Hydrogen sulphide (H2S), otherwise known as hydro-sulfuric acid, sulphuretted hydrogen, stink damp, dihydrogen monosulfide, sulfur hydride, hepatic gas, sewer gas, is a naturally occurring gas. A by-product of decomposition, it is found in natural gas, crude petroleum, volcanic gas and hot springs.There are anthropogenic sources of H2S such as food processing, pulp & paper mills, tanneries, wastewater treatment plants, petroleum refineries and solid waste disposal. The industries most frequently associated with hydrogen sulfide exposure are oil & gas (from extraction to refining), wastewater treatment, and pulp and paper.
H2S is a highly toxic, poisonous, irritating, flammable, colourless gas with a characteristic rotten egg odour. At high concentrations H2S has a sweet odour but it may be undetectable by humans at those levels due to olfactory fatigue (may occur at approx. 100 µL/L with a 2 – 15 min exposure) or olfactory paralysis (approx. 150 µL/L). As H2S is slightly heavier than air (specific gravity of 1.189) it may accumulate in low-lying, unventilated areas.
Despite being able to smell H2S at low concentration levels, the most minimal impact on humans (possible bronchial irritation in asthmatics) is not until approximately 500 times the odour detection limit for a duration of 30 minutes or more.
It is imperitive that proper gas detection equipment is used rather than relying solely on sense of smell. A single or multiple-gas detector for personnel can be carried by the individual and/or as a fixed system for area monitoring. If a hazardous volume of H2S is detected and workers must remain in the area a self contained breathing apparatus must be used. Regardless of the type of gas detector or monitor used, routine maintenance, regular calibration and bump testing are required to ensure instruments are working properly.
Explosive limit of H2S in air ranges from 4.3 to 46% (43,000 µL/L to 460,000 µL/L). In air at Patm, H2S has and autoignition temperature of 260˚C (US)/ 270 ˚C (EUR) and a reactivity rating of O (scale of 0-4) (Source: Centers for Disease Control and Prevention [CDC], 2000).
This concentration level is much higher than the PEL and, despite the high flammability of the gas, it is therefore not the primary hazard associated with H2S.
Respiratory stress and/or death
Humans can be exposed to H2S via respiration or contact with skin or eyes. Given that H2S is naturally occcurring in small amounts within the human body, any absorbed H2S will be quickly metabolized by the body and excreted. Due to insufficient data, it has not been shown that prolonged, low level exposure causes adverse health affects.
Acute exposure can cause varying symptoms depending on concentration of H2S and the health of the person affected (ie: asthmatics). At high concentrations, H2S blocks the oxidation process of tissue cells, reduces the oxygen-carrying capacity of blood, depresses the central nervous system, causes respiratory failure and asphyxiation. At acute, lower-level concentrations it causes nausea, fatigue, loss of appetite, headaches, irritation.
Health Effects on Humans:
Chart source: WHO 2003
H2S Concentration (µL/L)
No observed adverse effect level
Bronchial constriction in asthmatic individuals
4 – 21
Fatigue, loss of appetite, headache, irritability, poor memory, dizziness
Death (likely a result of respiratory failure/arrest)
Occupational Health Exposure Standards:
Source: OSHA & American National Standards Institute (ANSI Standard No. Z37.2-1972)
Permissible Exposure Limit:
General Industry Ceiling Limit: 20 ppm
General Industry Peak Limit: 50 ppm (up to 10 minutes if no other exposure during shift)
Construction 8-hour Limit: 10 ppm
Shipyard 8-hour limit: 10 ppm
Immediately Dangerous to Life and Health: 100 ppm/ 30 mins
Recommended Exposure Limit (10 min ceiling): 10 ppm
Immediately Dangerous to Life and Health: 100 ppm
Short Term Exposure Limit: 5 ppm / 15 min.