H2S Exposure & Workplace Exposure Limit Readiness Summary
SWMS variants reference your state’s WHS legislation. Instant download after payment.
Hydrogen sulphide is not brought onto a wastewater site — it is generated continuously by the assets themselves, wherever the sewage turns anaerobic, which is the normal state of any sewer with real detention time. That makes it the one hazardous chemical on the network that cannot be eliminated by substitution, and under the hazardous chemicals provisions (Part 7.1) of the WHS Regulation 2025 (NSW) a PCBU must ensure no person at the workplace is exposed to an airborne concentration above the exposure standard. The current values are 10 ppm as an 8-hour time-weighted average and 15 ppm as a 15-minute short-term exposure limit. Both apply at once, and a wet-well or maintenance hole entry is precisely the profile that sits comfortably inside the 8-hour average while breaching the short-term limit on a spike. Odour will not tell you where you sit: at roughly 100-150 ppm H2S paralyses the sense of smell, so the smell going away is the worst possible sign, not a reassuring one.
This document is the exposure-position summary that sits behind your entry procedures. It sets out why H2S behaves differently from other workplace gases, health effects across the concentration range from odour threshold to immediate collapse, the current legal position and what changes when workplace exposure standards become **Workplace Exposure Limits on 1 December 2026** with the H2S value still unresolved, where H2S actually concentrates across a wastewater system, the difference between confined space atmospheric testing and personal exposure monitoring, a pre-1 December 2026 action checklist, and an exposure assessment record table. The scope boundary is stated in the document itself and repeated here: this is an awareness and readiness summary to support planning and consultation. **It is not an exposure assessment and it does not establish your compliance position** — only monitoring data from your own tasks and assets can do that. Authored for New South Wales. Regulator: SafeWork NSW.
Hazards identified
14 hazards covered, sorted by priority.
The warning disappears precisely as the danger escalates; any control that depends on workers smelling the gas is a fatal control
A worker crosses from 'I can no longer smell it' to unconsciousness with no sensory warning at all
Atmospheres immediately dangerous to life or health, from which escape without impairment is not assured
Death within minutes without rescue; above 1000 ppm, immediate collapse after a single breath and respiratory paralysis
A reading taken at the opening can be safe while the base of the space — exactly where the worker descends — is lethal
Breach of the Part 7.1 duty, with judgement and odour offered as certainty where a regulator expects exposure data
Both limits apply at once; a short, sharp exposure on entry is the norm in this work, not the exception
The single most under-appreciated hot spot on a network; the peak is often here, not at the treatment plant
A different hazard class entirely, never to be approached on a wet-well exposure assumption
A draft revision considered 1 ppm TWA / 5 ppm STEL; a shift of that order would place routine wastewater atmospheres into exceedance across the sector
Instrument configuration and control design across the whole system requiring rework at short notice after the value is published
The entrant is protected for the moment of entry while the PCBU still has no answer on where shift exposure sits against the TWA and STEL
A single significant trade waste discharger changes the sulfide picture across the whole downstream system without anyone reassessing exposure
Conjunctivitis and respiratory injury on prolonged exposure, well below the concentrations workers associate with danger
Control measures
Hierarchy-of-controls order: elimination → substitution → isolation → engineering → administrative → PPE.
- 1Treat the exposure standard as a ceiling, not a target — compliance with the number does not discharge the duty, which is to eliminate or minimise exposure so far as is reasonably practicable.
- 2Apply both current values together: 10 ppm as an 8-hour time-weighted average and 15 ppm as a 15-minute short-term exposure limit, recognising that entry work breaches the short-term limit on a spike while the average still reads compliant.
- 3Never rely on odour as a warning or an all-clear — H2S paralyses the sense of smell at roughly 100-150 ppm, and the smell disappearing signals greater danger, not less.
- 4Test at depth, not at the opening: H2S is heavier than air and pools at the base of the space, so a clear reading at the lip proves nothing about where the worker is going.
- 5Identify every task and location across the network, pump stations and treatment plant where workers may be exposed to H2S, rather than sampling at random.
- 6Target monitoring at the actual hot spots — rising main discharge points, pump station wet wells, maintenance holes downstream of a rising main, drop structures and junctions, treatment plant inlet works, digesters and biogas structures, and sludge holding and thickening.
- 7Establish current exposure by personal monitoring at the highest-risk tasks — wet well entry, maintenance hole entry downstream of a rising main, and inlet works — carried out by a competent person.
- 8Keep the two monitoring exercises distinct: atmospheric testing before and during every entry is a Part 4.3 duty about immediate safety, while personal exposure monitoring establishes what workers are actually exposed to across a shift. A gas monitor protects the entrant; an exposure assessment answers the regulator.
- 9Assess results against the current standard (10 ppm TWA / 15 ppm STEL) and against the lower values under consideration, so the exposure position is known under either outcome.
- 10Review controls through the hierarchy — can entry be eliminated by CCTV, remote inspection or non-entry cleaning, and can generation be reduced by odour control dosing, ventilation or detention-time management.
- 11Verify gas detection: correct sensors, in-date calibration, bump testing before use, and alarm set points referenced to the exposure standard — then confirm those set points will still be appropriate if the limit is lowered.
- 12Review the trade waste profile on the network for high-strength organic dischargers, and account for warm sewage temperature, long rising main detention, flat terrain and low flows as generation drivers.
- 13Do not plan on a number that has not been decided — monitor Safe Work Australia for the final adopted H2S Workplace Exposure Limit and review controls once it is published; establishing exposure data now is useful regardless of where the value lands. Confirm rescue arrangements are non-entry and have been drilled on the actual assets.
- 14Consult workers on the H2S risk and the controls per Section 47 of the WHS Act 2011 (NSW), record the consultation, and review this document whenever the assets, method, personnel or exposure standards change, after any incident, or at minimum every 12 months — with review under Section 302 where it supports a SWMS for high risk construction work.
Applicable Codes of Practice
The governing benchmark for this document — the duty to ensure no person is exposed above the workplace exposure standard, and the role of air monitoring where exposure is uncertain.
The benchmark for the pre-entry and during-entry atmospheric testing duty this document distinguishes from personal exposure monitoring.
The technical standard underpinning atmospheric limits and testing in the wastewater spaces where H2S exposure occurs.
Underpins the gas detection verification step in the readiness checklist — correct sensors, calibration, bump testing and alarm set points.
The standards for respiratory protective equipment where exposure cannot be reduced below the standard by higher-order controls.
The source of the current H2S values of 10 ppm TWA and 15 ppm STEL, and the list being replaced on 1 December 2026 with the transition period ending 30 November 2026.
High-Risk Construction Work triggered
This is a hazardous chemicals exposure document, not a construction work document, so it is not gated on the high risk construction work categories in Section 291 of the WHS Regulation 2025 (NSW). The duty it addresses sits in Part 7.1: hydrogen sulphide is a hazardous chemical, and a PCBU must ensure that no person at the workplace is exposed to an airborne concentration above the workplace exposure standard — currently 10 ppm as an 8-hour time-weighted average and 15 ppm as a 15-minute short-term exposure limit, both applying at once and both applying until 30 November 2026. Where the PCBU is not certain on reasonable grounds that exposure is below the standard, air monitoring is how the position is established; judgement and odour are not a basis for certainty in a sewer, and the standard is a ceiling that must not be reached rather than a target that may be occupied. From 1 December 2026 the workplace exposure standards list is replaced by a Workplace Exposure Limit list agreed by WHS ministers, with the harmonised transition period ending 30 November 2026. The rename is deliberate — a limit is a value that must not be exceeded. The H2S value itself is not yet settled: Safe Work Australia's draft revision considered a TWA of 1 ppm and a STEL of 5 ppm based on ACGIH recommendations, with the final Australian value pending a regulatory impact assessment and the currently published values unchanged in the meantime. Because routine wastewater atmospheres frequently exceed 1 ppm, water and wastewater is the sector most exposed to that outcome. Separately, where the exposure occurs during entry to a wastewater confined space, the Part 4.3 confined space duties apply in full and independently of construction status, and an H2S exposure causing death or serious injury is a notifiable incident under Sections 35-38 of the WHS Act 2011 (NSW), prosecuted as a Category 1 or Category 2 offence.
Who this is for
- →Local councils and water utilities operating sewer networks, sewage pump stations and wastewater treatment plants who need to establish their H2S exposure position before the Workplace Exposure Limit transition.
- →WHS managers and HSE advisors responsible for the hazardous chemicals duty, exposure monitoring programs and alarm set points across a wastewater system.
- →Treatment plant operators, network controllers and wastewater maintenance supervisors briefing crews on why odour is not a warning.
- →Asset and operations managers planning odour control dosing, detention-time management and non-entry inspection ahead of a potentially lower limit.
- →Civil, drainage and pipeline contractors working on wastewater assets under contract to a utility who need to understand where H2S concentrates on the network.
What you receive
- ✓An exposure and readiness summary authored for New South Wales — the WHS Act 2011 (NSW), the WHS Regulation 2025 (NSW) Part 7.1 hazardous chemicals and Part 4.3 confined spaces, and SafeWork NSW as regulator.
- ✓Section 1 — Why H2S is different from other workplace gases: continuous generation by your own assets so it cannot be eliminated by substitution, odour failing as a warning, heavier-than-air pooling, and the pattern of killing rescuers.
- ✓Section 2 — Health effects by concentration: an indicative table from the 0.01-1.5 ppm odour threshold, through irritation at 20-50 ppm, IDLH at approximately 100 ppm, olfactory paralysis at 100-150 ppm, pulmonary oedema risk at 200-300 ppm, knockdown at 500-700 ppm, to immediate collapse above 1000 ppm — with the gap between losing the smell and losing consciousness called out explicitly.
- ✓Section 3 — The current legal position: a table of the 10 ppm TWA and 15 ppm STEL and the date they apply until, the Part 7.1 duty, why both limits apply at once, and why the standard is a ceiling rather than a target.
- ✓Section 4 — What is changing on 1 December 2026: the WEL list replacing the WES list, the transition ending 30 November 2026, the unresolved H2S value including the 1 ppm TWA / 5 ppm STEL draft consideration, international comparison, and an honest PCBU position on planning against a number not yet decided.
- ✓Section 5 — Where your exposure actually is: a location table covering rising main discharge points, wet wells, maintenance holes downstream of a rising main, drop structures and junctions, inlet works, digesters and biogas structures and sludge holding, plus the generation factors that make it worse.
- ✓Section 6 — Your monitoring obligation: the distinction between Part 4.3 pre-entry and during-entry atmospheric testing and personal exposure monitoring by a competent person, and why the two are not substitutes.
- ✓Section 7 — A pre-1 December 2026 action checklist with Yes / N/A columns, covering task identification, personal monitoring, assessment against both current and lower values, hierarchy review, gas detection verification, alarm set points, trade waste profile, non-entry rescue drilling, Section 47 consultation and monitoring Safe Work Australia for the final value; plus Section 8, a blank exposure assessment record table (date, task/location, TWA result, STEL result, assessed by), and a references list.
- ✓Microsoft Word (.docx) format, unbranded, editable, with end-user fill fields — supplied so your organisation owns and completes it for your own assets.
Worked example
A regional NSW council is asked by its board whether the December 2026 exposure limit change affects it. The WHS manager says no: the crews carry four-gas detectors, the alarms have never sounded on the 10 ppm set point, and nobody has been hurt. That answer contains three errors and no data. First, the detectors are set to the current standard, so a value that has been under review — with 1 ppm TWA and 5 ppm STEL considered — would put routine atmospheres the council has never recorded into exceedance overnight. Second, an instrument that protects an entrant at the moment of entry says nothing about shift exposure against a TWA or a STEL; the council has confined space testing records and zero exposure assessments, so under Part 7.1 it cannot say on reasonable grounds that anyone is below the standard, only that no alarm went off. Third, its highest exposures are almost certainly not where it thinks: the peak is at the rising main discharge points, where turbulence strips dissolved sulfide out of solution, and a new high-strength organic trade waste discharger upstream has quietly changed the sulfide picture across the whole downstream system. Then a crew opens a maintenance hole below that rising main, smells nothing, and reports the space as clean — the reading nobody questions, because the crew does not know that the smell stopping is the signal, not the all-clear. When SafeWork NSW asks how the council knows its workers are below the exposure standard, the honest answer is that it does not, and it has had until 30 November 2026 to find out.
Related legislation
- Work Health and Safety Act 2011 (NSW) — Section 19 primary duty of care; Section 47 consultation with workers on the H2S risk and controls; Sections 35-38 notifiable incidents (an H2S exposure or asphyxiation is notifiable).
- Work Health and Safety Regulation 2025 (NSW) — Part 7.1 hazardous chemicals: the duty that no person at the workplace is exposed to an airborne concentration above the workplace exposure standard, and air monitoring where exposure is uncertain.
- Work Health and Safety Regulation 2025 (NSW) — Part 4.3 confined spaces: atmospheric testing before and during entry, applying to every confined space entry independently of construction status.
- Work Health and Safety Regulation 2025 (NSW) — Section 291 (high risk construction work) and Section 299 (preparation and content of a SWMS), with review under Section 302, where wastewater confined space entry is carried out as construction work.
- Safe Work Australia — Workplace exposure standards for airborne contaminants (H2S 10 ppm TWA / 15 ppm STEL, applying until 30 November 2026) and the Workplace exposure limits for airborne contaminants list applying from 1 December 2026; AS 2865 (confined spaces), AS/NZS 1715 and 1716 (respiratory protective equipment) and AS/NZS 60079.29.2 (gas detectors).
Frequently asked questions
Does buying this document make us compliant?
No. This is a documentation set, not compliance, and the document says so itself in its own scope statement: it is an awareness and readiness summary to support planning and consultation, it is not an exposure assessment, and it does not establish your compliance position. Only monitoring data from your own tasks and assets can do that. What you get is the exposure position set out accurately for New South Wales — the current values, what changes on 1 December 2026, where H2S actually concentrates on a wastewater system, and a checklist of the steps to take before the deadline. Where exposure is uncertain, you still need to engage a competent person to carry out personal exposure monitoring. The document tells you what to do; it does not do it for you.
Our gas detectors have never alarmed. Doesn't that prove we're below the standard?
No, and the document draws this distinction deliberately. Atmospheric testing before and during a confined space entry is a Part 4.3 duty and answers one question: is it safe to enter right now. Personal exposure monitoring is a different exercise, carried out by a competent person, and establishes what a worker is actually exposed to across a shift measured against the 8-hour time-weighted average and the 15-minute short-term exposure limit. A detector that never alarmed tells you no alarm sounded at its set point. It does not tell you where your shift exposure sits against the TWA or the STEL, and it is not the evidence a regulator is asking for. The two are not substitutes: a gas monitor protects the entrant, an exposure assessment answers the regulator.
What exactly happens to the H2S limit on 1 December 2026?
The national workplace exposure standards list becomes the Workplace Exposure Limit list from 1 December 2026, with the harmonised transition period ending 30 November 2026. The rename is deliberate — a limit is a value that must not be exceeded. The H2S value itself is not settled. Safe Work Australia's draft revision considered a TWA of 1 ppm and a STEL of 5 ppm based on ACGIH recommendations, with the final Australian value pending a regulatory impact assessment; the currently published values of 10 ppm TWA and 15 ppm STEL remain unchanged in the meantime. The document is explicit that you should not plan on a number that has not been decided. Because routine wastewater atmospheres frequently exceed 1 ppm, water and wastewater is the sector most exposed if the limit drops that far — which is exactly why establishing exposure data now is worth doing regardless of where the value lands.
Why does the document say our worst exposure probably isn't at the treatment plant?
Because H2S is not evenly distributed across a wastewater system, and the section on where exposure actually sits names rising main discharge points as the single most under-appreciated hot spot: turbulence at the discharge strips dissolved sulfide out of solution, so the peak is often there rather than at the plant. Maintenance holes downstream of a rising main get the same stripping effect, in a chamber a crew opens from a public street. Wet wells are the classic fatality location because the gas is heavier than air and accumulates at the base. Inlet works are typically the worst location on the plant, and digesters are a different hazard class entirely. Generation is driven up by warm sewage, long rising main detention, flat terrain, low flows and high-strength organic trade waste — so a single significant trade waste discharger changes the sulfide picture across the whole downstream system. Targeting monitoring at those locations gives a far better picture than sampling at random.