An increase in mucin 5AC, a mucus secretion associated with chronic bronchitis and asthma, was also found in both e-cigarette and cigarette users.
Dr Mehmet Kesimer, who lead the study said: "There is confusion about whether e-cigarettes are “safer” than cigarettes because the potential adverse effects of e-cigarettes are only beginning to be studied.
"Our results suggest that e-cigarettes might be just as bad as cigarettes."
While he admitted that "comparing the harm of e-cigarettes with cigarettes is a little like comparing apples to oranges," he said: "Our data shows that e-cigarettes have a signature of harm in the lung that is both similar and unique, which challenges the concept that switching from cigarettes to e-cigarettes is a healthier alternative."
The study, published in the American Journal of Respiratory and Critical Care Medicine, examined 44 sputum samples from e-cigarette users, current cigarette smokers, and non-smokers, the authors made it clear that most of the e-cigarette smokers were formerly cigarette smokers, making it difficult to clearly identify whether these results were solely related to e-cigarette use.
The news comes after Public Health England concluded that the devices were 95 per cent less harmful than smoking normal cigarettes in 2015.
In May, users were hit by new restrictions under a new directive cracking down on all smoking products.
The legislation, which limits the potency of vaping liquids and reduces the size of refill packets, was introduced last year, but companies were given 12 months to adopt the changes.
Supporters say the new rules, which include a reduction in the size of refills and tanks and cartridges are needed to dissuade people from picking up a vaping habit, while critics argue the new laws will put smokers off switching to e-cigarettes.
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