Hydrogen sulfide: why sewer gas can be dangerous
Updated October 30, 2026 · 2-minute read · Old House Sewer Lines
Hydrogen sulfide, the main gas behind the rotten-egg "sewer gas" smell, is dangerous because it gathers in low, enclosed spaces and because at higher levels it knocks out the sense of smell that would warn you. At very high concentrations it causes sudden collapse.
The federal Agency for Toxic Substances and Disease Registry (ATSDR) describes hydrogen sulfide as a flammable, colorless gas that smells like rotten eggs and is often called sewer gas or "stink damp" (ATSDR ToxFAQs). It is heavier than air, so ATSDR says exposure risk is greater close to the ground and in enclosed spaces.
The nose stops working
People can smell hydrogen sulfide at very low levels, far below dangerous ones. That early warning fails as the level rises. ATSDR says people lose the ability to smell it after about 2 to 15 minutes at about 100 parts per million (ppm), which makes odor an ineffective hazard warning. An odor that seems to weaken says nothing reliable about the level in the air. ATSDR adds that levels above 500 ppm can cause sudden unconsciousness or death.
What OSHA's table shows
The Occupational Safety and Health Administration publishes a table of effects by concentration (OSHA, Hydrogen Sulfide Hazards). It also lists the NIOSH "immediately dangerous to life or health" level at 100 ppm.
| Level | Effect OSHA lists |
|---|---|
| 3–5 ppm | Odor becomes more offensive |
| Above 30 ppm | Odor described as sweet or sickeningly sweet |
| 100 ppm | Coughing, eye irritation, loss of smell after 2–15 minutes |
| 500–700 ppm | Collapse in 5 minutes |
| 700–1000 ppm | Collapse within 1 to 2 breaths, breathing stops |
| 1000–2000 ppm | Nearly instant death |
Workplace limits are far lower: OSHA lists a 20 ppm ceiling for general industry and a 10 ppm eight-hour limit for construction. The gas also burns. OSHA gives its explosive range in air as 4.3 to 45 percent.
Around a house
Inside a home, sewer gas typically gets in by way of a dry trap, a loose cleanout cap or a blocked vent. North Dakota State University Extension lists headaches, nausea, dizziness and drowsiness as symptoms when it backs into a house (NDSU). For causes and fixes, see sewer gas smell in the house. When only hot water smells, the water heater's anode is the usual suspect, per NDSU; see rotten egg smell from hot water or sewer.
The highest levels are found where gas collects with little air: manholes, sewer pits and septic tanks. EPA says only trained specialists should clean or repair septic tanks because they may hold dangerous gases (EPA). The NYS Department of Health lists hydrogen sulfide and flammable methane among confined-space hazards. See why no one should climb into a manhole.
Hydrogen sulfide: what changes as the level rises
Key facts
- Hydrogen sulfide is flammable, colorless, smells like rotten eggs and is heavier than air (ATSDR).
- Around 100 ppm, smell gives out after roughly 2 to 15 minutes of exposure (ATSDR).
- Above 500 ppm it can cause sudden unconsciousness or death (ATSDR).
- OSHA lists the NIOSH IDLH at 100 ppm and an explosive range of 4.3 to 45 percent in air (OSHA).
- Only trained specialists should clean or repair septic tanks (EPA).