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.