Levels of a toxic gas in the brain could influence whether or not we get dementia and epilepsy, a study reveals. The gas, hydrogen sulfide (H2S), is created inside the body in small doses – and is perhaps best known for carrying the torrid smell of rotten eggs. H2S is produced in the brain and several smooth muscles, including the thoracic aorta (the section of the aorta in the chest) and the ileum (the final section of the small intestine). By testing rat brain cells, the scientists found that H2S is involved in blocking a key brain cell gateway that helps the brain to communicate effectively.