New Study: Secondhand Bong Smoke is full of Hazardous Fine Particles

As more states and locales legalize the smoking of cannabis at special events, cannabis stores, restaurants and hotels, nonsmokers’ exposure to secondhand cannabis smoke and vape aerosol are also increasing.  However, we still don’t have much scientific data on how much secondhand smoke cannabis use creates and what the exposure levels might be.  A new paper in JAMA Network Open shows that smoking cannabis with a bong (a water pipe) can release extremely high concentrations of fine particles creating exposures that are very likely to harm the health of others.  Researchers measured the fine particulate matter under 2.5 micrometers in diameter (PM_2.5) in a living room where people were smoking cannabis with a bong.  Over eight experiments, Nguyen and Hammond showed that the average PM_2.5 concentration was 1,300 µg/m³.  This is 37 times higher than the US EPA 24-hour standard for safe concentrations (37 µg/m³) and 1050 µg/m³ higher than the concentrations that the EPA deems hazardous.  

Bongs are popular for home and party use because they lower the temperature of the smoke, are easy to share and are perceived as safer than joints and hand pipes.  Bongs and other water pipes bubble the smoke that the smoker inhales through water.  This removes some of the water-soluble toxicants and larger particles, but still allows most of the smoke chemicals and PM_2.5 through to the smoker.  The smoke released as the cannabis smolders in the bowl of the pipe goes directly into the air.  Regardless of what is lost in the water, it's the fine particles (the PM_2.5) that make it far into the lungs and can still cause pulmonary and cardiovascular harm. 

This paper is a valuable complement to a 2021 publication by Ott et al., that compared the fine particle emissions from just three puffs from joints, bongs, hand pipes, cannabis vape pens or tobacco cigarettes.  This study found that joints released the most PM_2.5 (7.8 milligrams/minute), followed by the bong (5.2 mg/min), hand pipe (4.2 mg/min), vape pen (3.4 mg/min) and tobacco cigarette (2.2 mg/min).   This suggests that if the situation Nguyen and Hammond studied had included people smoking joints, the PM_2.5 concentrations might have been even higher.  They didn’t have the smokers puff in any particular way, they just let the people smoke as they wanted to and monitored the air.  Another recent study showed that PM_2.5 concentrations in a cannabis store that only allowed vaporizing, dabbing and vape pens were lower (84 µg/m³ average), but still above safe levels.  

We already know that breathing PM_2.5 from any form of combustion is dangerous and that secondhand cannabis smoke can increase the risk of heart attack.  What we are learning is that cannabis use can put even more smoke into the air than cigarette use.  We need to take this seriously.  Cannabis advocates support legalizing social cannabis use in public places because it gives people who use medical cannabis and can’t smoke at home, a safe and legal place to do it.  This argument glosses over the fact that many medical cannabis users can get relief from edible products while everyone needs clean air to breathe.  We need to support laws that protect workers and the public from exposure to secondhand cannabis smoke. 

Suzaynn Schick PhD and Matt Springer PhD