The UK should rethink restrictions on nicotine levels in e-cigarettes, researchers argue, saying people using lower nicotine concentration e-liquids take longer and more frequent puffs, increasing their exposure to potentially harmful compounds.

Addressing a parliamentary inquiry into e-cigarettes last week, smoking cessation expert Dr Lynne Dawkins said there was no evidence for increased harms of nicotine at levels above the current cap of 20 mg/ml.

“[The cap] seems arbitrary to me,” said Dr Dawkins, an associate professor at the Centre for Addictive Behaviours Research at London South Bank University.

However, lower concentration nicotine e-liquids may more harmful than higher concentrations, with research from her group suggesting people compensated for the reduced nicotine by increasing puff duration and frequency.

“… [T]hat is costly financially, as you are using more e-liquid, and it may also come with a health cost if increased exposure [to carbonyl compounds] translates to long-term health risk,” she told the inquiry.

In a written submission, Dr Dawkins and colleagues explained more intensive puffing patterns can lead to the atomiser coil in the e-cigarette overheating, resulting in increased carcinogen production.

In their latest study of 20 vapers using high vs low nicotine concentrations (manuscript under review), they found urinary levels of formate – a metabolite of formaldehyde – were significantly higher when people used a low concentration e-liquid with a higher power setting.

Nicotine has an effect on blood pressure and heart rate, but this was not problematic for most people, the inquiry heard. Some studies suggested nicotine might be harmful to the adolescent brain but the findings came almost exclusively from animal studies, Dr Dawkins said.

“In general, the cap on nicotine concentrations is sending the wrong message – that nicotine is problematic and addictive,” she said. Rather any harms from vaping appeared to be linked to the flavourings and the heating of the solvents.

The ongoing inquiry coincides with the recent publication of an updated evidence review into e-cigarettes, which found many thousands of UK smokers incorrectly believed vaping was as harmful as smoking (see our previous coverage here.)

The review also found e-cigarette use in the UK has plateaued in recent years at just under 3 million users.

Dr Dawkins questioned why more smokers were not making the switch to e-cigarettes and why half of all smokers who tried them were not fully transitioning.

“This may be partly due to the nicotine concentrations, and it may be partly due to the health messages, but we need to make them as attractive as possible to smokers. That means having a variety of different products … and having different concentrations of nicotine available,” she told the inquiry.