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E-cigarettes linked to lung cancer in mice

Tuesday 8 October 2019

"E-cigarette smoke could cause lung cancer – despite being tobacco-free, experts warn," reports The Sun. However, The Sun's headline does not mention that this was based on a study using mice and the findings cannot be automatically applied to humans.

The number of people using e-cigarettes, or vaping, has grown rapidly over the past decade and they have helped many people to give up smoking cigarettes.

However, as e-cigarettes have only been available for a relatively short time, there is still uncertainty over their potential health effects, particularly with long-term use.

This laboratory study involved 85 mice who were divided into 3 groups. Over the course of 1 year, each group was exposed to either:

  • e-cigarette smoke (including nicotine)
  • e-cigarette vapour (without nicotine)
  • filtered normal air

The researchers found that 9 out of 40 mice exposed to e-cigarette smoke developed lung tumours. Only 1 of the mice breathing normal air developed a lung tumour during the study, and none of the mice breathing nicotine-free vapour developed a lung tumour.

More than half of the mice exposed to e-cigarette smoke also showed signs of potentially cancerous changes in their bladder.

At the moment, it's not possible to say that e-cigarettes definitely cause lung cancer or changes in the bladder. This is because humans are not biologically identical to mice. We do not know if the findings represent what would happen in humans, or whether these laboratory exposures are the same as a person vaping.

We need to continue to monitor people using e-cigarettes and gather evidence on the potential health effects of vaping.

Advice from the National Institute for Health and Care Excellence (NICE) on e-cigarettes states: "the evidence suggests that e-cigarettes are substantially less harmful to health than smoking but are not risk-free. The evidence in this area is still developing, including evidence on the long-term health impact".

Where did the story come from?

This study was conducted by researchers from New York University School of Medicine and funded by the US National Institutes of Health. The study was published in the peer-reviewed scientific journal PNAS, and is free to read online.

As mentioned, The Sun's headline was potentially misleading as it did not mention that the study only involved a small sample of mice. The newspaper's full reporting was accurate though. The Mail Online and Daily Mirror did make this point clear in headlines.

What kind of research was this?

This was a study of mice looking to see whether exposing mice to e-cigarette smoke or vapour over a long period of time led to tumour development.

The researchers had conducted a previous study where they exposed mice to e-cigarette smoke containing nicotine over 12 weeks and found this led to signs of abnormal changes in the lungs and bladders of mice. They wanted to follow this up and see what happened after a much longer exposure.

Animal studies can give an indication of the potential biological and health effects from an exposure. But we cannot be sure that the laboratory exposure scenario or disease processes directly represent what would be seen in humans.

What did the research involve?

The study involved 85 male mice (6 to 8 weeks old) who were randomised into 3 groups. The first group, containing 40 mice, was exposed to a steady concentration of e-cigarette smoke. This is made up of nicotine dissolved in a "carrier chemical" (isopropylene glycol and vegetable glycerine).

The second group, containing 18 mice, was exposed to a steady concentration of vapour containing the carrier chemical only and no nicotine. Both the 1st and 2nd groups were kept in separate exposure chambers and were exposed for 4 hours a day, 5 days a week, for 54 weeks.

The third group, containing 18 mice, was kept in the animal room of the laboratory and exposed to filtered air. The researchers observed what happened to the mice over the course of the year. Any that died during that period were examined in the laboratory. At the end of the study, all mice were examined to look for signs of tumour development in any organs.

What were the basic results?

When the organs of each mouse were examined at the end of the study, lung tumours were found in 9 of the 40 mice exposed to e-cigarette smoke (22.5%). All were adenocarcinomas, the most common type of lung cancer in humans.

None of the mice exposed to the carrier vapour alone developed lung tumours, while 1 mouse exposed to filtered air had a lung tumour.

There were no bladder tumours in any of the mice. However, the researchers observed there was an overgrowth of cells in the bladder lining of 23 of the 40 mice exposed to e-cigarette smoke (57.5%) compared with only 1 of the mice exposed to the carrier vapour and none who were exposed to filtered air only.

How did the researchers interpret the results?

The researchers say: "These findings raise the possibility that ECS [e-cigarette smoke] is a lung and bladder carcinogen in addition to nicotine."

They also cautioned that: "While it is well established that tobacco smoke poses a huge threat to human health, whether ECS poses any threat to humans is not yet known and warrants careful investigation."

Conclusion

The use of e-cigarettes has become widespread. Evidence has shown they can decrease cigarette smoking rates, and they're understood to be considerably less harmful than smoking cigarettes.

But we are not certain that e-cigarettes are completely "safe". The difficulty is that they have not been on the market for long enough for us to build sufficient and clear evidence on their long-term safety.

This animal study supports the researchers' previous laboratory findings that the combination of nicotine and carrier vapour in e-cigarette smoke seems to cause cancerous cell changes in the lungs and bladders of mice. The study benefits from exposing mice to the smoke over a long period to allow for tumour development. It leads to the question of whether this could reflect – perhaps in fast-forward, considering the short lifespan of mice – what could be happening in humans.

However, this animal study alone cannot tell us whether e-cigarettes cause lung or bladder cancer in humans. We're not biologically identical to mice and the effects might not be the same.

We also do not know whether the ambient exposure concentrations used in this study are the same as someone vaping. If there is a harmful effect, we do not know whether it might affect only the person vaping or whether, like cigarette smoke, passive exposure to e-cigarette smoke could also be a risk to health.

At this stage we need to continue to gather evidence on the health effects of e-cigarettes in humans. It could be many years before we're able to understand any risks associated with long-term use of e-cigarettes.

One way to reduce any potential risk would be for people to only use e-cigarettes as a short-term measure to help them stop smoking, rather than using them long term as an alternative to cigarettes.

Find out more about using e-cigarettes to stop smoking.

Analysis by Bazian
Edited by NHS Website

Links to the headlines

E-cigarette smoke could cause lung cancer – despite being tobacco free, experts warn

The Sun, 7 October 2019

E-cigarette smoke caused lung cancer in mice

Mail Online, 7 October 2019

Vaping linked to cancer as one in four mice exposed to e-cigarette smoke develop disease

Mirror Online, 7 October 2019

Links to the science

Tang M-s, Wu X-R, Lee H-W et al.

Electronic-cigarette smoke induces lung adenocarcinoma and bladder urothelial hyperplasia in mice

The Proceedings of the National Academy of Sciences (PNAS). Published online 7 October 2019