young people.

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Dylan Nelson from Wisconsin, who has asthma and has been vaping for about a year, was hospitalised with pneumonia last month after he started having trouble breathing.

The 26-year-old described feeling as if he were breathing through a straw. He was coughing, his heart was racing and his breathing was hard and fast.

Mr Nelson said he spent days in the hospital, some of that time attached to a ventilator. His mother, Kim Barnes, said when a nurse told her it might be related to vaping, it was a wake-up call for her.

Now, she wants to convey that sense of urgency to other parents: “You need to sit your kids down and tell them the dangers of this stuff. If you’re an adult, wise up – this is not good.”

Doctors had seen “scattered cases” of lung illnesses tied to vaping before, but they had not identified a pattern until now, said Ms Chapman,

“I think it’s important to understand that vaping is assumed to be safe, and yet we know so little about it,” she said.