E-cigarettes may make lungs vulnerable to infection

17th January 2017

“Vaping may not be as safe as smokers think, research suggests,” The Guardian reports. New research found that mice exposed to e-cigarette vapours comparable to a typical human level experienced mild lung damage and a reduced immune response to infection.

This may be due to the fact that the vapour produced by e-cigarettes contains free radicals (atoms and molecules that are toxic to cells).

Mice exposed to e-cigarettes daily for two weeks had increased levels of macrophages in their lungs. Macrophages are a type of white blood cell that remove damaged and dead cells, and are evidence of cell damage. These mice also had a worse response when infected with either a bacteria that causes pneumonia or the flu virus.

E-cigarettes vapour contains 1% of the amount of free radicals that are produced from normal cigarettes, so it is not clear from this study what effect this would have for humans.

Are e-cigarettes safer than normal cigarettes? Almost certainly. Are they 100% safe? Probably not.

If you are planning to quit smoking, especially if you have a lung condition such as chronic obstructive pulmonary disease, other types of nicotine replacement therapies (NRTs), such as patches, may be a safer option.

Where did the story come from?

The study was carried out by researchers from John Hopkins University in Maryland, the University of Tennessee Health Science Center and Louisiana State University. It was funded by the Flight Attendant Medical Research Institute and the US National Institutes of Health.

The study was published in the peer-reviewed medical journal PLOS ONE on an open-access basis, so the article is free to read online here.

The study was accurately reported in the UK media.

What kind of research was this?

This was an animal study looking at the effects of e-cigarettes on the immune system. The authors say that many people perceive e-cigarettes to be a healthy alternative to cigarettes. However, there have only been limited studies in humans and animals on their safety.

They report that on current evidence, the US Food and Drug Administration (FDA), the World Health Organization (WHO) and the European Respiratory Society do not consider them a safe alternative. This study in mice was designed to look specifically at the effect of e-cigarettes on the immune system and their ability to fight a bacterial and viral infection of the lungs after exposure to e-cigarette vapour.

What did the research involve?

Eight-week-old mice were randomly assigned to be either exposed to e-cigarettes or normal air, and then their response to viral and bacterial infections was compared.

The mice in the e-cigarette group were put in a chamber in which a smoke machine delivered the equivalent of a two-second puff of menthol e-cigarettes (1.8% nicotine) every 10 seconds for one-and-a-half hours. This occurred twice per day for two weeks. The researchers stated that this level of exposure was comparable to the exposure you would expect to see in an “average” e-cig user.

The vapour was analysed to see if it contained free radicals, which are toxic to cells. A subset group were exposed in the same way to traditional flavoured e-cigarettes, also containing 1.8% nicotine. This level of vapour exposure gave them an average blood level of cotinine, a breakdown product of nicotine, of 267ng/ml. In humans who smoke cigarettes and e-cigarettes, the level is usually between 200ng/ml and 800ng/ml, so this was fairly low exposure.

At the end of the last exposure, both groups of mice were infected via the nose with either a bacterial infection of Streptococcal pneumonia or the viral infection influenza. Macrophages from the lungs of some mice from each group were grown in dishes and also exposed to these infections, to study their response.

The researchers then compared the level of infection and the immune response in mice exposed and not exposed to e-cigarettes.

What were the basic results?

The e-cigarette vapour contained free radicals. The level was just less than 1% of that found in cigarette smoke.

Compared to the mice exposed to air, lungs from mice exposed to e-cigarettes had:

  • increased levels of oxidative stress (evidence of damage from free radicals)
  • a 58% increase in the number of macrophages, indicating that they needed to remove damaged cells

After infection with Streptococcal pneumonia:

  • mice exposed to e-cigarettes had higher levels of the bacteria in their lungs
  • the macrophages from mice exposed to both types of e-cigarettes were less able to deal with the infection when grown in dishes, which may be why there was a higher level of bacteria in the lungs

After infection with influenza, mice exposed to e-cigarettes:

  • had higher levels of the virus four days after the infection
  • lost more weight 10 to 12 days after infection, which was taken as an indicator of more severe illness
  • two out of the 10 mice died, compared to none in the air-exposed group

After infection with a higher dose of influenza, 60% of mice exposed to e-cigarettes died, compared to 30% in the air-exposed group.

How did the researchers interpret the results?

The researchers concluded that, “E-cig exposure is not a safe alternative to cigarette smoking”. They say that the exposure caused “airway inflammation, oxidative stress, and impaired anti-bacterial and anti-viral responses that include increased bacterial burden and viral titers in the lungs, impaired bacterial phagocytosis [removal of bacteria by immune cells], and increased virus-induced morbidity and mortality”.


This animal study has shown that the vapour from e-cigarettes contains free radicals, which are toxic to cells and are linked to cancer. Exposure to e-cigarettes caused inflammation in the lungs of mice, with increased numbers of macrophages, which mop up damaged and dead cells. Mice exposed to e-cigarette vapour also had a reduced ability to fight both bacterial and viral infections.

Taken together, this is convincing evidence suggesting that e-cigarettes are not harmless. However, the effect of e-cigarettes was not compared to normal cigarettes in this study, so it is not clear how much safer they might be. The study was also not conducted in humans. An animal study such as this can give us a good idea of the effects that a chemical could have in humans. However, mice and humans do not have identical biology, so we cannot be certain that the effects would be identical.

The authors worryingly report that in the US, despite e-cigarettes being marketed as an aid to help people stop smoking, they are gaining popularity among people who have never smoked. This could soon be the case in the UK. Studies such as this show that they could still be bad for your health, and contribute to the growing body of research investigating e-cigarettes, both in terms of their effectiveness as aids to stop smoking, and their possible health effects.

There are many other ways to help you stop smoking that do not potentially expose the lungs to harm (though they may still have side effects). These include nicotine patches, gum and inhalers, as well as medication designed to reduce cravings for cigarettes, such as Zyban (bupropion). Read more about stop smoking treatments, as well as the NHS Stop Smoking Services that are available.



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