Hydrogen sulfide (H2S) is a colorless, flammable gas that smells like rotten eggs at low concentrations. It forms wherever organic material breaks down without oxygen, which is why it shows up in so many confined spaces.
Where H2S comes from
- Sewers, manholes, lift stations and wastewater treatment (decomposing sewage and sludge).
- Manure pits and agricultural storage.
- Oil and gas wells, tanks and pipelines (sour crude and gas).
- Pulp and paper, tanneries, and some chemical processes.
- Disturbing sludge in a tank can release trapped H2S suddenly, even after the air tested clean.
H2S is heavier than air (specific gravity about 1.19), so it collects in low spots and at the bottom of spaces. It's also flammable, with a lower explosive limit of about 4% by volume.
Exposure limits
| Organization | Limit |
|---|---|
| OSHA general industry PEL | 20 ppm ceiling, with a peak of 50 ppm allowed for a single 10-minute period once per 8-hour shift if no other measurable exposure occurs |
| OSHA construction PEL | 10 ppm 8-hour TWA |
| NIOSH REL | 10 ppm ceiling (10 minutes) |
| ACGIH TLV | 1 ppm 8-hour TWA, 5 ppm STEL |
| IDLH (NIOSH) | 100 ppm |
Many 4-gas monitors alarm at 10 ppm, and many employers set their permit's acceptable entry condition at or below that level. Check readings with the gas reading checker.
Effects by concentration
OSHA describes the effects of H2S exposure roughly as follows:
| Concentration | Possible effects |
|---|---|
| 0.01 to 1.5 ppm | Odor threshold; rotten-egg smell |
| 2 to 5 ppm | Prolonged exposure may cause nausea, tearing, headaches or loss of sleep |
| 20 ppm | Possible fatigue, loss of appetite, headache, irritability, poor memory, dizziness |
| 50 to 100 ppm | Slight conjunctivitis and respiratory tract irritation after about 1 hour |
| 100 ppm | Coughing, eye irritation, loss of smell after 2 to 15 minutes (olfactory fatigue) |
| 100 to 150 ppm | Loss of smell (olfactory fatigue or paralysis) |
| 500 to 700 ppm | Staggering, collapse in 5 minutes, serious eye damage in 30 minutes, death after 30 to 60 minutes |
| 700 to 1,000 ppm | Rapid unconsciousness ("knockdown") within 1 or 2 breaths; breathing stops; death within minutes |
| 1,000 to 2,000 ppm | Nearly instant death |
Testing and monitoring
- Test from outside before entry, at the bottom and other levels, using a sampling pump.
- Entrants wear a personal H2S or 4-gas monitor in the breathing zone.
- Monitor continuously when disturbing sludge, opening lines or cleaning, since releases can be sudden.
- Bump test daily. H2S sensors can lose sensitivity, and H2S can inhibit catalytic LEL sensors. See bump test vs. calibration.
Controls
- Isolate incoming flows and lines, and drain and flush where possible.
- Ventilate with supply air delivered to the bottom of the space, and keep it running.
- Use supplied-air respirators where concentrations can't be kept acceptable or could reach IDLH.
- Plan rescue with retrieval systems so no one enters to help without protection. Many H2S deaths include would-be rescuers.
Sources and official references
Last reviewed: October 25, 2026 · Free general education based on OSHA 29 CFR 1910.146 and 29 CFR 1926 Subpart AA. It does not replace the site-specific training, written program and hands-on evaluation your employer must provide, and it does not issue a certificate. Nothing you enter on this page is saved or sent.