H2S: The Silent Killer

• By Hamed Khoshniat
• May 09, 2008

On Jan. 16, 2002, two workers were killed and eight others were injured when they inhaled hydrogen sulfide gas leaked from an underground process sewer at the Georgia-Pacific Naheola mill in Pennington, Ala. Among the injured were workers who attempted to assist their colleagues from the deadly cloud. The gas was the product of a sudden, uncontrolled chemical reaction taking place in the sewer as the men worked above.

The accident occurred in the vicinity of a tank truck unloading station at the Naheola mill, which makes products such as tissue paper, towels, and cardboard from wood pulp. Nine of the victims were employees of Burkes Construction Inc. who were working on a maintenance project near the unloading station. The 10th victim was a driver from a local trucking company. The plant’s pulp-making process involves treating wood chips with chemicals such as sodium hydrosulfide (NaSH), with NaSH solution delivered to the mill periodically by tank trucks. In the preceding 24 hours, 15 trucks had delivered NaSH to the unloading station; the main cause of the accident was losing up to 5 gallons of NaSH into a collection pit during each delivery. As the NaSH deliveries continued, several Burkes employees were working on a project which would require them to stand in the collection pit. To assist them, a mill operator opened a valve to drain the contents of the pit into the wastewater system. Unknown to the operator, the collection pit drained directly into a sewer line where sulfuric acid (H₂SO4) was being added to treat the mill effluent.

As soon as NaSH from the collection pit contacted the acidic contents of the sewer, it began reacting to form H₂S gas. H₂S gas leaked through the seal of a fiberglass manhole cover near the Burkes workers, and all 10 victims were exposed to the escaping H₂S gas. Three workers were overcome almost immediately and fell to the ground. Instead of evacuating the area, three of the remaining workers attempted to drag the fallen men to fresh air without any respiratory protective equipment. Two of them passed out in the in the course of assisting the others. Mill workers farther away saw the victims collapsing and called in emergency help. Two Burkes workers died rapidly of H₂S poisoning. Seven other Burkes employees and one driver working for another company were injured from exposure to the gas. Six country paramedics who transported the victims later reported symptoms consistent with H₂S exposure—evidently due to gas release from the victims and their clothing—but none required hospitalization.

Results of the CSB Investigation
The U.S. Chemical Safety and Hazard Investigation Board (CSB) reported the following items as the root causes of the accident:

1) The unexpected chemical reaction (between NaSH and H₂SO4) producing H₂S gas.

2) In 1995, the drain from the collection pit was connected to the sewer line, where acid was periodically added.

3) The plant did not identify the unloading station as an H₂S risk area and did not install any gas warning device.

4) There had been earlier indications of problems with the fiberglass manhole cover from which the hydrogen sulfide escaped.

5) The plant management did not heed material safety data sheet (MSDS) information about NaSH and its unexpected reactions, so workers did not know that the chemical will react with acids to generate H₂S gas and should be prevented from entering potentially acidic sewers.

6) H₂S was not identified as a hazard in the immediate area of the pulp mill where the incident occurred.

7) The victims did not know how to respond appropriately when the gas release occurred, and several attempted to aid injured co-workers without protective equipment.

8) The plant did not have an H₂S emergency response training rescue procedure for company employees and contract workers.

Protection Against H₂S
How do you protect yourself against H₂S? The selection of an appropriate respiratory protection (SCBA, escape mask, airline hose apparatus, etc.) guarantees your survival, depending on the nature of the job (e.g. doing maintenance work, coming to someone’s rescue, or urgent escape from a hazard). You must wear respiratory protection that is best suited for the purpose.

What do you need to know for safely working on site? Have at least one stand-by person qualified to perform first aid and CPR. Always take into consideration the wind direction to escape from the hazard area. Never take shortcuts. Know where the SCBA is located in your workplace. Learn how to work with an SCBA, a fire extinguisher, and a portable H₂S gas alert device. Never enter (or work) without suitable respiratory protection and an H₂S gas alert device in a hazard area.

Employ the buddy system anywhere there is the potential for sour gas to be present above the threshold limit value. Learn emergency actions to escape from the hazardous area and to assist in the recovery of H₂S victims. Recognize H₂S leakage alarms and actions to be taken. Before entering an H₂S confined area, protect yourself with appropriate PPE. Never attempt to rescue a victim with your escape mask. If you work on a vessel or equipment where there is the possibility of H₂S leakage, always wear a hose/air-line breathing apparatus with a mini-air escape cylinder.

This article originally appeared in the May 2008 issue of Occupational Health & Safety.