A forensic autopsy case of suicide using baking soda and citric acid in a bathtub.

A mixture of baking soda (sodium hydrogen carbonate) and citric acid is used for cleaning purposes, but, in recent years, this mixture has been introduced on the internet as a method for suicide. Here, we report a case in which the two agents were mixed in a bathtub to generate carbon dioxide (CO). A man in his early 60s was found dead in his clothes in a bathtub with a lid. Postmortem changes were somewhat progressed, but only pulmonary edema was observed, with no injuries or lesions that could have led to cause of death, and no drug toxicants detected. Empty containers of baking soda (789 g) and citric acid (850 g) were found at the scene. We conducted a reproduction experiment showing that CO reached a lethal concentration (~ 60%) within 12 s, but the O concentration was about 10%, which is not a lethal concentration. Therefore, it is highly likely that the man died of CO intoxication within a short period of time and before he would have suffered asphyxiation due to oxygen deficiency. Normally, this result can only be obtained with the cooperation of an investigative agency and a large-scale reproduction experiment, but we were able to diagnose the cause of his death, including a differential diagnosis of asphyxia due to oxygen deficiency, by means of a reproduction model at 1/35 scale that could easily be performed in the laboratory.