Sewer Smell in Basement: Causes, Health Risks & 2026 Fixes

Sewer gas isn't just an unpleasant odor — it's a mixture of methane, hydrogen sulfide, ammonia, and carbon dioxide that can cause headaches, nausea, and in concentrated amounts, fire risk. If you smell rotten eggs or raw sewage in your basement, treat it as a problem to solve this week, not next month. The good news: about half of all sewer-smell complaints trace back to a single bone-dry floor drain trap that you can fix in 60 seconds with a glass of water. The other half require a more careful diagnostic walkthrough. This guide gives you both.

Before you call a plumber or appliance tech, work through this list in order. The first three causes account for roughly 70% of all calls in this category, and most are diagnosable in under 15 minutes by a homeowner with no special tools.

Work through these steps in order. Stop the moment one of them resolves the problem — don't keep going and risk creating a new issue. If you reach the end without success, the problem is in professional-repair territory and a licensed technician should be your next call.