New research published in the medical journal JAMA Network Open reveals smoking cannabis from a bong at home generates four times as much secondhand smoke as cigarettes.
Researchers found after smoking with a bong or a water pipe for 10 minutes, fine particulate matter increased by more than 10-fold in a 200-square-foot room.
They found smoking weed for 30 minutes in the same-sized room led to a more than a 28-fold rise in fine particulate matter levels.
Researchers found higher air pollutant levels persisted for at least two hours after smoking had finished.
"We did not simulate the study. This was a real hangout, a real social event," study co-author S. Katherine Hammond said. "The smoking lasted 1.5-2 hours, but we measured air concentrations before and after."
Researchers say nonsmokers are exposed to even higher concentrations of secondhand smoke materials when 'hot-boxing' - a practice in which cannabis smokers produce high volumes of smoke in an enclosed environment.
Researchers are confident in their findings, saying "contrary to popular beliefs, bong smoking is not safe."
The study also found 27 percent of young adults believe secondhand cannabis smoke is safe. Researchers say it's clear that idea needs to be addressed.
"This study's finding suggests secondhand cannabis smoke in the home is not safe and the public perceptions of secondhand cannabis smoke in the home is not safe, and the public perception of secondhand cannabis smoke safety must be addressed."