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 in the UK.
There is no similar law for vaping, although some pub chains 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 are to 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 as unhealthy."
While e-cigarettes don’t produce toxic tobacco smoke, which causes lung cancer, they generate other compounds that could be potentially dangerous to health in humans.
Formaldehyde intake from just 100 daily puffs of one of these devices is higher than the amount inhaled by a smoker of 10 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.