H2S exposure limit in Canada by province, plus the ACGIH TLV
The ACGIH H2S exposure limit is 1 ppm (8-hour) and 5 ppm (15-minute). Alberta, Ontario, BC and Quebec use higher values. See each limit and the risks.
H2S exposure limit in Canada and why the gas is so dangerous
The ACGIH H2S exposure limit is 1 ppm as an 8-hour average and 5 ppm as a 15-minute short-term limit (ACGIH). Canada has no single legal limit for hydrogen sulfide: the applicable values depend on your jurisdiction (CCOHS).
Hydrogen sulfide (chemical formula H2S, also listed as hydrosulfuric acid) is a colourless gas with a rotten-egg smell (NIOSH). Workers also call it sour gas, sewer gas or hydrogen sulphide (CCOHS).
H2S ppm limits by jurisdiction
Ppm means parts per million. TWA is a time-weighted average, and STEL is a short-term exposure limit, usually over 15 minutes. A ceiling must not be exceeded at any time (CCOHS).
| Jurisdiction | 8-hour limit | Short-term or ceiling |
|---|---|---|
| ACGIH TLV | 1 ppm | 5 ppm STEL |
| Federal (Canada Labour Code) | 1 ppm | 5 ppm STEL |
| Manitoba | 1 ppm | 5 ppm STEL |
| Alberta | 10 ppm | 15 ppm ceiling |
| Ontario | 10 ppm | 15 ppm STEL/C |
| Saskatchewan | 10 ppm | 15 ppm (15-minute) |
| British Columbia | none | 10 ppm ceiling |
| Quebec | 8 ppm | 10 ppm ceiling |
Federally regulated workplaces follow the ACGIH TLVs "as amended from time to time" under COHSR section 10.19. Manitoba also uses ACGIH values, fixed at the 2019 edition. The H2S TLV was already 1 ppm and 5 ppm then (OSHA).
Alberta lists 10 ppm over 8 hours and 15 ppm marked "(c)" in OHS Code Schedule 1. That mark means a ceiling, not a STEL, so 15 ppm must not be exceeded at any time.
Ontario has H2S in its own Regulation 833 table at 10 ppm TWA and 15 ppm STEL/C, so the ACGIH value does not apply there. Saskatchewan uses 10 ppm (8-hour) and 15 ppm (15-minute). WorkSafeBC sets one 10 ppm ceiling and nothing else.
Older Quebec documents still quote 10 ppm and 15 ppm. Do not use those. The current values are an 8 ppm 8-hour limit (VEMP) and a 10 ppm ceiling (IRSST, CNESST REPTOX). Check the current provincial values before setting workplace limits.
In Atlantic Canada and the territories, check your own regulator. CCOHS explains how each jurisdiction adopts its limits.
US values, for comparison
OSHA has no 8-hour TWA for H2S in general industry. It sets a 20 ppm ceiling, with one 50 ppm peak of up to 10 minutes only if there is no other measurable exposure that shift, and a 10 ppm TWA for construction and shipyards (OSHA). NIOSH recommends a 10 ppm ceiling over 10 minutes and puts the immediately dangerous to life or health (IDLH) level at 100 ppm (NIOSH, IDLH).
H2S poisoning symptoms by concentration
OSHA lists these effects by concentration and exposure time (OSHA hazards). These are hazard descriptions, not safe exposure times.
- 0.01 to 1.5 ppm: odour threshold.
- 2 to 5 ppm: long exposure can cause nausea, tearing eyes, headaches and lost sleep.
- 20 ppm: possible fatigue, headache, irritability, poor memory and dizziness.
- 50 to 100 ppm: slight conjunctivitis ("gas eye") and airway irritation after 1 hour.
- 100 ppm: coughing, eye irritation and altered breathing. Drowsiness after 15 to 30 minutes. Death may occur after 48 hours.
- 200 to 300 ppm: marked conjunctivitis and airway irritation after 1 hour. Pulmonary edema may follow prolonged exposure.
- 500 to 700 ppm: staggering and collapse in 5 minutes, death after 30 to 60 minutes.
- 700 to 1000 ppm: "knockdown" within 1 or 2 breaths, breathing stops, death within minutes.
- 1000 to 2000 ppm: nearly instant death.
Why H2S is so dangerous at work
Smell is not a reliable warning. At 100 ppm, olfactory fatigue sets in after 2 to 15 minutes (OSHA hazards). At high levels the smell can vanish instantly, so do not trust your nose (OSHA fact sheet).
The gas is heavier than air (OSHA). It collects in basements, manholes, sewer lines and manure pits (OSHA fact sheet). In Canada the main sources are oil and gas facilities, kraft pulp and paper mills, wastewater treatment and mining (Health Canada). It is extremely flammable (CCOHS). Its explosive range is 4.3% to 45%, and burning it makes sulfur dioxide (OSHA).
Rescuers die as well. CCOHS estimates 60% of confined-space deaths are would-be rescuers (CCOHS). Never attempt rescue without training and suitable respiratory protection. Entry into an IDLH atmosphere requires pressure-demand self-contained breathing apparatus (SCBA), or supplied air with an escape supply (OSHA fact sheet).
To check H2S limits in other countries, see the RegAffairs AI answer on H2S exposure limits.