As many as 50 people in at least six states have come down with breathing illnesses that may be linked to e-cigarettes or other vaping products.
No deaths have been reported, but at least a few have come close.
Some patients have likened onset of the illness to a heart attack, and others to the flu. Symptoms have included shortness of breath, fatigue, chest pain and vomiting. Doctors say the illnesses resemble an inhalation injury, with the body apparently reacting to a caustic substance that someone breathed in.
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Dr. Melodi Pirzada, a pediatric lung specialist at NYU Winthrop Hospital in New York, said she's seen two cases this summer — one of them an athletic 18-year-old who almost died.
"We're all baffled," Pirzada said. The only common factor was they had been vaping, she said.
Wisconsin health officials on Thursday said they're seen 15 confirmed cases, with another 15 illnesses under investigation. New York state officials are investigating 10, Illinois has seen at least six, and Minnesota doctors this week said they have four more. California and Indiana have also been looking into reported illnesses.
Health officials have only been counting certain lung illnesses in which the person had vaped within three months. Most are teens, but some adult cases have also been reported. No single vaping device or liquid is associated with the illnesses.
Dylan Nelson, a 26-year-old Wisconsin man, went to see a doctor when he first became ill. He has asthma, was diagnosed with pneumonia and was treated and released.
Within a few days, he could barely breathe. He went to a hospital and was put on a breathing tube. His two brothers kept a round-the-clock vigil over him in the ensuing days, and at one point one called his mother to the hospital, saying: "Mom, I don't think he's going to make it. ... He can't die without his mom."
He rallied and was released from the hospital late last month.
But "he still has lung damage and heart damage," and doctors still don't know how much they'll heal, said his mother, Kim Barnes of Burlington, Wisconsin.
Electronic cigarettes have been described as a less dangerous alternative to regular cigarettes, but health officials have been worried about kids using them. Most of the concern has focused on nicotine, which health officials say is harmful to developing brains and might make kids more likely to take up cigarettes.
But some vaping products have been found to contain other potentially harmful substances, including flavoring chemicals and oils used for vaping marijuana, experts say.
Wisconsin officials did do not know the names of the products the sick people had vaped, but added they could include several substances, including nicotine and THC — the main active chemical in marijuana.
Dr. Anne Griffiths, a lung specialist who saw all four of the reported Minnesota cases, said each had vaped different products.
"I really do think the primary cause of these illnesses is what's been inhaled," said Griffiths, of Children's Minnesota.
Among the many questions: Why are cases only surfacing now, when e-cigarettes have been around for years and are now being used by more than 10 million Americans?
It's possible illnesses previously weren't recognized as being related to vaping, Griffiths said. She said she found several similar case reports published in medical journals.
"My sense is this isn't new. It's new that we're recognizing it," she said.
Nelson's mother is convinced vaping is to blame.
She said her son's friends have continued vaping, even after they saw what happened to him, because they don't vape the brand of THC-containing cartridges he had used.
"Everybody's vaping. They're all vaping. And they all think it's safe," Barnes said.
AP reporter Steve Karnowski contributed to this report.