Health officials in Milwaukee, Wis., issued a stern warning for residents who use e-cigarettes: Stop vaping “immediately.”

Officials with the City of Milwaukee Health Department (MHD) said in a statement Wednesday 16 people have been hospitalized with chemical pneumonia – “inflammation of the lungs due to aspiration or inhalation of irritants,” according to the MDH – after they vaped nicotine or marijuana.

“At this time, the specifics of the products are unknown. Residents are again strongly encouraged to not utilize any THC products containing e-liquid,” officials added.

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Wisconsin was one of the first states to report vaping-related lung illnesses. As of Thursday, state health officials said there were 32 ongoing cases, including 11 that are still under investigation.

The majority of those interviewed by officials said they vaped THC products prior to becoming ill. THC is the psychoactive ingredient in marijuana that causes the “high” sensation. That said, “while most cases have reported vaping THC products, DHS is continuing to investigate all possible causes,” officials noted.

In a statement provided to Fox News on Friday, the Centers for Disease Control and Prevention (CDC) said there were 215 potential cases of respiratory illnesses linked to vaping across 25 states.

Teens across the country have primarily been affected. The first vaping-related death was reported last week in Illinois.

Both the CDC and the Food and Drug Administration (FDA) are investigating what may be behind the illnesses, though a specific product has not yet been identified. An unnamed source reportedly told The Washington Post both state and federal health officials are looking at “contaminants or counterfeit substances,” specifically in THC-containing vaping products, as a possible source.

"More information is needed to better understand whether there’s a relationship between any specific products or substances and the reported illnesses. At this time, there does not appear to be one product involved in all of the cases, although THC and cannabinoids use has been reported in many cases," the CDC said in a statement. "At this time, the specific substances within the e-cigarette products that cause illness are not known and could involve a variety of substances. We continue to gather information about the names of the products used, where they were purchased, and how the products were used. That information is critical to help determine whether patterns emerge on which we can take additional action."

The negative health effects associated with cigarettes and cigars have long been documented, leading to a decline in use among teens in the U.S. in recent years.

But the same is not true for e-cigarette use. In 2018, nearly 1 in every 20 middle school students (4.9 percent) reported using electronic cigarettes in the past 30 days. That's an increase from less than 1 percent in 2011.

Last year, the Surgeon General of the United States, Jerome Adams, declared vaping among American teens an “epidemic."

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"This is an unprecedented challenge,” Health and Human Services Secretary Alex Azar said at the time.

Though e-cigarettes are often touted as a "safer" alternative to cigarettes, a recent study found that may not be true.

The study, led by University of Pennsylvania researchers, claimed there are damaging effects on a user’s blood vessels after just one use.