US media reported several other states were also reporting cases (Representational)

US health officials are investigating the cases of dozens of people, mainly teens, who were hospitalized with severe lung injuries in recent weeks after vaping, though the precise cause of their illnesses remains a mystery.

Health departments in Illinois, Minnesota and Wisconsin have issued statements describing cases involving patients who exhibited coughs, shortness of breath, dizziness and fatigue, symptoms that led them to hospitals where they had to be intubated.

Officials in the three states, which have reported at least 30 confirmed cases and 22 under investigation, said it was too early to say whether the illnesses were connected, but were working with each other and the Centers for Disease and Control and Prevention to investigate further.

Some of them involved "dabbing," or vaping marijuana products. US media reported several other states were also reporting cases.

Thomas Haupt, a respiratory disease epidemiologist with Wisconsin's Department of Health Services, told AFP on Thursday: "We're in the process of interviewing all of our patients, to try to find out any commonalities.

"At this particular point, vaping is the only thing that they have in common, but we're trying to cast a broader net and just make sure that we are not missing something as well."

The findings have baffled experts, since e-cigarettes have been available in the US since 2006 and scientific research so far suggests they are probably a less toxic alternative to smoking.

Their use among adolescents has skyrocketed in recent years: some 3.6 million middle and high school students used vaping products in 2018, an increase of 1.5 million on the year before, according to the report by the Centers for Disease Control and Prevention (CDC).

Haupt told AFP that the Wisconsin cases weren't confined to teens -- the confirmed cases included users into their mid-thirties while the suspected cases included people in their fifties.

Many had to receive mechanical assistance in breathing, while imaging had revealed infiltrates, and "ground glass opacities" that indicate abnormalities in the alveoli.

But they responded well to steroid treatment and were later discharged, and will receive follow-up scans a few weeks from now to determine whether there has been any permanent damage.

"This is an eye-opening investigation for us," he said, adding that the patients had no underlying illnesses that would put them at risk for complications.

"We always encourage parents to talk to their children about this. If they do vape, it has to come from a reputable dealer," he added.