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Health Bulletin - Clean Air - Hydrogen Sulphide Project (H2S)

H2S - TLV-TWA[1] = 10 parts per million (ppm); STEL[2] = 15 ppm

The aim of this project is to increase awareness of the hazard of hydrogen sulphide (H2S), and to increase the level of control measures used.

H2S is a colourless gas, smelling strongly of rotten eggs at concentrations of up to 30 ppm and reportedly having a sickeningly sweet odour at concentrations of up to 100 ppm. The odour threshold is very low (approx. 0.0005 ppm) compared with the STEL2 of 15 ppm. It is extremely toxic, and the most poisonous of the naturally occurring gases. Its relative density (where air = 1) is 1.189, so it is heavier than air. It is highly flammable and forms explosive mixtures with air between 4.3 and 45.5% by volume. The product of combustion is sulphur dioxide, which is also extremely toxic. H2S is very soluble in water (up to 3 volumes of gas per volume of water), and agitating a solution of H2S will liberate the gas. It is therefore important not to disturb pools of water when it is suspected that they may contain dissolved hydrogen sulphide gas.

H2S has its major effects on the respiratory system, where its asphyxiant action is due to paralysis of the nerve centres in the brain controlling breathing. In addition to the effects on the respiratory system, low concentrations of 20 - 150 ppm cause irritation of the eyes. Slightly higher concentrations may cause irritation of the upper respiratory tract, and if exposure is prolonged, pulmonary oedema may result. The irritant action has been explained on the basis that H2S combines with the alkali present in moist surface tissues to form sodium sulphide, a caustic agent. Collapse, unconsciousness and death can occur following exposure to concentrations of just a few hundred ppm for 2 minutes, or 100 - 150 ppm for 8 to 48 hours, although individual factors and the presence of other contaminants may predispose a person at lower levels or shorter exposure times.

H2S is an insidious poison since the sense of smell may become fatigued. The odour and irritating effects do not offer dependable warning to workers who may be exposed to gradually increasing amounts. This so-called "olfactory fatigue" occurs at about 50 ppm, and can develop within 10 minutes at about 100 ppm. Olfactory fatigue can lead a person who is exposed to the gas to believe that its concentration is decreasing, when it is actually increasing. The only reliable method for detecting hydrogen sulphide is a calibrated gas detector.

Sources of hydrogen sulphide

H2S is a potential hazard in stagnant water, tanneries, tanning abattoirs, mining, oil refineries and production, pulp mills, metal processing, brewing, sewage treatment and the fishing industry. It is also present as a natural gas in the Rotorua area, and is responsible for the rotten egg smell found in that region. It is formed by the decomposition of plant and animal matter by bacteria in an oxygen-deficient environment. Confined spaces tend to lead to the accumulation of the gas.

Hydrogen sulphide is often encountered in large amounts in most geothermal areas in New Zealand. On reaching the surface, most of the gas is released from the geothermal fluid along with steam at boiling temperature. At lower temperatures, much of the H2S gas remains dissolved in the geothermal fluid. Where geothermal energy is used, H2S gas is removed from the geothermal fluid and is vented at a safe height above ground for dispersal by wind. On still, foggy days, H2S tends to accumulate in low places in dangerous concentrations.

Control Measures

Adequate ventilation remains the most effective control measure for hydrogen sulphide. Ventilation able to capture the gas at source is the best option. When a person is required to enter a confined space, the space should be cleared of contaminating hydrogen sulphide gas by the introduction of mechanical ventilation. No person should enter the area without continuous mechanical ventilation and the use of a calibrated gas detector, indicating that the area is safe to enter. Where this is not practicable, personal protective equipment should be worn, as detailed below.

Personal Protective Equipment

Where it is necessary to enter an area of known high H2S concentration, or where the concentration is unknown, and ventilation is impracticable, self-contained breathing apparatus or an air-line respirator must be used. Air-purifying respirators are not appropriate for high or unknown concentrations of toxic gases.

Eye protection, providing a gas seal, should also be worn where a person needs to work in an area with significant levels of H2S present. In many cases appropriate respiratory protection incorporates appropriate eye protection.

Case studies - Hydrogen sulphide

  1. In 2001, a worker descended into a 25,000 L plastic tank containing a chicken manure and straw mixture, used as a substrate for culturing mushrooms. He took with him a high pressure hose, intending to clean down the inside of the tank. After several minutes, a colleague climbed up the ladder that had been placed against the outside, but couldn't see the other worker inside due to the darkness. When he shouted his name and received no response, he decided to go inside to see what the problem was. Luckily, just before going in, one of the managers had walked past so the second worker told him of his intention to go into the tank because the other worker had fallen. When he reached the bottom of the ladder that had been lowered into the tank for access, the second worker found his colleague lying in the effluent. He attempted to rescue him, but he himself collapsed as he started to lift him back up the ladder. The manager had by this stage climbed up the outer ladder to see what was going on, and noticed that both men were lying in the bottom of the tank. The manager then decided to go in, but no sooner was he in than he started to feel unwell, so he retraced his steps, alerted his brother and between them they cut an access hole in the side of the tank and rescued the two men. The man who had been in the tank the shorter period of time began breathing for himself almost immediately upon rescue. The other man did not commence breathing for himself until the ambulance arrived a significant time later. Both were evacuated to hospital by helicopter, where they each spent approximately one week. Both were off work for a month, and one had problems with his sight for some months. Measurements taken the following day in an identical tank showed that hydrogen sulphide was present at 82 ppm, and oxygen was present at a normal concentration of 20.9%. It is likely, though not confirmed, that carbon dioxide was also present at an elevated level. The concentration of hydrogen sulphide in the tank where the two men were working is unknown, but likely to have been of the order of several hundred parts per million, given the speed at which they became unconscious. Pathological blood analysis confirmed that high levels of H2S had been present. When the sampling tube was first lowered into the tank, no hydrogen sulphide was apparent, but after stirring up the contents using a high pressure hose, and taking further measurements, the 82 ppm value was obtained. This illustrates the ability of this gas to be released from solution upon agitation.
  2. A contractor was engaged to repair a pump situated at the bottom of a 12 metre deep well used for garden reticulation at a manufacturing workplace. Another contractor was engaged to test the atmosphere and provide emergency back-up and first aid, in line with confined space procedures. The pump contractor was wearing a harness, attached to a frame and pulley system. The atmospheric testing initially indicated that the atmosphere was safe. The pump contractor worked on the pump for about an hour, in visual and voice contact with several people above. It was then decided to remove the pump. The reticulation pipe was disconnected and allowed to drain back into the well. The people above noticed a rotten egg smell (hydrogen sulphide gas), and mentioned it to the pump contractor, who said it was normal. He was then observed to lose consciousness. He was immediately pulled to the surface (in about 10 seconds) and given oxygen, after which he regained consciousness. The atmosphere in the well was re-tested and found to contain levels of hydrogen sulphide significantly above the occupational exposure standards of 10 ppm (8h TLV-TWA) and 15 ppm (15 min STEL). It was considered that non-operation of the pump had allowed large volumes of hydrogen sulphide to build up in the pipes due to breakdown of organic matter, and the gas entered the well when the pipes were disconnected. Blowing air into the well, or the use of an air-line respirator would have prevented this accident, as would the use of a personal or multi-gas monitor. (Source: Government of Western Australia, Department Consumer and Employment Protection)
  3. Three men died in February 1999 in an oxygen-deficient tidal manhole in a 300 mm sewer pipeline in Auckland. It is believed that one of the men entered the manhole, to remedy an apparent blockage, and collapsed shortly afterwards. The second man entered to effect a rescue, but it appears that he too, got into difficulty, and the third man then entered to assist him. The job of removing such blockages was normally done from above ground using a water blasting technique. It was speculated, based on the evidence available, that they experienced some difficulty in manipulating the hose into the blocked pipeline, from above. There were three separate companies involved in the operation - all three had written policies in place at the time of the accident which were consistent with the standard for confined space work, but safety equipment had been left back at one of the business's depots. Because there was insufficient gear to clear the blockage, it was agreed that the work would not be done that day. However it appears that for some reason, one of the three men remaining on site decided to descend into the manhole. Testing of the atmosphere was undertaken some five days after the event. This revealed an atmosphere that was severely deficient in oxygen, and containing other gases including hydrogen sulphide, carbon dioxide, carbon monoxide and methane. The pattern of readings suggested that less oxygen was present at high tide. The pathology report concluded that death resulted from hydrogen sulphide intoxication whilst working in a sewer.
  4. In the UK in 2001, a road tanker load (approx. 20 tonnes) of waste alkali solution was transferred to a treatment tank where it was mixed with waste acids to control the pH level. During the process, the acid reacted with the polysulphide contaminants in the waste alkali, releasing an estimated 183m3 of hydrogen sulphide gas. The gas escaped from the tank and settled at ground level. Emergency services attended the scene where they remained for approximately 8 hours. There was 1 fatality from asphyxiation and 3 other injuries to employees who were all taken to hospital suffering from effects of exposure to the gas.
  5. An employee of an Australian dairy company was temporarily affected by hydrogen sulphide and methane gas while cleaning a confined space inside a waste dairy tank. The company was convicted for breaches of the Regulations.
  6. A 65-year-old dairy farmer, his two sons, a grandson and a nephew died when they entered a manure pit containing an oxygen-deficient atmosphere. On the day of the accident, one of the sons entered the pit to repair the agitator shaft. He collapsed, so the nephew yelled to his 8-year-old brother to get help. Before help arrived, each of the victims entered the pit to help those who had been overcome and each was overcome in turn. All five victims died of asphyxiation due to the lack of oxygen. The decomposing manure had not been disturbed for several days and would have been releasing flammable and highly toxic gases such as methane and hydrogen sulphide. Both these gases would have displaced the oxygen.
  7. An employee of a contractor was overcome by hydrogen sulphide when he was carrying out maintenance work at a small wastewater treatment plant. He had been working near the opened top of an influent chamber, measuring approximately 2m in diameter and 5m deep, but when a piece of pipe he was working with fell into the bottom of the chamber, he climbed down a ladder to retrieve it. The victim lost consciousness just as he was about to come back up. He was pulled from the chamber approximately an hour after he collapsed, however had been administered oxygen from 15 minutes after he collapsed. Hydrogen sulphide levels taken when inspectors arrived exceeded the WES values, however the plant had been restarted by this time. In a simulation exercise carried out some time later, a reading of 1070 ppm was obtained at the 4.5 m level - approximately the position of the victim when he was rescued. He spent 4 days in hospital, 2 of which he was maintained in a drug-induced coma. Several months after the accident, the victim was reported as making a reasonably good recovery, however there were still some outstanding health issues to be resolved.

Workplaces Hydrogen sulphide

  • Sewage treatment - when organic matter is broken down, especially in an oxygen-deficient atmosphere, H2S is likely to be present.
  • Farming - offal pits and effluent tanks will often have very high levels of H2S present, as well as oxygen-deficient atmospheres.
  • Geothermal - in New Zealand most geothermal areas are rich in H2S, which usually "flashes off" to the atmosphere when it reaches the surface along with steam, however on still, foggy days, it tends to accumulate in low places in dangerous concentrations. H2S should also be considered when inspecting confined spaces in a geothermal-rich area.
  • Tanneries - H2S is a by-product of the pickling part of the process. The residual sulphides and hydrosulphides, remaining after the dehairing process, give rise to H2S when they react with sulphuric acid used in the pickling. Ventilation to remove H2S at source, or process modification using peroxide to oxidise residual sulphide to sulphur are control measures available.
  • Fishing industry - accumulated decaying fish will produce H2S, especially when in an oxygen-deficient atmosphere.
  • Abattoirs - Treatment of waste-water from the cleaning up of slaughtered animals can lead to the production of H2S.

Footnotes:

1 - TLV - TWA = Threshold Limit Value - Time Weighted Average = an 8 hour exposure limited designed to protect the worker from the effects of long-term exposure.

2 - STEL = Short-Term Exposure Limit = the 15 minute average exposure standard, designed to protect the worker against adverse effects of irritation, chronic or irreversible tissue change, or narcosis that may increase the likelihood of accidents.

Issued by the Department of Labour, New Zealand
http://www.osh.dol.govt.nz

No. 26 - January 2007