Breathing in second-hand cannabis smoke from a bong exposes people to four times more toxic pollutants than those from a standard cigarette, a study has found.
The risks of second-hand cigarette smoking have been known for years, but this paper is thought to be one of the first to show marijuana fumes also pose a risk.
Researchers at the University of California, Berkeley monitored PM2.5 levels — a particle type that can worsen asthma and heart disease — in a room over the two hours bong smoking took place, and for two hours afterwards.
Patton Nguyen, a master's of public health student at the university and lead study author, warned that 'bong smoking was not safe'.
He said toxic fumes released from the devices could pose a risk to children, pregnant women and grandparents if they also inhaled them.
A total of 18 US states — including California, New York and Arizona — have already legalized the smoking of Cannabis delivery for recreational reasons, with 20 others having legalized it more medical purposes.
Pressure is also building on the federal government to legalize the drug recreationally.
The above graph shows the PM2.5 levels in the air before, during and after smoking a bong during the eight sessions alongside the average (blue dotted line).
The recommended levels from the Environmental Protection Agency are included (red dotted line)
PM2.5 particles in the air can irritate the lining of the nose and throat, and raise the risk of asthma and heart problems studies suggest (file photo)
The Centers for Disease Control and Prevention (CDC) warns marijuana smoke contains many of the same toxic and cancer-causing chemicals as in cigarettes.
Previous studies showed the compound responsible for the marijuana 'high' can be passed to children and infants through second-hand smoking.
This could pose a risk to youngster's mental development, and trigger problems with attention, motivation and memory.