Mosquito coils are considered to be safe insecticides for humans and mammals, although some studies highlight concerns when they are used in closed rooms. Coils sold in China and Malaysia were found to produce as much smoke PM2.5 as 75-137 burning cigarettes and formaldehyde emission levels in line with 51 burning cigarettes.[2] Other studies in rats conclude that mosquito coils are not a significant health risk, although some organisms may experience temporary sensory irritation like that caused by smoke from the combustion of organic materials such as logs. In one study, rats were directly exposed to a coil's smoke for six hours a day, five days a week for thirteen weeks.