They've been touted as a slightly healthier alternative to puffing on tobacco.

Only a few weeks ago, UK health bodies suggested electronic cigarettes should be in hospital shops to encourage smokers to wean themselves off their habit.

But a new study has discovered toxic levels of heavy metals in e-cigarette aerosols, once again raising doubts over just how safe vaping really is.

Researchers from Johns Hopkins Bloomberg School of Public Health analysed e-cigarette vaporisers borrowed from 56 daily vapers, and found many were being exposed to potentially toxic levels of chromium, nickel, and lead.

Their research comes on the back of a preliminary study they'd conducted in 2016, which had detected elevated levels of nickel and chromium in the urine and saliva of e-cigarette users.

High concentrations of these heavy metals have been linked to a variety of health conditions in the past, including cardiovascular disease, brain damage, and a variety of cancers.

While the studies don't go as far as to connect vaping with any of these health problems, it's no great leap to infer there's an increase in risk.

Most e-cigarette vaporising devices work by using a battery-powered heating element to turn a liquid solution into an aerosol.

In this most recent study, the researchers went straight to the contents of the liquid surrounding the heating coil, as well as the vaping liquid in the dispensary and the aerosols the device produced.

They tested for a total of 15 metals this time. While the concentrations of metals in the dispensary were hardly worth worrying about, the aerosols and liquid surrounding the heating element were a whole other matter.

"We don't know yet whether metals are chemically leaching from the coil or vaporising when it's heated," says the study's senior author, Ana María Rule.

Just under half of the devices analysed produced aerosols with lead levels that exceed Environmental Protection Agency limits. The average levels of nickel, chromium, and manganese were also in an unsafe margin.

"These were median levels only," says Rule.

"The actual levels of these metals varied greatly from sample to sample, and often were much higher than safe limits."

That means some e-cig devices might be fine. Others could be making your vape far more toxic than you'd like.

Humans have been inhaling tobacco smoke for centuries, but converting nicotine into a breathable mist is a fairly new habit - and the US Food and Drug Administration (FDA) is still making up its mind on what to do about it.

In 2016, it aimed to regulate the devices in the same way it regulated any other tobacco product. Last year it decided vaping could be a way to step nicotine addicts down into a slightly-less-unhealthy habit.

Rule is adamant that results like these need to be taken into account in regulations.

"It's important for the FDA, the e-cigarette companies and vapers themselves to know that these heating coils, as currently made, seem to be leaking toxic metals – which then get into the aerosols that vapers inhale."

The study comes on the back of findings that flavourings added to vaping liquids can also come with added health risks.

None of this should be taken to mean cigarettes are a safer choice - traditional smoking will still give you a lung full of heavy metals, such as cadmium and lead.

But it is a good moment to remember safer doesn't mean safe. And there's still plenty of debate over exactly what risks are involved with vaping.

This research was published in Environmental Health Perspectives.