Vaping has sent nearly 100 people, mostly teens and young adults, to the hospital with lung illnesses in the last couple of weeks. 

The Centers for Disease Control and Prevention (CDC) is investigating 94 possible cases of "severe lung illness associated with vaping," which have been reported in 14 states as of Aug 17, according to a statement from the agency. It's unclear what's causing the illnesses or even if there is a link between patients' symptoms and the kinds of products they used.

But the patients have reported vaping various substances, including nicotine and marijuana. Some of the patients said they bought their e-cigarette products on the street, according to a previous Live Science report. What could be harming them?

If that's the case, this chemically induced lung injury could lead to several severe lung conditions. One condition, known as  acute respiratory distress syndrome (ARDS), has been associated with the inhalation of toxic fumes. Another condition is chemical pneumonitis, or inflammation of the lungs that is caused by inhaling irritants, Siegel told Live Science. 

But "it is very unlikely that this is being caused purely by nicotine-containing e-liquids," he added. Rather, the reaction is probably caused by a "contaminant that is present in certain formulations of cannabis products" sold for vaping, possibly on the streets, he said. 

For example, pneumonitis has been linked to the practice of "dabbing," or inhalation of butane hash oil, a concentrated form of marijuana that contains high levels of the drug's active compound, THC. Heating butane hash oil to high temperatures may result in the formation of chemicals that are toxic to the lungs, according to a report published in the journal Respiratory Medicine Case Reports last January. 

But other experts say that noncannabis e-cigarettes, those that contain nicotine instead of THC, could also cause such reactions. A group of researchers recently found that nicotine releases potentially harmful enzymes — molecules that have a role in breaking down proteins in the body — into the lungs. Those researchers published their results on Aug. 7 in the American Journal of Respiratory and Critical Care Medicine.

These enzymes are known to cause lung damage, said Robert Tarran, the senior author of that study and a professor at the University of North Carolina School of Medicine. At least in part, "I think that … the teenagers are taking in very high amounts of nicotine that is then affecting the immune cells in the lung," he said.

Previous research into e-cigarettes has found multiple chemicals in the vapor that can damage cells and trigger inflammation in the lungs and in the bloodstream, said Dr. Laura Crotty Alexander, an associate professor of medicine at the University of California, San Diego. "It could be a particular chemical in the e-cigarette vapor or multiple different chemicals leading to acute lung injury," Crotty said.

An acute lung injury means that the cells of the lungs have been damaged and the immune system has responded to try and stop the damage and heal the lungs. Because of inflammation and cell damage, gas exchange in the body is "impaired" such that patients are short of breath, deprived of oxygen and often coughing, Alexander said. This could be caused by store-bought or street-bought e-cigarettes, she said.

A study published today (Aug. 20) found that right after vaping a non-nicotine-containing e-cigarette, participants had much less oxygen flowing through their blood. "We could only conjecture at this point" about whether this finding could help explain the mysterious vaping-related illness, said, Felix Wehrli, senior author of the study and a professor of radiologic science and biophysics at the University of Pennsylvania Perelman School of Medicine.

One possibility is that these patients' lungs could not take up oxygen properly, he said. If that was the case, the lungs might have resorted to taking up more oxygen from the blood. This phenomenon would also explain why Wehrli and colleagues saw a reduction of oxygen flowing through blood vessels after vaping in their study, he said.

In part because e-cigarettes are often thought of as being safer than regular cigarettes, since they don't contain tobacco, there has been a surge in e-cigarette use in recent years, especially among teens.

Most e-cigarette liquids contain three major components: propylene glycol, glycerol and nicotine, Alexander said. But when you heat these components into a vapor, you create completely different chemicals, some of which are toxic, such as formaldehyde and acrolein. "But because there's new e-cigarettes and e-liquids coming on market at least every month, it's impossible to keep up with all the different chemicals that are being sold and breathed in," she said. 

What's more, the Food and Drug Administration isn't actively regulating these devices, she said. The administration doesn't require companies to label products with all the ingredients, and the ones that are labeled are often wrong, she said. As for the e-cigarettes sold on the street, what's in them is even more unclear, she said. So her advice to teenagers is "if somebody hands you one of these devices at a party, do not use it," she said. "Because you have no idea what's in it."

In any case, according to the CDC statement, there isn't conclusive evidence that an infectious disease is causing the illnesses. The investigation is ongoing, and the agency said it will share more information as it becomes available.