Federal health officials are investigating potential links between lung illnesses and e-cigarettes amid the soaring popularity of so-called vaping among young people.
Since June 28, there have been at least 94 cases reported across several states involving people with "severe pulmonary disease" possibly tied to vaping, with the majority of those affected being teenagers and young adults, according to the U.S. Centers for Disease Control and Prevention.
At least 30 of those cases were reported in Wisconsin, the CDC said in a statement Friday.
The CDC said it is working with health departments in Wisconsin, Illinois, California, Indiana and Minnesota to investigate a potential link between breathing problems and the use of e-cigarettes, which are thought to be healthier than traditional means of smoking.
"Additional states have alerted CDC to possible (not confirmed) cases and investigations into these cases are ongoing," the statement said. "There is no conclusive evidence that an infectious disease is causing the illnesses."
While some cases in each of the states are similar and appear to be linked to e-cigarette product use, more information is needed to determine what is causing the illnesses, the CDC said.
The U.S. Food and Drug Administration also is investigating reports of seizures among e-cigarette users.
Seizures are a potential side effect of nicotine toxicity, but a recent uptick in "reports of adverse experiences with tobacco products that mentioned seizures occurring with e-cigarette use (e.g., vaping) signal a potential emerging safety issue," the FDA said in April.
ABC News' Ella Torres contributed to this report.