Update 30/11/17: Diacetyl was banned in eliquids / ecigarettes in the UK in 2016 under the EU Tobacco Products Directive
"Smokers who use e-cigs 'are risking harm to their lungs'," the Daily Mail reports after US researchers discovered some brands contained diacetyl, a buttery flavouring linked to lung disease in people who worked in microwave popcorn factories.
Two other chemicals linked to lung damage were also found in the cigarette alternatives, calling their safety into question.
Diacetyl was detected in 39 of the 51 flavours tested, ranging from barely measurable levels to concentrations of 239 micrograms per e-cigarette.
Diacetyl, a safe food flavouring, has been used to give microwave popcorn its buttery taste. But it has also been implicated in the case of eight popcorn factory workers who developed a lung condition called severe bronchiolitis obliterans after breathing it in.
Nicknamed "popcorn lung", bronchiolitis obliterans causes scarring of the lungs and loss of function that can become so severe the only treatment option may be a lung transplant.
Much of the alarm caused by this study hangs on the strength of evidence from previous reports linking these chemicals to lung damage.
However, this study didn't look at this issue directly, so whether there is a link between e-cigarettes and "popcorn lung" is currently unknown.
More information is needed on the potential causal link between these chemicals and lung disease, particularly the doses at which any damage might occur.
If you're trying to quit smoking, other forms of nicotine replacement therapy, which are regulated, are available on prescription from your GP or pharmacists.
The study was carried out by researchers from the Harvard T. H. Chan School of Public Health, and was funded by the US National Institute of Environmental Health Sciences.
The media generally covered the story accurately. The Daily Mail, for example, gave some balance by stating that not everyone agreed with the warning.
The paper said experts writing in the British Medical Journal said, "Many of the conclusions were premature and based on weak evidence".
This laboratory study set out to learn whether three chemicals suspected of causing lung problems were present in flavoured e-cigarettes.
It may come as a surprise to many that we don't know exactly what is in e-cigarettes and the effect they have on health, especially given that there are more than 7,000 different "e-cigarette flavours" and the devices are used by millions.
E-cigarettes are currently unregulated, meaning there is no standardisation or medical control over what is in them, although this is set to change in 2016.
And because they haven't been around long, there is precious little research informing us about the effect they have on health. As such, there is a lot of debate about whether they are good or bad for health.
One side says e-cigarettes are a much safer alternative to regular tobacco smoking as they don't contain cancer-causing chemicals, just the additive substance nicotine. A recent evidence review by Public Health England concluded they are 95% safer than regular cigarettes.
The other side of the argument says the vapour may still damage lungs in as yet unknown ways, and the widespread uptake of vaping may normalise smoking again, leading to more people – especially young adults – taking up the tobacco version.
Marketing of sweet and fruit e-cig flavours to kids and young adults has also come under the spotlight, especially when they can choose between flavours like cherry crush and alien blood.
Irrespective of which side of the debate you're on, one thing is for sure: we lack information about what is in e-cigs and how they affect health. Most research has focused on the nicotine in the devices, rather than the chemical content beyond this.
This study wanted to find out whether three chemicals the researchers suggest are linked to lung damage – diacetyl, 2,3-pentanedione and acetoin – were present in widely available e-cigarettes.
The study reminds us that inhaling diacetyl was linked to a cluster of eight popcorn factory workers developing a serious lung condition called severe bronchiolitis obliterans.
This is an irreversible loss of lung function that can become so severe that the only treatment option may be a lung transplant. However, the evidence of this link is not under scrutiny here.
An investigation into the popcorn workers indicated diacetyl was the biggest chemical in the butter flavourings, but two other chemicals – 2,3-pentanedione and acetoin – were also present in the popcorn factory. And all three were potentially involved in lung damage.
The research, based in the US, selected 51 types of flavoured e-cigarettes sold by leading brands, targeting those with flavours deemed appealing to kids or young adults, such as fruit or cocktail flavours cherry crush or pina colada.
The e-cigarettes were hooked up to a lab-built smoking device that smoked the e-cigarettes to completion using 8-second draws with 15-30-second breaks between puffs (Occupational Safety and Health Administration Method 1012). The device had various filters attached, allowing levels of the three chemicals to be collected and analysed.
High- and low-flow rates were used in case this changed the proportion of chemicals coming out of the e-cigarette. The measures were used to calculate the total chemical mass emitted from the e-cigarette cartridge. Blank cartridges were used to act as controls and establish baseline measurements.
Several flavours were tested more than once and the results averaged – for example, testing two e-cigarette cartridges from the same pack.
At least one of the flavouring chemicals were detected in 47 of the 51 unique flavours tested (92%). This included several e-cigarette flavours that are not sweet or fruit flavoured, such as classic and menthol. Many flavours had more than one of the chemicals.
Diacetyl was detected in 39 of the 51 flavours tested (76.5%), ranging from barely measurable levels to concentrations of 239 micrograms per e-cigarette.
The other two chemicals of interest – 2,3-pentanedione and acetoin – were detected in 23 and 46 of the 51 flavours tested (50%) at concentrations up to 64 and 529 micrograms per e-cigarette respectively.
The Harvard researchers concluded: "Due to the associations between diacetyl, bronchiolitis obliterans and other severe respiratory diseases observed in workers, urgent action is recommended to further evaluate this potentially widespread exposure via flavoured e-cigarettes."
This study shows three chemicals reportedly linked to serious lung damage are present in many flavoured e-cigarettes in the US, raising concerns about their safety.
Diacetyl was banned in eliquids / ecigarettes in the UK in 2016 under the EU Tobacco Products Directive
However, this issue is not black and white. The researchers' concerns have been rightly raised, and their conclusion that urgent research needs to follow this study seems logical given the apparent lack of knowledge in this area.
Still, much of the potential health risk and alarm factor of this study hangs on the strength of previous reports suggesting these chemicals can cause lung damage.
But this particular study didn't address this directly. We don't yet know whether these chemicals, in their vaporised forms and inhaled at the levels typical in e-cigarettes, cause any lung damage.
This study simply looked at how often the three chemicals were detected in e-cigarettes, assuming a link with potential lung damage from other studies that were not appraised here.
What we really need is more information on the potential causal link between these chemicals and lung disease, particularly the doses at which any damage might occur, which may or may not be present in e-cigs.
Many e-cigarette users may be asking themselves whether they should stop vaping. This study only provides indirect evidence of a risk linking chemicals in e-cigarettes to lung disease, but, if this is true, the consequences could be serious.
At present, there are more questions than answers. There's little doubt not smoking tobacco or e-cigarettes is the best way to reduce your risk.
Nicotine replacement therapies, which are regulated as medical products, such as patches and gum, are designed to wean you off your nicotine addiction altogether, rather than replacing one source of nicotine with an (admittedly much safer) alternative.