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The Dangers of Hydrogen Sulfide Gas (H2S)

In the city of Portage La Prairie in April of 1969, two CUPE members and a city official went to the sewage lagoons to check a valve on a feeder line. One member of the crew entered the valve chamber and collapsed. A second member went in to rescue him and also collapsed. The third member summoned help. By the time the fire department arrived both workers were dead. Air samples taken at the time showed hydrogen sulfide levels in excess of the measuring instrument’s upper limits.

There have been numerous workers killed through exposure to this gas in many different occupational settings. Hydrogen sulfide (Hydrogen sulphide)  is a poisonous gas and the leading cause of death through gas inhalation in the workplace. One whiff of a sufficiently high concentration can cause death.

What is Hydrogen sulfide?

Hydrogen sulfide (H2S) is a colourless gas which at lower concentrations has a distinctive rotten-egg odour. Most exposure to it occurs by inhalation. Hydrogen sulfide is generated as a by-product in many industrial processes or by the decomposition of organic (previously living) matter. It is slightly heavier than air and is therefore especially dangerous in low-lying areas and confined or enclosed workspaces. At high temperatures (260 degrees Celsius, 500 degrees Fahrenheit) hydrogen sulfide reacts explosively.

Hydrogen sulfide goes by a number of names including: hydrogen sulphide ,dihydrogen sulfide, sulphur hydribe, hydrosulphuric acid, sulfuretted hydrogen and hepatic gas. Its common names are stink damp, sour gas or sewer gas.

Deadly Poison

Hydrogen sulfide is a deadly poison. When inhaled it goes through the lungs and enters the bloodstream. To protect itself the body tries to break down the hydrogen sulfide as fast as possible into a harmless compound. Poisoning occurs when the amount absorbed in the blood exceeds the rate at which it is eliminated.

Exposure to high concentrations of hydrogen sulfide (acute exposure) can cause immediate coma and death from respiratory failure. Hydrogen sulfide builds up in the blood, paralyzes the nerve centres in the brain which stops the lungs from working. If not caught in time, acute poisoning is deadly.

Sub-acute exposure (exposure to lower levels) may result in headaches, dizziness, loss of balance, agitation, nausea, diarrhea. Chronic poisoning (repeated exposure to low levels) may result in slowed pulse rate, fatigue, insomnia, cold sweat, eye infections, loss of weight, skin eruptions. If a worker is suffering from any of these symptoms, hydrogen sulfide may be responsible. The presence of hydrogen sulfide may be responsible. The presence of hydrogen sulfide should be suspected and tested for.

Who is at risk?

  • sewage treatment plant workers
  • sewer workers
  • workers in manholes
  • tunnel workers
  • well diggers
  • workers in chemical laboratories

Enclosed and confined spaces are the danger areas prone to the rapid buildup of hydrogen sulfide gas. A confined space is any workplace (not necessarily small) which is wholly or partially enclosed and from which immediate escape is difficult (sewers, manholes, pits, tunnels, vats, tanks, ovens, grain elevators and open-topped spaces more than 4 feet in depth). Enclosed areas (wet wells, grit chambers, pump stations’ valve chambers) although easier to get in and out of, are still as dangerous in the accumulation of deadly gas. A room or a building can be considered an enclosed space.

Levels of Exposure

Hydrogen sulfide is measured in parts per million (ppm). The American Conference of Governmental Industrial Hygienists (ACGIH) has recommended a Threshold Limit Value (TLV)-Time Wrighted Average (TWA) of 1 ppm. 1 ppm is equal to 0.0001 per cent by volume, or only one-ten thousandth of one per cent of the total air volume. This is a time-weighted average exposure for a normal eight hour work day and a 40 hour work week to which nearly all workers may be repeatedly exposed without adverse effect.

There is a Short Term Exposure Limit, the  STEL which  is a 15 minute time-weighted average exposure which should not be exceeded at any time during a work day even if the 8-hour time-weighted average is within the TLV. Exposure at STEL should not be repeated more than 4 times per day.

In many Canadian jurisdictions exposure limits are similar to the Threshold Limit Values recommended by the ACGIH.

The recommended limit of 10 ppm in Alberta does not guarantee worker safety. If should not be used as a guideline demarcating safe and dangerous concentrations of hydrogen sulfide. Because of wide variations in individual susceptibility some workers may experience problems at concentrations at or below the threshold limit.

The safest exposure to Hydrogen sulfide is no exposure at all.

Common effects of exposure

Hydrogen sulfide at low levels has a distinctive rotten-egg odour and workers mistakenly assume that the absence of smell means that they are not exposed to it. Smell is a poor warning sign of hydrogen sulfide.

At higher concentrations a sweet smell may be noted, but at even greater concentrations, hydrogen sulfide can “paralyze” the sense of smell and the ability to smell is lost. Some workers are congenitally (by birth) unable to smell hydrogen sulfide. That is why the air should always be monitored by instruments designed to detect hydrogen sulfide.

Parts per million – Effects

0.13 – This is the odour threshold. Odour is unpleasant. Sore eyes.

4.6 – Strong intense odour, but tolerable. Prolonged exposure may deaden the sense of smell.

10-20 – Causes painful eye, nose and throat irritation, headaches, fatigue, irritability, insomnia, gastrointestinal disturbance, loss of appetite, dizziness. Prolonged exposure may cause bronchitis and pneumonia.

30-100 – Sickeningly sweet smell noted.

50 – May cause muscle fatigue, inflammation and dryness of nose, throat and tubes leading to the lungs. Exposure for one hour or more at levels above 50 ppm can cause severe eye tissue damage. Long-term exposure can cause lung disease.

100-150 – Loss of smell, stinging of eyes and throat. Fatal after 8 to 48 hours of continuous exposure.

200-250 – Nervous system depression (headache, dizziness and nausea are symptoms). Prolonged exposure may cause fluid accumulation in the lungs. Fatal in 4 to 8 hours of continuous exposure.

250-600 – Pulmonary edema (lungs fill with fluid, foaming in mouth, chemical damage to lungs).

300 – May cause muscle cramps, low blood pressure and unconsciousness after 20 minutes.

300 to 500 – ppm may be fatal in 1 to 4 hours of continuous exposure.

500 – Paralyzes the respiratory system and overcomes victim almost instantaneously. Death after exposure of 30 to 60 minutes.

700 – Paralysis of the nervous system.

1000 – Immediately fatal.

Some workers may experience abnormal reflexes-dizziness, insomnia and loss of appetite that lasts for months or years. Acute poisoning which does not result in death may produce long-term symptoms such as loss of memory or depression, paralysis of facial muscles.

What can be done?

A program for worker protection

1.  Monitoring and ventilation
First, workplace air should be monitored and hydrogen sulfide should be controlled so that no worker is exposed to levels above 10 ppm or the legal limit in your jurisdiction. All areas where toxic gases are detected should be ventilated with a fresh air-blower. The ventilation system should be non–sparking and inspected every six months. Any areas with sources of gas should be closed, blanked off, locked and tagged.

Modern continuous direct-reading electronic gas monitors with strip charts or circular chart recorders, sensitive to low levels of hydrogen sulfide, should be installed permanently in key locations near the ground. Such devices should have sound alarms that are set to warn workers when 10 ppm is reached.

Portable monitors should be clipped onto the worker’s belt and carried into confined spaces as a supplement or when fixed ones are not appropriate. These should warn workers with audible alarm and coloured lights and have the capacity to be used continuously for more than 8 hours without recharging batteries.

2.  Preventing Work Practice
All entries to confined spaces should be posted with specific work procedures and safety checklist or permit system. The permit system would provide written authority for entering the area, list all potential hazards and the safety equipment necessary to ensure worker safety and identify that the workers involved have received proper current training in confined entry procedures. Safety equipment, first aid, rescue equipment and instructions should be in the immediate vicinity.

Before entering an unknown or confined area, sampling should be done with a remote monitor on a wand attached to a toxic gas meter. The monitor should reach the lowest point in the space and monitoring should continue throughout the course of work.

No one should enter a confined space without at least one standby person stationed outside who is in constant contact with the worker inside. A third person should survey the operation. Workers should be equipped with gas monitors.

A harness, safety belt and wrist lock attached to a lifeline should be worn in the event that an unconscious worker must be pulled out of a confined space. An aerosol type horn that can be blasted in case of emergency should also be worn. Rescuers should also be equipped with proper safety equipment.

3.  Respirators and Respirator Problems
A suitable breathing apparatus (one approved by the U.S. National Institute for Occupational Safety and Health – NIOSH- or Canadian Standards Association – C.S.A.- standard Z94.4) must be worn when entering a confined space. NIOSH recommends any supplied-air respirator or self-contained breathing apparatus for levels of hydrogen sulfide of 10 to 100 ppm; up to 250 ppm – any supplied-air respirator with a full facepiece. Check for current recommendations that change over time. Self contained respirators have air supplied from a cylinder worn on the back while supplied-air respirators have air supplied by a long line from a large stationary tank.

When the atmosphere is dangerous workers should wear a self-contained breathing apparatus. Air purifying filter or cartridge respirators easily leak, have a limited lifetime and should not be used when the atmosphere is dangerous. Air supplied respirators with masks hooked up to an air hose connected to a blower are not suitable for tight areas and may foul or break.

Appropriate respirators must always be used with proper fitting, be easily accessible, maintained and cleaned regularly. (See C.S.A. standard M1982 Selection, Care and Use of Respirators).

4.  Record-keeping
Air monitoring should be conducted at each location where hydrogen sulfide may be released and a survey log kept. If concentrations are below the recommended ceiling the records must contain the basis for this conclusion.

5.  Training
A comprehensive education/training program should be developed for workers which should include: knowledge of hazards from exposure; symptoms and how to deal with them; all emergency procedures (first aid, resuscitation, evacuation, meaning of alarms); training in the location of monitors, respirators. This training would be done quarterly and before a worker starts on the job. Workers should be certified as trained before being assigned to enter confined spaces. In Western Canada, the common programs on H2S are Energy Safety Canada H2S Alive and H2S Blended Renewal.  For those who want H2S awareness programs, you can enroll in a classroom or online seminar on this basic subject.

6.  Medical examinations
Workers should be given medical examinations annually and prior to starting a job. Particular attention should be paid to eyes, nervous and respiratory systems. Checkups should include the ability to use respirators and hear to and see warnings.

For more information contact:

CUPE National Health and Safety Branch
1375 St-Laurent Boulevard

Tel: (613) 237-1590
Fax: (613) 237-5508


1. Molly Coye, “Hydrogen Sulfide: A powerful and deceptive killer”, Oil, Chemical and Atomic Workers Union Health and Safety.

2. NIOSH: “A Recommended Standard for Occupational Exposure to Hydrogen Sulfide” states that “Conclusive evidence of adverse health effects from chronic exposure to hydrogen sulfide at low concentrations is lacking.” U.S. Department of Health, Education, and Welfare, National Institute for Occupational Safety and Health, p.1. In the OCAW published pamphlet “Hydrogen Sulfide: A powerful and deceptive killer”, Molly Coye, M.D., p.7, indicates that chronic poisoning may indeed occur. She notes that many of the symptoms are not specific to hydrogen sulfide and that other exposures may cause similar effects. Nevertheless hydrogen sulfide should be suspected. Publication No. 15, pp. 8-10

3. Coye, op. cit., pamphlet gives the example of the lethal concentration of 700 ppm: “700 ppm is equal to 0.07 per cent by volume, or only 7 one-hundredth of one per cent of the total air volume”. Since 700 ppm = .07 per cent = 7 one-hundredth of one per cent, therefore, 10 ppm = .001 per cent = 1 one-thousandth of one per cent.

4. The chart is based on information from the main resources listed. Part of this chart is reproduced in theOCAW pamphlet, “Hydrogen Sulfide: A powerful and deceptive killer”, p.6. Also see, “Chemical Hazard Summary”, Canadian Centre for Occupational Health and Safety, Hamilton, No.12, August 29, 1985, pp.5 and 6.

5. Key, Henchel, Buxtler, et. al., Occupational Diseases a Guide to their Recognition, U.S. Department of Health, Education, and Welfare, National Institute for Occupational Safety and Health, Washington, 1977, p.424.


ACGIH – American Conference of Governmental Industrial Hygienists
NIOSH – National Institute for Occupational Safety and Health
ppm – parts per million
TLV – Threshold Limit Value
TWA – Time Weighted Average
STEL – Short Term Exposure Level

Main references used

“Chemical Hazard Summary: Hydrogen Sulfide”, No.12, August 29, 1985, Canadian Centre for Occupational Health and Safety, Hamilton.

“Hydrogen Sulfide: A powerful and deceptive killer”, Molly Coye, Oil, Chemical and Atomic Workers International Union Publication no.15, 1977.

“The Killer: H2S”, Alberta Workers’ Health and Safety Compensation, Occupational Health and Safety Division.

Occupation Diseases: A guide to Their Recognition, Key, Henschel, Butler, et al., U.S. Department of Health, Education, and Welfare, National Institute for Occupational Safety and Health, Washington, June 1977.

NIOSH/OSHA: Occupational Health Guidelines for Chemical Hazards, National Institute for Occupational Safety and Health/Occupational Safety and Health Administration, January 1981.

“A Recommended Standard for Occupational Exposure to Hydrogen Sulfide”, U.S. Department of Health, Education, and Welfare Public Health Service, National Institute for Occupational Safety and Health, Cincinnatti.

“Risky Business: An AFSCME Health and Safety Guide for Sewer, and Sewage Treatment Plant Workers”, The American Federation of State, County and Municipal Employees, Washington, D.C.

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