Vapour from e-cigarettes has a fraction of the toxins of traditional cigarettes, and much less nicotine, a new study has found.
But the amount of nicotine in every puff can differ widely from what's on the label.
The research by Dr Murray Laugesen, published in the New Zealand Medical Journal today, showed e-cigarettes are much safer than smoking traditional tobacco.
The study looked at nicotine and toxicant levels in 13 brands of e-cigarette bought over the internet in 2013.
They were compared to a first-generation model studied in 2008, and a traditional Marlboro tobacco cigarette.
Results showed e-cigarettes contained 200 times less toxic aldehydes, including formaldehyde, than Marlboros.
They were also significantly lower in aldehydes than first generation e-cigarettes from 2008.
Levels of nicotine in 'high strength' e-cigarettes, which were labeled as containing 16mg to 18mg, actually ranged from 5mg to 46mg.
The vapour delivered 19mcg to 93mcg of nicotine per puff, compared to 147mcg per puff for the Marlboro cigarettes.
Dr Laugesen said the difference between labeled and actual nicotine content highlighted a lack in quality control in the products.
"If government does decide to regulate e-cigarettes, that's something they should consider," he said.
E-cigarettes containing nicotine are illegal to sell in New Zealand, but can be purchased online from overseas for personal use.
They're also illegal to sell when marketed as a smoking cessation aid.
Dr Laugesen said the government is waiting for more evidence, but research already shows they are significantly safer than tobacco cigarettes.
"In the meantime we have 5000 people dying every year from smoking conventional cigarettes."
As the excise tax on cigarettes is raised every year, raising the cost of tobacco, smokers would be looking to quit, he said.
"Nicotine e-cigarettes do not cause cancer, do not kill people. They've been on sale since 2007 in the west and there are no reports of cancer or suchlike from them."