The patients are in different parts of the country and have used different brands of products in recent weeks.
The chemical is an oil derived from vitamin E.
Investigators at the US Food and Drug Administration found the oil in cannabis products in samples collected from patients who fell ill across the United States.
FDA officials shared that information with state health officials during a telephone briefing this week, according to several officials who took part in the call.
That same chemical was also found in nearly all cannabis samples from patients who fell ill in New York in recent weeks, a state health department spokeswoman said.
Vitamin E is found naturally in certain foods, such as canola oil, olive oil and almonds.
The oil derived from the vitamin, known as vitamin E acetate, is commonly available as a nutritional supplement and is used in topical skin treatments. It is not known to cause harm when ingested as a vitamin supplement or applied to the skin.
Its name sounds harmless, experts said, but its molecular structure could make it hazardous when inhaled.
Its oil-like properties could be associated with the kinds of respiratory symptoms that many patients have reported: cough, shortness of breath and chest pain.
"We knew from earlier testing by New York that they had found vitamin E acetate, but to have FDA talk about it from their overall testing plan, that was the most remarkable thing that we heard," said one official who took part in the briefing but was not authorised to speak publicly.
The FDA also told state officials Wednesday that its lab tests found nothing unusual in nicotine products that had been collected from sick patients, according to another person who took part in the call.
State health departments are reporting new cases weekly. As of 27 August there were 215 possible cases reported by 25 states. Additional reports of lung illnesses are under investigation, according to the Centres for Disease Control and Prevention, which is leading the investigation.
On Wednesday, Oregon health authorities said a middle-aged adult who died in late July of a severe respiratory illness had used an e-cigarette containing marijuana oil purchased from a legal dispensary.
It is the second death linked to vaping nationwide and the first to be linked to a product bought at a store. Illinois officials reported the first death last week, but did not specify what kind of product was used.
State and federal health authorities have said they are focusing on the role of contaminants or counterfeit substances as a likely cause of vaping-related lung illnesses. Many patients have told officials and clinicians that they bought cannabis products off the street.
Many of those who have fallen ill say they have vaped products containing marijuana, but some also used traditional nicotine e-cigarettes. Authorities said they are not ruling out adulterants in nicotine vaping products.
Although the discovery of a common chemical in lab tests offers a potential lead, officials cautioned that they are a long way from understanding what exactly is making so many people sick.
The FDA analysed 12 viable nicotine samples and 18 viable THC products, state officials said. Vitamin E acetate was found in 10 of the 18 THC products.
"This was the only thing that seemed to show up in 10 of the 18 cannabis products," said one state official.
The results seem to confirm findings from New York State. Late last week, its lab found "very high levels of vitamin E acetate in nearly all" its cannabis samples tested. At least one vape product containing vitamin E acetate has been linked to each patient who submitted a product for testing, the department said.
"Vitamin E acetate is not an approved additive for New York State Medical Marijuana Programme-authorised vape samples and was not seen in the nicotine-based products that were tested. As a result, vitamin E acetate is now a key focus" of New York's investigation, New York State Health Commissioner Howard Zucker said in a statement on Thursday.
As of Thursday, New York had received 34 reports from doctors of severe pulmonary illness among patients who ranged in age from 15 to 46 who were using at least one cannabis-containing vape product before becoming sick.
The second report of a death has emphasised the danger of this lung disease. "It was surprising that the patient suddenly appeared without any other underlying health conditions and became ill enough to die from this syndrome," said Ann Thomas, a physician with the Oregon Health Authority.
Vaping refers to the increasingly popular practice of inhaling vapour from an e-cigarette device, which frequently involves heating a liquid that can contain nicotine, marijuana or other drugs.
Mystery lung illness linked to vaping reported in nearly 100 people
Vitamin E acetate is basically grease, said Michelle Francl, a chemistry professor at Bryn Mawr College.
Its molecular structure means that "you have to heat it up pretty hot" for it to vaporise.
Its boiling point is 363 degrees Fahrenheit, which is well above the 212 degrees F boiling point for water, and nearly four times higher than normal human body temperature.
Once the oil is heated hot enough to vaporise, it can potentially decompose, and "now you're breathing in who-knows-what," Ms Francl said.
When that vapour cools down in the lungs, it returns to its original state at that temperature and pressure, she said, which means "it has now coated the inside of your lungs with that oil".
In Utah, clinicians have treated several patients with acute lung injuries who were diagnosed with a rare condition known as lipoid pneumonia, with symptoms including chest pain and difficulty breathing.
Those patients had abnormal immune cells filled with lipids, doctors said.
Unlike the human digestive tract, which can break down and get rid of foreign substances, the lungs aren't designed to handle anything except gases, experts said.
Laura Crotty Alexander, a lung inflammation and e-cigarette researcher at the University of California in San Diego School of Medicine, said it is not clear whether the chemical itself or its by-products could be toxic.
"We haven't looked at the toxicity of vitamin E in the lungs," she said. "The lungs are designed to exchange gas molecules; they're not designed to be exposed to other chemicals."
When the lung cells die, that often provokes an inflammatory response and "other cells need to come in and clean up the cell debris", Ms Alexander said. But the lungs are very delicate.
When extra cells enter, "they get in the way of gas exchange", which makes it more difficult for oxygen to get into a person's bloodstream.
The inflammation can cause liquid to accumulate in the lungs, making it difficult for someone to breathe, Ms Alexander explained.
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