Andy Ramkumar, who works at Gotham Vape in Queens, vapes at the store on September 17, 2019 in New York City. Spencer Platt | Getty Images News | Getty Images

Doctors researching the cause of a sudden respiratory illness that's killed at least 16 people in the U.S. since July say a mix of "toxic chemical fumes," not oils as previously expected, may be what's making patients sick, according to a new study. Researchers from the Mayo Clinic examined lung biopsies from 17 patients suspected of having a perplexing vaping illness that's sickened more than 805 people since April. Doctors have previously said it resembled a rare form of pneumonia caused by the accumulation of fatty substances known as lipids. However, none of the cases examined by the Mayo Clinic researchers showed any evidence of lipoid pneumonia, according to the study published Wednesday in the New England Journal of Medicine. It's the first formal study examining tissue samples of patients who have fallen ill from vaping.

"It seems to be some kind of direct chemical injury, similar to what one might see with exposures to toxic chemical fumes, poisonous gases and toxic agents," said Dr. Brandon Larsen, a surgical pathologist at Mayo Clinic Arizona and a lead author of the study. "Based on what we have seen in our study, we suspect that most cases involve chemical contaminants, toxic byproducts or other noxious agents within vape liquids." The pathologists said research on lung injuries linked to vaping is still in its early stages and that the findings should be interpreted with caution. The vaping illness outbreak has now spread to 46 states and one territory, according to the Centers for Disease Control and Prevention. Public health officials in 13 states have reported deaths. The CDC has dispatched more than 100 doctors and investigators to identify the specific cause of the deadly illness. Early symptoms include coughing, shortness of breath, fatigue, chest pains, nausea, vomiting and diarrhea. Most patients have reported a history of using e-cigarette products containing THC, according to the CDC. However, many patients have reported using THC and nicotine, the agency said. Some have reported the use of vaping products containing only nicotine. Of the 17 patients in the study, 71% had vaped with marijuana or cannabis oils. All of the patients had acute lung injury and two of the patients died, the researchers said.