Contrary to popular opinion, cannabis smoke from a bong is a potential health hazard for non-smokers, according to peer reviewed research published Wednesday in JAMA Network Openas Congress prepares to vote this week on legislation that would decriminalize the drug.

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Home cannabis bong smoking “significantly increased” levels of fine particulate matter—pollutants numerous scientific studies have linked to issues including cancer, breathing problems and heart attacks—compared to background levels, according to researchers from the University of California, Berkeley.

The researchers measured the concentration of fine particulate matter where a nonsmoker might sit in a household living room during a social cannabis smoking session eight times.

On six occasions, the levels of fine particulate matter increased 100-fold to 1,000-fold from background levels, recording jumps of more than 20-fold from already “high background” levels for the remaining two.

After just 15 minutes of smoking, the average levels of the pollutant were more than twice the threshold deemed hazardous by the Environmental Protection Agency, the researchers said, and a single smoking session a day would generate an average concentration roughly five times higher than in the average cigarette-smoking home.

The concentration of fine particulate matter decayed slowly after smoking stopped, the researchers found, with rates still more than 10 times background levels 12 hours afterwards.

The researchers said the study suggests that, “contrary to popular beliefs, bong smoking is not safe,” and the practice actually generates concentrations of fine particulate matter four times greater than cigarette or tobacco hookah smoking.