Hydrogen sulfide is a highly flammable hazardous gas that occurs naturally in crude petroleum and natural gas. It also is present in sewage, created through the breakdown of organic matter, and can cause negative health effects when inhaled. Hydrogen sulfide is heavier than air so it tends to be more highly concentrated in low-lying areas that are poorly ventilated, such as basements, manholes, sewer lines, and underground telephone or electrical vaults. ... Read more »