You've heard about the dangers of secondhand cigarette smoke, but what about secondhand bong smoke? The haze after a bubbly bong hit may appear harmless, but a study published in JAMA Network Open found bystanders may inhale air pollutants at concentrations more than twice federal air quality limits. Researchers at the University of California, Berkeley, measured fine particulate matter (PM2.5) in a real-world setting where a group of young adults socially smoked cannabis with a bong for two hours in an ordinary household living room. Within the first 15 minutes of smoking, PM2.5 concentrations surpassed air quality levels deemed safe by the Environmental Protection Agency. "There's negative attitudes to secondhand tobacco smoke but not really to secondhand cannabis smoke," said lead study author Patton Nguyen, an industrial hygienist and a graduate of UC Berkeley School of Public Health. "What we want this study to do is really elucidate and help people understand that there are public health concerns." Fine particulate matter, or PM2.5, in the air can travel deep into the respiratory tract, reaching the lungs and affecting their function.