Nationally, there's so much concern about the rise of vaping-related lung illness that the Centers for Disease Control and Prevention on Monday activated its round-the-clock emergency operations center to handle the crisis.

More than 300 cases have been reported nationwide, amid the exploding popularity of e-cigarettes, which heat a liquid to produce an aerosol that's then inhaled.

Here in Massachusetts, public health officials have received 38 preliminary reports of cases that could fit the CDC criteria for the newly identified vaping illness.

The state Department of Public Health says none of these cases is classified as probable or confirmed at this point. But it's likely most will be, says Dr. Jonathan Winickoff, a pediatrician and researcher at Massachusetts General Hospital — because there is no lab test for the illness, only a description.

The situation is developing, but here are some basics:

What Are We Talking About Here?

According to the CDC — which has a dedicated web page on the topic here — at least 36 states have reported more than 300 cases of a severe lung disease linked to vaping, or e-cigarettes. At latest count, seven people had died nationwide.

"We have seen a number of cases at Children's admitted to the ICU with respiratory failure," says Dr. Eleanor Muise, a lung specialist trainee at Boston Children's Hospital.

Specialists at Children's have also seen less severe cases, which can include more chronic symptoms such as persistent coughs.

Dr. Muise says in the severe cases, it's as if the patients' lungs are having a massive allergic reaction to an inhaled product.

How Is Vaping-Related Illness Defined?

It's severe lung disease that arises in patients with a history of e-cigarette use and no sign of an infection.

As described in a series of cases, the CDC says, patients' lungs looked abnormal when scanned. Early symptoms included a cough, chest pain, shortness of breath, fever, nausea and abdominal pain. Steroids tended to help patients; antibiotics did not. Some needed breathing support ranging from oxygen to being put on a ventilator.

Dr. Winickoff says from his perspective as a pediatrician, an adolescent comes in with a cough, and the usual attempts to treat them for an infection don't work.

"What we're hearing is that these kids are air-hungry," he says. "They feel like they can't breathe, they can't get enough oxygen into their bodies. In some cases it's because there's so much swelling and fluid in the lungs that the oxygen in the air doesn't get into the bloodstream. And it almost feels like they're drowning."

He says it's important for parents and doctors to be vigilant about asking kids if they use vaping products, so their oxygen levels can be monitored if necessary — and they can be treated with steroids before their condition gets worse.

What Causes Vaping-Related Lung Illness?

The CDC says it's not clear: "No consistent e-cigarette product, substance, or additive has been identified in all cases, nor has any one product or substance been conclusively linked to pulmonary disease in patients." Among substances vaped are cannabis and nicotine.

Dr. Sucharita Kher, the medical director of the Tufts Medical Center adult pulmonary clinic, says that so little is known about vaping-related lung illness that more is being learned every day.

"I think it's not a disease per se," she says, "it's more of a syndrome," a group of symptoms.

It's not certain whether it's caused by the act of inhaling the vapor, or chemicals in it, or chemical changes from heating the liquid — often called e-juice — used in an e-cigarette. Or it may be related to the doses the vaper gets, or some additive, she says.

"So there's not one thing," Dr. Kher says. "It could be any of these that could be affecting it, and I think that's why this case-finding process that the CDC and Mass. DPH have adopted is just so important, so they can look at every case and see what happened in each case."

Theories abound, including the possibility that Vitamin E oil in cannabis could be a culprit. Most patients — but not all — had vaped cannabis products.

But Dr. Alicia Casey, a pediatric pulmonologist at Children's, says many people "have the misperception that a lot of the acute lung injury cases that we’ve been seeing over the summer [come from] black market or doctored products — and that is not the case."

For now, the CDC recommends that "people consider not using e-cigarettes," and particularly stay away from street purchases.

And Dr. Casey says: "I'm a pediatric pulmonologist, and I care a lot about the lungs, and there isn't anything you can inhale in this way that would be safe."

Companies tell customers that vaping devices are safe, but they are not, she says. This summer's spate of lung injury and more long-term data beginning to accumulate show that "it absolutely is not safe," she says.

Now What?

Public health officials in Massachusetts and nationally are investigating and tracking the cases, and are requiring that health care staffers report all cases of vaping-related lung illnesses to them for the next 12 months.

When a possible case is identified, the CDC asks for extensive information, including the substance the patient had vaped, where it was bought and the device used.

Dr. Kher notes one immediate change in medical practice: It's long been routine to ask patients if they smoke, but it's now going to become much more routine to ask patients if they vape as well.

Who's At Risk?

Many high school students, among others. According to state data, about 20% say they've vaped in the past 30 days — six times higher than the adult rate. The illness can take days or weeks to develop, it seems, and may start slowly. The CDC encourages anyone who has symptoms to seek medical help.