KNOXVILLE, Tenn. — The Centers for Disease Control said as many as 450 people across 33 states may be suffering from vaping-related lung illnesses Friday.

That number includes at least six reported cases in Tennessee, according to the state health department. One of those cases was in Knox County

While officials have not pinpointed the exact cause of these illnesses, the CDC said they believe a chemical is responsible. 

The pattern of sick patients with similar illnesses and intensified nationwide investigations is similar to when Americans were learning about cigarettes in the 1950s, according to Stanford University History of Science Professor Robert Proctor.

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"It wasn't until the 1950s and 1960s that it was proven that they cause lung cancer, heart disease and other diseases," Proctor said. "With publicity, people started to learn the truth and so they started trying to quit."

In 1964, then Surgeon General Luther Terry issued a report linking cigarette smoking to dangerous side effects — decades after smoking became widespread in the United States.

Proctor studied the impact that report and others had on the tobacco industry in his 2014 publication, "The Changing Public Image of Smoking in the United States: 1964–2014."

With e-cigarettes, however, the effects are not as well-known. Only 13 years have passed since modern e-cigarettes were first introduced to the United States, according to the Consumer Advocates for Smoke Free Alternatives Association

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"We haven't had the kind of time to find out what the long term effects are going to be," Proctor said. "With these recent deaths, there's going to be more research and people are going to start trying to find the truth."

A single cause to the outbreak has not been determined. Many who were sicked, but not all, reported vaping THC products. One of other the potential links health officials are investigating are unregulated street-sold cartridges. 

"Regardless of the ongoing investigation, people who use e-cigarette products should not buy these products off the street and should not modify e-cigarette products or add any substances that are not intended by the manufacturer," the CDC said Friday.

While state and federal health officials have pledged to find the chemicals or agents causing these health problems, Proctor said it may take some time to discover all the possible health impacts. 

"It's not good to force anything into your lungs and it took decades for people to learn how deadly cigarettes are," he said. "It'll take time to find out how deadly e-cigarettes are."

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Currently, the CDC only recommends vaping as an alternative for people who are trying to quit smoking tobacco products, saying it poses less known health risks compared to smoking. They still urge vaping should never be taken up by people who don't smoke, youths, young adults, and pregnant women.