Over the years, much has been reported about health risks of second-hand tobacco smoke, so it should be no surprise that second-hand cannabis smoke is also harmful. Researchers at the University of California San Diego note that cannabis is often smoked indoors, putting non-smokers such as children at risk for exposure. The study measured cannabis in the urine of young children in more than two hundred homes. It found odds of detectable cannabis in children were five times higher in households with reported cannabis smoking. That smoke contains known cancer-causing chemicals that could have long-term health risks. The message in JAMA Network Open seems simple: Indoor smoking of cannabis around kids is risky.