What is sewer gas?

Sewer gas forms when organic waste breaks down in drain pipes, septic tanks, and sewers. The decomposition process releases a mixture of methane, hydrogen sulfide, ammonia, and carbon dioxide. The rotten egg smell most people associate with sewer gas comes from hydrogen sulfide.

The odor itself is unpleasant but not dangerous at normal household levels. More concerning are the health risks from prolonged exposure to high concentrations. Hydrogen sulfide and methane can cause headaches, dizziness, nausea, and respiratory irritation. In extreme cases or enclosed spaces, sewer gas poses serious risks including asphyxiation.

Plumbing systems are designed with two main defenses against sewer gas entering homes. Traps (U-shaped or S-shaped sections of pipe) hold standing water that acts as a seal, blocking gases from flowing backward into living spaces. Vents (typically running through roof lines) allow gases to escape safely to the outside atmosphere while maintaining proper drainage flow. When traps dry out, become damaged, or vents become blocked, sewer gas can enter the home through drains, creating foul odors and potential health concerns.

A persistent sewer smell indoors often signals a failed trap seal, a blocked vent, or a damaged drain line. Addressing these issues promptly is important for air quality and safety. Professional plumbers can diagnose and repair the source of unwanted gas entry.