E-cigarettes emit toxic vapours which break safety guidelines when used indoors, research suggests.
Non-vaping customers in pubs can be exposed to unhealthy levels of formaldehyde, which causes cancer, and acrolein, a toxin which irritates the eyes and skin.
That is the finding of a study based on as few as three people an hour using the devices in a bar.
The dangers of passive smoking led to the 2007 smoking ban.
There is no similar law for vaping, although some pub chains including JD Wetherspoon have voluntarily banned electronic cigarettes.
Last year the World Health Organisation called on Britain to consider a total ban in public spaces due to the risk from second-hand vapour'.
Researchers led by Berkeley University in California found toxic chemicals released into bars exceeded their state's guideline safety levels.
The study states: E-cigarettes likely represent a lower risk to health than traditional combustion cigarettes, but they are not innocuous.'
Following a similar study last year, co-author Dr Hugo Destaillats, from Berkeley, said: Regular cigarettes are super unhealthy. E-cigarettes are just unhealthy.'
While e-cigarettes don't produce toxic tobacco smoke, which causes lung cancer, they generate other compounds that are potentially dangerous to human health.
Formaldehyde intake from just 100 daily puffs of one of these devices is higher than the amount inhaled by a smoker on ten conventional cigarettes a day. The chemical, used to embalm dead bodies, has been suggested to raise the risk of leukaemia and brain cancer in people such as funeral directors who work with it regularly.
Formaldehyde, along with acrolein, is produced when the main ingredients of electronic cigarettes, propylene, glycol and glycerin, are heated. The person vaping is exposed but they also endanger others when exhaling the vapour into the environment.
The study authors calculated levels of this vapour based on the findings of previous studies on vaping in bars. They took into account indoor air volumes and the number of hourly users, which ranged from just over three to 13.
The results showed, despite e-cigarette vapour evaporating quickly, that formaldehyde in these bars hit an average level of 135 micrograms per cubic metre and acrolein reached 28 micrograms both exceeding the California guidelines.
The study, published in the journal Environmental Science & Technology, also judged the risk to people who use e-cigarettes, based on 250 puffs a day, which was found to be typical usage for many vapers in a previous study.
This level of use exposed people to unsafe levels of formaldehyde and diacetyl a chemical used to flavour many e-cigarettes and blamed for an incurable condition called popcorn lung' in which the organs become so scarred that a lung transplant can be needed.
Before the 2007 smoking ban, passive smoking had been a serious problem. One famous victim was Roy Castle, a non-smoker who died of lung cancer after years playing the trumpet in smoke-filled pubs.