H₂S Safety

Overview

H₂S is colourless, flammable, heavier than air, and deadens the sense of smell — never rely on smell.

Controls are continuous monitoring, ventilation, supplied-air apparatus for entry/rescue, and wind awareness for upwind escape.

Health effects by concentration

~10 ppm — occupational exposure limit over an 8-hour shift.
~100 ppm — IDLH region; loss of smell and danger to life.
~500–700 ppm — rapid unconsciousness within minutes.
>700–1000 ppm — immediate collapse and death.

Step-by-step procedure

Identify where H₂S may be present and include it in the permit.
Issue each worker a calibrated personal H₂S monitor with alarms.
Set alarm thresholds — low around 10 ppm, high around 15 ppm.
Ventilate the area; for confined spaces, gas-test before and during.
Check wind direction; brief the upwind escape route and muster point.
On any high alarm, stop work and evacuate upwind immediately.
Use positive-pressure SCBA / supplied air for entry/rescue — never a filter respirator at high/IDLH levels.
Run regular H₂S drills and account for everyone at muster.

Key controls

Continuous personal monitoring — never enter without a working monitor.
Supplied-air (SCBA) staged and ready for entry or rescue.
Wind socks and known upwind escape routes to a safe muster point.

Roles & responsibilities

Role	Responsibility
Worker	Wears a monitor, watches the wind, evacuates upwind on alarm.
Gas Tester / Standby	Tests the atmosphere, maintains the alarm, watches over entrants.
Rescue Team	Enters only with SCBA — never on breathable air.

Common mistakes to avoid

Relying on smell — H₂S deadens smell at deadly levels.
Rushing in to rescue without SCBA — rescuers become victims.
Using a filter respirator in a high or IDLH atmosphere.

Legal requirements (Thailand)

Occupational Safety, Health and Environment Act B.E. 2554 (2011) — the governing workplace-safety law in Thailand.
Ministerial Regulation on OSH for work in confined spaces B.E. 2562 (2019).

Frequently asked questions

What is H₂S Safety?

Hydrogen sulphide (H₂S) is a colourless, flammable gas that is heavier than air, so it collects in low areas such as pits, sumps and the bottom of vessels. At low concentrations it has a 'rotten egg' smell, but it rapidly deadens the sense of smell — so smell must never be used to judge whether it is safe.

Who is responsible?

Worker: Wears a personal monitor, watches the wind and evacuates upwind on alarm.; Gas Tester / Standby: Tests the atmosphere, maintains the alarm and watches over entrants.; Rescue Team: Enters only with SCBA — never attempts a rescue on breathable air.

What are the most common mistakes to avoid?

Relying on smell — H₂S deadens the sense of smell at the very levels that are deadly. Rushing in to rescue a collapsed colleague without SCBA — rescuers become victims. Using an air-purifying (filter) respirator in a high or IDLH atmosphere.

References — Confined-space & gas-testing procedures, IDLH data; Thai OSH Act B.E. 2554 confined-space ministerial regulation.