A health-surveillance system put in place after the terrorist attacks of September 2001 has been used to pinpoint the cause of the vaping-related lung injuries that have killed 54 Americans and sent more than 2,500 people to the hospital.

Using this system, researchers at the Centers for Disease Control and Prevention have found that the lung injuries rose sharply in June of this year.

Dr. Anne Schuchat, the CDC's principal deputy director, says this sharp spike strongly points to a single culprit for most of these cases: vitamin E acetate, an additive found in illicit cannabis-containing vapes.

"That doesn't mean that there aren't other chemicals that can or are causing lung injury," Schuchat said during a telephone news conference. But backed with additional data about vitamin E acetate found in lung samples from people who were injured after vaping, she attributes the bulk of the outbreak to that additive.

The CDC is still trying to understand the mechanism by which this substance injures lung tissue. It may interfere with a natural fluid in the lung called surfactant, which helps make lung tissue stretchy. Or a byproduct may be a toxic chemical.

Emergency-room doctors in Wisconsin first noticed an outbreak of these lung injuries in June. They alerted state and federal health officials, who quickly started investigating the extent of the outbreak as they looked for what could be causing it.

Investigators homed in on vaping products containing marijuana extracts, specifically those that were purchased online or on the street, rather than through dispensaries. Further research focused on vitamin E acetate, which is used in some of these preparations to dilute the much more expensive THC oil.

It was important to understand the timing of the outbreak as part of this investigation. Health officials wanted to know when exactly the problems first cropped up, and whether they had missed many cases before those initial reports. That's why they turned to the surveillance data, collected from more than 3,200 emergency rooms from most states.

They found a gradual increase in emergency-room visits among people who vaped or used e-cigarettes, starting in January of 2017.

"Those visits weren't limited to lung injury, so there may be other health effects that people were experiencing," says Kathleen Hartnett, an epidemiologist at the CDC who is first author of a studybeing published by the New England Journal of Medicine. People may have also been going to the emergency room because of nicotine poisoning, or intoxication from THC, the main psychoactive ingredient in marijuana.

Narrowing their search to people under the age of 35, they detected a sharp spike in June of 2019 — the same time that the doctors in Wisconsin reported their first cases.

The number of cases climbed from June into September, when they peaked. They have since declined, but the CDC still reports about 100 cases a week, and the death toll continues to tick up.

The surveillance system doesn't automatically flag health concerns – often scientists dive into the data once their suspicion is raised, as it was in this case. The outbreak only became apparent in the data once researchers defined more closely what they were looking for and focused on one age group.

"While electronic health data are increasingly powerful, they really can't yet replace the experience of clinicians like the ones in Wisconsin, who most definitely saved lives when they... reported these first cases," Hartnett says.

The US Food and Drug Administration also announced Friday a crackdown on illicit vaping cartridges. The FDA and the Drug Enforcement Administration seized 44 websites advertising the sale of these products, as part of "Operation Vapor Lock," its response to the lung injuries.

You can contact NPR Science Correspondent Richard Harris at rharris@npr.org.

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