Sewer gas is a mixture of gases that can escape from underground sewage systems. It is primarily composed of methane, hydrogen sulfide, and carbon dioxide. These gases can be produced by the decomposition of organic matter in sewage. When sewer gas accumulates, it can create unpleasant odors and pose health risks. Exposure to sewer gas can lead to symptoms like headaches, dizziness, and nausea. In high concentrations, hydrogen sulfide can be particularly dangerous, as it is toxic and can cause respiratory issues. Proper ventilation and maintenance of sewer systems are essential to prevent the buildup of these harmful gases.