Sewer gas is a mix of gases produced by decomposing waste — mostly nitrogen and carbon dioxide, with smaller amounts of hydrogen sulphide (the rotten-egg smell), ammonia and methane.
At the concentrations that make a house smell, sewer gas is primarily a nuisance: headaches, nausea and eye or throat irritation are possible with prolonged exposure, especially in poorly ventilated rooms. The distinctive smell is actually protective — your nose detects hydrogen sulphide at extraordinarily low concentrations, well below harmful levels.
Situations that deserve more caution:
- Confined, unventilated spaces — under-floor areas, pits and cellars where gas can accumulate. Higher concentrations of hydrogen sulphide are genuinely hazardous, and one of its nastier properties is that it deadens your sense of smell as levels rise.
- Very strong or suddenly worsening smells, which suggest an open drain, broken pipe or failed seal rather than a dry trap.
- Methane accumulation in enclosed spaces is a flammability concern, though this is rare in normal homes.
Sensible response: ventilate the room, refill any dry floor waste traps with water, and check for obvious causes like a rocking toilet. If the smell persists, treat it as a drainage fault to be found and fixed — a smoke test or camera inspection can trace exactly where gas is escaping. What you should not do is live with it long-term; a permanent sewer smell always has a cause.