An 18-year-old Pennsylvania woman was hospitalised with acute respiratory distress and respiratory failure just three weeks after she'd taken up vaping - which, according to her doctors, directly caused her condition.

It's the first reported case of vaping causing respiratory failure. According to the woman's doctors, it was caused by an allergic reaction to the chemicals in the vapour she was inhaling.

Vaping - wherein flavoured liquid, sometimes containing nicotine, is heated into a vapour and inhaled like smoke - has been promoted as a safer alternative to smoking and an effective quitting aid.

However, science is still out on the long-term effects of the practice, and how safe it really is.

Vaping-related cases of pneumonia and 'hypersensitivity pneumonitis' - an allergic reaction to inhaled substances - are rare, but have been reported in the literature before.

The teen in question went to the hospital after experiencing a cough, shortness of breath and pain around her lungs for two days. Her history included mild asthma and an allergy to Brazil nuts, neither of which were thought to have caused her current condition.

She had no runny nose, no fever, and no heart abnormalities. The doctors admitted her to the intensive care unit and prescribed antibiotics - but her condition rapidly deteriorated. She entered respiratory failure, and needed to be intubated.

"She was unable to get enough oxygen into her blood from her lungs and required a mechanical ventilator to breathe for her until her lungs recovered," Daniel Weiner, one of the doctors who worked on her case, told CNN.

Her condition was so severe that tubes had to be inserted into her lungs via her chest to drain off the fluid that was accumulating.

After tests ruled out an infection, she was treated with methylprednisolone, a steroid used to suppress the immune system in the event of a severe allergic reaction.

Typically, hypersensitivity pneumonitis is associated with microbes, such as can be found on mouldy grains or hay, or in bird droppings. When these microbes get into the lungs, it can cause an immune response to flare up.

Because immune cells migrate from your blood to the affected area when you experience an infection or allergic reaction, the blood vessel barrier can become "leaky". This can cause fluid to build up.

Although it's just one case, the study highlights how little we know about the dangers of vaping, the researchers said. Particularly in the younger population, amongst whom e-cigarette use is growing.

Indeed, recent studies have found some areas of concern. One found that some of the flavourings used to sweeten e-liquids could be potentially damaging to the immune system. Another found that heavy metals from the heating coil are making their way into the inhaled vapour (and the vapers' lungs).

And while Public Health England has reported that vaping is a safe and effective way to quit smoking, the US National Institute of Health has found that it's possibly not so effective - and the jury is still out on whether it's safe.

Meanwhile, however, this particular patient is all better. Five days after she first came to the hospital, she was able to breathe on her own, and was eventually discharged from hospital.

The doctors wrote up her case report and published it in the journal Pediatrics.