Secondhand marijuana smoke from bongs more dangerous than cigarette smoke: study

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Secondhand cannabis smoke from a bong is more dangerous than cigarette smoking, according to researchers. 

In a study published Wednesday in the journal JAMA Network Open, authors from the University of California, Berkeley School of Public Health wrote that bong smoking “is not safe.” 


“Decades ago, many people thought [secondhand tobacco smoke (SHTS)] presented no health risk to nonsmokers. Scientific research since then changed this perception and led to smoke-free environments. Incorrect beliefs about [secondhand cannabis smoke (SHCS)] safety promote indoor cannabis smoking,” they said. 

“Nonsmokers are exposed to even higher concentrations of SHCS materials during ‘hot-boxing,’ the popular practice in which cannabis smokers produce high volumes of smoke in an enclosed environment. This study’s findings suggest SHCS in the home is not safe and that public perceptions of SHCS safety must be addressed.”

The group found that concentrations of fine particulate matter (PM2.5) generated in a home during social cannabis bong smoking to which a nonsmoking resident might be exposed were greatly increased compared with background levels, and that PM2.5 decayed only gradually after smoking ceased.

Following 15 minutes of smoking, average PM2.5 was more than twice the Environmental Protection Agency (EPA) hazardous air quality threshold.

People congregate on Union Square May 1, 2021, in New York for an annual cannabis rally to celebrate legalization of recreational marijuana in New York state.
(Lev Radin/Pacific Press/LightRocket via Getty Images)

“If one assumes the exposure concentrations were at the mean levels observed, a single home smoking session with no other exposures would generate an estimated mean daily concentration that greatly exceeds the average in cigarette-smoking homes, nonsmoking homes and the U.S. EPA daily standard,” the researchers said. 

In order to reach these conclusions, the Environmental Health Sciences Division members’ levels of PM2.5 were measured before, during and after eight cannabis social-smoking sessions in a household living room. 

An aerosol monitor was placed where a nonsmoker might sit to record the levels.

Home cannabis bong smoking significantly increased PM2.5 from background levels by at least 100-fold to 1,000-fold for six of eight sessions. The other two sessions had high background levels and significantly increased PM2.5 more than 20-fold.

A teenager smokes marijuana out of a bong while with friends at the Vancouver Art Gallery during the annual 4/20 day, which promotes the use of marijuana, in Vancouver, British Columbia, April 20, 2013. 

A teenager smokes marijuana out of a bong while with friends at the Vancouver Art Gallery during the annual 4/20 day, which promotes the use of marijuana, in Vancouver, British Columbia, April 20, 2013. 
(REUTERS/Ben Nelms)


During the first 10 minutes of smoking, mean PM2.5 concentrations increased to 410 micrograms and 570 micrograms after 15 minutes. After half an hour, that level was 1,000 micrograms, and it reached as high as 2,500 micrograms in one session.

The concentration during smoking increased to an average of 1,300 micrograms. 

During two-hour smoking sessions, the mean five-minute peak PM2.5 concentration was 1,700 micrograms and remained half of that 90 minutes after smoking ended. 

“Each half hour after smoking ceased, mean concentration declined to 78% of peak value, then 60%, then 40% and, after 110 minutes, 31%,” the authors wrote. “In the [one] session monitored for 12 hours after smoking stopped, PM2.5 remained elevated at 50 [micrograms], more than 10 times the background concentration. Cannabis bong smoking in the home generated [four] times greater PM2.5 concentrations than cigarette or tobacco hookah smoking.”

Limitations to the study include that cannabis smoking was not directly observed. 

The study was overseen by professor Katharine Hammond. 

“You recall the orange days of wildfires a year and a half ago? When the sky was orange the concentrations of particles were 10 times higher than that,” she told the Bay Area’s KRON 4 Wednesday.

PM2.5, the main component of wildfire smoke, has been found to be linked to lung disease and cardiovascular issues. 


According to the Centers for Disease Control and Prevention (CDC), marijuana is the most commonly used federally illegal drug in the U.S.

The agency said 48.2 million people, or about 18% of Americans, used it at least once in 2019.

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