The confirmed deaths have occurred in 15 states: Alabama, California, Delaware, Florida, Georgia, Illinois, Indiana, Kansas, Minnesota, Mississippi, Missouri, Nebraska, New Jersey, Oregon and Virginia. More deaths are under investigation. State health departments have released very little information about the patients who died. But they have tended to be older.

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Anne Schuchat, the CDC’s principal deputy director, said in a briefing with reporters Thursday that the median age of the patients who died is about 50, with a range from people in their 20s to those in their 70s. Compared to those who have fallen ill, a higher proportion of the deaths have been in women, she said. She did not have details about their underlying medical conditions. But individuals with such conditions would probably not withstand this type of acute lung injury, she said.

Schuchat said it is difficult to know how long this outbreak will continue. With hundreds of new cases being reported each week, CDC officials are intensifying their recommendation that people refrain from using e-cigarettes or vaping, particularly products containing THC, and especially THC products bought on the street.

“We haven’t seen a measurable drop-off in the occurrence of new cases,” she said. “We don’t feel individuals have changed their behaviors or the products are gone. … I cannot stress enough the seriousness of these lung injuries.”

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The CDC is working with the Food and Drug Administration and the Drug Enforcement Administration to find the sources and causes of the outbreak and remove them from circulation. But Schuchat said that is likely to be difficult and time-consuming.

“I’m not optimistic that tomorrow, we’ll be able to pull all the risky stuff off the market,” she said.

Officials have said they suspect the cause of the lung disease to be some kind of chemical exposure but have not identified a single e-cigarette or vaping product, brand or specific substance that has been definitively linked to the growing national outbreak. Among 578 patients with information on substances used in e-cigarette, or vaping, products in the three months before they had symptoms, about 78 percent said they used THC-containing products. Officials have said those products appear to play an important role in the outbreak.

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The mystery over the exact cause deepened this week with a letter published Wednesday in the New England Journal of Medicine (NEJM) by pathologists from the Mayo Clinic who studied lung biopsies from 17 patients in the outbreak. Most had vaped marijuana or cannabis oils. Researchers found none of the patients had evidence of lipoid pneumonia, a rare form of pneumonia typically associated with elderly people accidentally inhaling oils into their lungs.

Those findings were in conflict with a study of five patients in North Carolina, published last month by the Centers for Disease Control and Prevention. In the North Carolina patients, clinicians found immune cells filled with oil and diagnosed the patients with lipoid pneumonia. Clinicians at the University of Utah also found the same type of cells, known as lipid-laden macrophages, in the lungs of six patients. Their findings were also published last month in a letter in the NEJM. The Utah clinicians suggested that the presence of these same lipid-containing immune cells could be a marker for vaping-related lung injury.

Kevin Davidson, a pulmonologist at WakeMed Health & Hospitals in Raleigh, N.C., and the senior author of the CDC study on lipoid pneumonia, said the discrepancy could be related to the way the most recent biopsies were processed. The routine way of processing tissues can remove lipids, and when that happens, it masks the characteristics of lipoid pneumonia. “You won’t be able to see the lipid” in the immune cells, he said.

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The need for special handling of biopsies was raised during a call between federal health officials and state officials and clinicians in late August, Davidson said. At that time, a pathologist from the FDA mentioned the need for special processing. The CDC’s website also states that if a lung biopsy is obtained, the routine pathology processing, which includes formalin-fixation and paraffin-embedding, “can remove lipids.”

One particular oil is a key focus of investigators: vitamin E oil, also known as vitamin E acetate. Experts in the legal marijuana industry have said it has been used on the marijuana black market to stretch THC oil that is used to fill vape cartridges. It is colorless and odorless, has similar viscosity to THC oil, and is much cheaper.

Vitamin E acetate, which is sold legally, is commonly used as a nutritional supplement and in skin-care products. It’s not harmful when ingested or applied to the skin. But health officials have warned it could be hazardous when inhaled, potentially causing the sorts of symptoms many patients have reported: cough, shortness of breath and chest pain.

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Vitamin E acetate has been found in THC products taken from sickened patients and tested by state labs and the FDA’s forensic lab, officials have said. Once the oil is heated hot enough to vaporize, it can potentially decompose. In addition, when that vapor cools down in the lungs, it returns to its original state, coating the inside of the lungs with oil droplets. Experts don’t know whether the chemical itself or its byproducts could be toxic.

The FDA has tested over 440 samples from 18 states. But scientists have not been able to collect enough material from every sample for testing, said Judy McMeekin, deputy associate commissioner for regulatory affairs. Preliminary tests showed some products contained THC at concentrations ranging from 14 to 76 percent, which would suggest that other substances were added to the product. Some samples also showed THC and vitamin E acetate, with the vitamin E oil in concentrations from 31 to 88 percent, she said.