Smoking Marijuana With A Water Bong Doesn’t Effectively Filter Compounds From Smoke, Study Suggests

For decades, marijuana consumers have debated whether using a bong, where smoke is pulled through water before inhalation, is any safer than inhaling smoke from a joint. Conventional wisdom has long held that water filtration makes for a cleaner, less harmful consumption experience.

But a new study, by authors affiliated with the University of Wisconsin-Madison and in Thailand, concludes that “bong water does not seem to significantly filter out any compound from the smoke.”

For the study, researchers analyzed the chemical makeup of smoke from three popular cannabis strains—Bubble Gum, Silver Haze and Hang Over OG—when consumed through both joints and bongs. Using gas chromatography-mass spectrometry (GC-MS), a highly sensitive apparatus that identifies chemical compounds by their molecular weight, they looked for differences in the final combusted smoke.

The results for both consumption methods were nearly identical. Bong water didn’t completely remove any of the detected compounds in the range the instrument could measure. The study found no compounds that appeared only in joint smoke and not in bong smoke, suggesting the water did not fully capture any components within the tested size range.

Gas chromatography-mass spectrometry “results from both the bong and joint smoke show similar smoke composition. No compounds between 5 to 350 g/mol were completely filtered by the bong water.”

The researchers note in the paper, published as a pre-print on bioRxiv, that their methods couldn’t capture larger particles, aerosols or metals—in other words, things that water might catch. Still, the findings cast doubt on the idea that a bong meaningfully reduces exposure to harmful chemicals.

“Although the effectiveness of the filtration of the bong is not clear, this study sheds light on the chemical composition of cannabis smoke,” they concluded.

The study also shows promise for compounds detected in higher concentrations. They note that the prevalence of β-cis-Caryophyllene, which was consistently present in the highest quantities, suggests “possible physiological importance despite limited research compared to THC and CBD.” They added that the compound “has a potential anti-inflammatory, antibiotic, antioxidant, anticarcinogenic and local anesthetic activity.”

The researchers argue that one of the biggest obstacles in cannabis science is the lack of standardized tools to measure what’s actually in smoke. Tobacco research, by contrast, relies on decades of standardized methods that make it possible to compare cigarettes across brands and countries.

“Establishing standardized analytical approaches could support more accurate assessments of cannabis quality, health risks, and therapeutic potential, while enabling comparisons across strains, cultivation methods, and global research efforts,” they wrote.

The authors caution about the methodological constraints, including sample size and smoke loss during collection. The Agilent GC-MS was limited in its ability to “detect larger particles, aerosols, and metal ions, restricted definitive conclusions regarding bong filtration effectiveness. However, the findings highlight that cannabis smoke contains a reproducible profile of compounds, both harmful and potentially beneficial,” they wrote.

The researchers stress that more standardized methods—such as better ways of measuring larger aerosols and analyzing the bong water itself—will be needed to draw firmer conclusions.

The study has not been peer reviewed. Additionally, the authors recently withdrew the paper “because there may be a conflicting bureaucracy issue due to the location this research was performed,” an update on the bioRxiv website says.

Bong water became a topic of policy interest in the U.S. when the Minnesota Supreme Court ruled in 2009 that it could be legally considered a drug, in part due to a state patrol officer testifying that marijuana users keep bong water “for future use…either drinking it or shooting it in the veins.” The state law changed this year when legislators and the governor enacted a law that ended the risk of decades in prison over drug residue.

Minnesota’s unique law came into national spotlight in 2024 when a Fargo woman faced the possibility of a 30-year prison sentence after being caught with bong water. The charges caused the Minnesota ACLU to represent her, given the draconian nature of the prosecutor’s actions.