Authorities in the US have added to their tally of vaping-related illnesses.

Health authorities say at least 19 people in the US have died from illnesses linked to e-cigarette use.

There appears to be an outbreak of disease occurring, but no cause has been discovered yet.

People in the US have access to an expanded range of vaping fluids due to the legality of cannabis-derived THC and CBD liquids in some areas.

Vaping advocates say the illnesses are likely linked to sub-standard manufacture of cannabis-derived liquids.

Clinicians in North Carolina have pointed to the inhalation of fatty substances (lipids) from aerosolised oils as a cause, but experts at the Mayo Clinic say patients' lungs had been exposed to other noxious fumes.

The Centres for Disease Control and Prevention (CDC) says 19 deaths in 15 states have been positively linked to vaping.

Only one case of lung injury has been reported outside the US, when Canadian authorities reported a youth had been hospitalised.

The lack of cases outside the US makes the situation even more mysterious.

Despite the lack of concrete findings and the low number of fatalities, the US Government has announced it will ban flavoured e-cigarette products in coming months.

International experts say the diseases appear to be coming from very specific vaping liquids or additives, and may not be linked to vaping generally.

“As in all disease outbreaks, it takes time to narrow down on the underlying cause: and in this example it is clearly something to do with vaping, something that often - but not exclusively - occurs in people who vape THC or other oils, and something that isn’t occurring in countries outside the USA,” says Professor John Britton from the University of Nottingham.

“So it is something in US vape fluids, or something about the particular e-cigarettes used by those affected, but remains something separate from vaping more generally and particularly from vaping nicotine.”

Professor Linda Bauld from the University of Edinburgh says the cases so far “were not chronic adverse effects - i.e. those that have built up over a long period and cause disease - but instead evidence of an outbreak that is similar to poisoning.”

“This provides further evidence that it is extremely unlikely, if not impossible, that flavoured nicotine e-liquids of the type that have been used by millions of people around the world for up to a decade (including in the UK) are causing these injuries. Instead contaminants look like they are to blame.

“Most of the evidence points to adulterants in cannabis vaping but other products may be involved.

“Recent bans in the USA of all flavoured vaping products are not going to prevent further cases like these if the culprit is contaminated products bought on the black market.

“Indeed bans may make the problem worse by restricting access and driving people to illicit sources.”