The number of vaping-related deaths has risen to 18 in the United States, with cases of illnesses linked to the practice topping 1,000 - health officials have said.
More than a third of those affected are under the age of 21, but those that have died have been older adults who apparently had more difficulty recovering.
An e-cigarette is a device that allows people to inhale nicotine in a vapour rather than smoke.
The devices do not burn tobacco and do not produce tar or carbon monoxide, two of the most damaging elements in tobacco smoke.
They work by heating a liquid that typically contains nicotine, propylene glycol and/or vegetable glycerine, and flavourings.
Doctors say the illnesses, which first appeared in March, resemble an inhalation injury.
Symptoms include severe, shortness of breath, fatigue, and chest pain, with most who fell ill saying they vaped products containing THC, the marijuana ingredient that causes a high.
The Centres for Disease Control and Prevention said 1,080 confirmed and probable cases have been reported in 48 states and one US territory.
Dr Anne Schuchat from the CDC said: "I cannot stress enough the seriousness of these injuries.
"This is a critical issue. We need to take steps to prevent additional cases."
It has been suggested patients' lungs are being clogged and inflamed by oils from vaping liquids, but a recent report in the New England Journal of Medicine pointed to the kind of chemical burns that might come from poisonous gases.
"There may be a lot of different nasty things in e-cigarette or vaping products, and they may cause different harms in the lung," Dr Schuchat said.
"We hope over the months ahead that we'll learn more about the spectrum of lung conditions that these exposures are having."
Around 275 cases have been added to the numbers each week, and about half of the newest batch were people hospitalised in the last two weeks.
The Food and Drug Administration is analysing products from 18 states, but neither that agency nor the CDC has pinpointed an electronic cigarette, vaping device, liquid or ingredient as the root cause.
The investigation has increasingly focused on THC vaping products and those with non-tobacco flavourings.
But until a cause is found, the CDC continues to advise Americans to refrain from using any vaping products.
Meanwhile, Public Health England (PHE) has said banning flavoured vaping products in the UK would drive people back to smoking.
Martin Dockrell, head of tobacco control at PHE, said: "E-cigarette flavours are an important advantage that vapes have over smoking and play an important part in encouraging smokers to switch."