H2S Exposure Limit
What is the exposure limit for H2S?
There is no single H2S limit; it depends on the jurisdiction. In the EU, and in most European countries that adopted the EU value, it is 5 ppm (7 mg/m³) as an 8-hour TWA and 10 ppm (14 mg/m³) as a 15-minute STEL. That figure is an indicative OELV (IOELV) set by Directive 2009/161/EU, the third IOELV list [1][2][3]. US federal OSHA does not set an 8-hour TWA for general industry. It sets a 20 ppm ceiling plus a 50 ppm peak [4].
Approach: I checked the national OEL registers for the US (OSHA, NIOSH, Cal/OSHA), Great Britain (EH40), Germany (TRGS 900), France, Spain, the Netherlands, Norway, Switzerland, Turkey, Korea, Japan (JSOH), Australia and Mexico. I cross-checked them against the IFA GESTIS international limit-value database. ACGIH values come only from secondary listings (OSHA and CCOHS), not from ACGIH itself.
Main limits for hydrogen sulphide (CAS 7783-06-4)
| Jurisdiction / body | 8-h TWA | Short-term | Status / note |
|---|---|---|---|
| EU (Dir. 2009/161/EU) | 5 ppm / 7 mg/m³ | 10 ppm / 14 mg/m³ (15 min) | Indicative OELV [5][6] |
| GB (EH40/2005) | 5 ppm / 7 mg/m³ | 10 ppm / 14 mg/m³ | WEL [7] |
| Germany (TRGS 900 AGW) | 5 ppm / 7.1 mg/m³ | Excursion factor 2(I) | Last changed 03/11 [8] |
| France | 5 ppm / 7 mg/m³ | 10 ppm / 14 mg/m³ | Binding (art. R. 4412-149 Code du travail) [9] |
| Spain (INSST LEP) | 5 ppm / 7 mg/m³ | 10 ppm / 14 mg/m³ | Noted as VLI [10] |
| Netherlands | 2.3 mg/m³ | none | Statutory since 1 Jan 2007, stricter than the EU value [3] |
| Belgium | 1.64 ppm / 2.3 mg/m³ | 4 ppm / 5.61 mg/m³ | [11][12] |
| Norway | 5 ppm / 7 mg/m³ | none in the national row | [13] |
| Switzerland (SUVA MAK) | 5 ppm / 7.1 mg/m³ | 10 ppm / 14.2 mg/m³ | [14] |
| Turkey | 5 ppm / 7 mg/m³ | 10 ppm / 14 mg/m³ | [15] |
| US OSHA, general industry (29 CFR 1910.1000 Table Z-2) | none | 20 ppm ceiling; 50 ppm peak, 10 min once only | [4] |
| US OSHA, construction and maritime | 10 ppm / 15 mg/m³ | none | 1926.55 / 1915.1000 [16] |
| US NIOSH REL | none | 10 ppm / 15 mg/m³ ceiling (10 min) | IDLH 100 ppm [16] |
| Cal/OSHA | 10 ppm | 15 ppm STEL; 50 ppm ceiling | [16] |
| ACGIH TLV (2009) | 1 ppm | 5 ppm STEL | [16][17] |
| Mexico (NOM-010-STPS-2014) | 1 ppm | 5 ppm | [18] |
| South Korea (MOEL Notice 2020-48) | 10 ppm | 15 ppm | [19] |
| Australia (Safe Work Australia WEL, approved 31 Jul 2026) | 10 ppm / 14 mg/m³ | 15 ppm / 21 mg/m³ | [20] |
| Japan | JSOH: 5 ppm / 7 mg/m³; MHLW: 1 ppm | none | [21][22] |
| Canada | Ontario: 10 ppm; Québec: 8 ppm | Ontario: 15 ppm; Québec: 10 ppm | [23][24] |
Things that change the answer:
- EU member states: the EU value is indicative, so each country's own transposed limit is the one that applies. Belgium and the Netherlands are stricter, at about 1.64 ppm (2.3 mg/m³) [3]. Austria applies 5 ppm as both its TWA and its short-term ceiling [25].
- REACH DNELs: the REACH registration DNELs listed in the Dutch database match the EU IOELV: 7 mg/m³ long-term and 14 mg/m³ short-term [3].
- US: the ACGIH TLV (1 ppm TWA, 5 ppm STEL) is much lower than the enforceable OSHA PEL. Many Canadian provinces broadly follow the TLVs, though the details vary by jurisdiction [17].
If you tell me which jurisdiction you need, I can pull the underlying legal instrument and any amendments that are pending.
What about Alberta?
In Alberta, the hydrogen sulphide (CAS 7783-06-4) limit is 10 ppm (14 mg/m³) as an 8-hour OEL and 15 ppm (21 mg/m³) as a ceiling. The source is the Alberta Occupational Health and Safety Code, Schedule 1, Table 2 [26].
Approach: I read the Alberta OHS Code Schedule 1, Table 2, as consolidated in force on 31 March 2025. The IFA GESTIS international database had no Alberta-specific hydrogen sulphide entry to cross-check against.
| Parameter | Alberta OHS Code, Sch. 1 Table 2 |
|---|---|
| 8-hour OEL | 10 ppm / 14 mg/m³ |
| 15-minute or ceiling | (c) 15 ppm / (c) 21 mg/m³ |
| Substance interaction notation | none |
How to read it:
- Ceiling, not STEL: the "(c)" marker in the table makes the 15 ppm value a ceiling, so exposure must not go above it at any time. A 15-minute STEL would be a time-weighted average instead [26].
- Compared with other jurisdictions: Alberta's numbers are the same as Ontario's (10 ppm TWA, 15 ppm short-term) [23]. They are well above the ACGIH TLV of 1 ppm TWA and 5 ppm STEL [17]. They are also above the EU IOELV of 5 ppm and 10 ppm [1].
I can also pull the hydrogen sulphide limits for other Canadian provinces, such as British Columbia or Saskatchewan, for a side-by-side comparison.
Why is it so dangerous at work?
Hydrogen sulphide is dangerous at work mainly because it is a fast-acting poison by inhalation, and it shuts off the one warning workers rely on. At low levels its rotten-egg smell is easy to notice. From about 100 ppm, though, the sense of smell fails within a short time, while the gas goes on to cause collapse ("knock-down"), respiratory paralysis and death. It is also extremely flammable and heavier than air, so it collects in low and enclosed spaces. The EU and GB both classify it as Acute Tox. 2 (H330, fatal if inhaled) and Flam. Gas 1A (H220), with the signal word "Danger" [27][28][29].
Approach: I read the CLP Annex VI and GB MCL harmonised classifications, the IFA GESTIS substance data sheet, the New Jersey Right to Know fact sheet (revised May 2012) and the OSHA Occupational Chemical Database entry.
1. The smell warning fails when it matters most
- People notice the odour at very low levels: 0.008-0.1 ppm according to NJDOH [30], and on average from 0.02 ppm according to GESTIS. Even at those levels, GESTIS notes that the nose adapts temporarily during continued exposure [31].
- From about 100 ppm, the sense of smell fails after a short time through exhaustion or paralysis. Because the warning disappears just as the danger rises, GESTIS calls this a considerable poisoning risk [31]. NJDOH says plainly not to rely on odour to judge exposure [30].
2. Effects get much worse over a narrow concentration range (GESTIS) [31]
| Concentration | Reported effect |
|---|---|
| 2 ppm | Bronchoconstriction in 2 of 10 asthmatics tested |
| ~10 ppm | Estimated threshold for eye irritation |
| ~100 ppm | Smell fails; eye, nose and throat irritation |
| ~150 ppm (30 min) | Headache, dizziness, diarrhoea |
| 300-500 ppm | Risk of lung damage, strong systemic effects, unconsciousness |
| 500 ppm | Rapid life-threatening effects on the central nervous system and heart; lethal after about 30 min |
| ≥ ~1000 ppm | Immediate collapse and respiratory paralysis |
100 ppm is the US IDLH (immediately dangerous to life or health) value [16]. GESTIS also notes that about 5% of deaths happen during transport to hospital [32].
3. Effects can be delayed or lasting
- Pulmonary oedema can appear late, so NJDOH recommends 24-48 hours of medical observation after overexposure [30]. GESTIS reports lung damage in about 15% of reported poisonings [31].
- After a knock-down, victims often seem to recover quickly, but further neurotoxic effects frequently follow. Survivors of severe poisoning can have long-lasting or permanent deficits in concentration, memory and balance. Loss of smell lasting more than 2 years has also been reported [31].
- Long-term low-level exposure is linked to eye and airway irritation, a reduced sense of smell, and nervous-system and cardiovascular effects. GESTIS notes that the epidemiology does not support firm dose-response conclusions [31]. NJDOH adds possible bronchitis and limited evidence of spontaneous abortion [33].
4. Where and how it behaves
- Heavier than air: the gas can travel some distance and ignite far from its source. NJDOH warns to keep it out of confined spaces such as sewers, and to check for explosive concentrations before entering a confined space [34][35].
- Common names: synonyms include "sewer gas" and "stink damp", and it occurs as a by-product of industrial and natural processes [16][30].
- Fire and explosion: the explosive range is about 4-45% in air, and it has NFPA ratings of 4 for health and 4 for fire [16]. Burning produces sulphur oxides, and it reacts violently with oxidisers [34].
- Rescuers at risk: GESTIS says first aiders must wear respiratory protection [32]. NJDOH calls for a supplied-air respirator above 10 ppm and SCBA where exposure above 100 ppm is possible [35].
These are the reasons the limits we discussed are set as ceilings or short-term values as well as 8-hour averages, including Alberta's 15 ppm ceiling. I can pull the confined-space and H2S-specific provisions of the Alberta OHS Code next if that would help.