Hydrogen Sulfide (H2S)
Hydrogen sulfide is a
colorless, flammable, extremely hazardous gas with a “rotten egg” smell.
Some common names for the gas include sewer gas, stink damp, swamp gas and
manure gas. It occurs naturally in crude petroleum, natural gas, and hot
springs. In addition, hydrogen sulfide is produced by bacterial
breakdown of organic materials and human and animal wastes (e.g.,
sewage).
Industrial activities
that can produce the gas include petroleum/natural gas drilling and
refining, wastewater treatment, coke ovens, tanneries, and kraft paper
mills. Hydrogen sulfide can also exist as a liquid compressed gas.
Hazardous properties of H2S gas
Hydrogen sulfide is
heavier than air and may travel along the ground. It collects in
low-lying and enclosed, poorly-ventilated areas such as basements,
manholes, sewer lines, underground telephone vaults and manure pits.
For work within confined
spaces, use appropriate procedures for identifying
hazards, monitoring and entering confined spaces.
The primary route of
exposure is inhalation and the gas is rapidly absorbed by the lungs.
Absorption through the
skin is minimal. People can smell the “rotten egg” odor of hydrogen
sulfide at low concentrations in air. However, with continuous low-level
exposure, or at high concentrations, a person loses his/her ability
to smell the gas even though it is still present (olfactory fatigue).
This can happen very
rapidly and at high concentrations, the ability to smell the gas can
be lost instantaneously. Therefore, DO NOT rely on your sense of smell to
indicate the continuing presence of hydrogen sulfide or to warn of
hazardous concentrations.
In addition, hydrogen
sulfide is a highly flammable gas and gas/air mixtures can be explosive.
It may travel to sources
of ignition and flash back. If ignited, the gas burns to
produce toxic vapors and gases, such as sulfur dioxide.
Contact with liquid
hydrogen sulfide causes frostbite. If clothing becomes wet with
the liquid, avoid ignition sources, remove the clothing and isolate
it in a safe area to allow the liquid to evaporate.
Health effects of H2S exposure
Hydrogen sulfide is both
an irritant and a chemical asphyxiant with effects on both oxygen
utilization and the central nervous system. Its health effects can vary
depending on the level and duration of exposure.
Repeated exposure can
result in health effects occurring at levels that were previously
tolerated
without any
effect. Low concentrations irritate the eyes, nose, throat and
respiratory system (e.g., burning/
tearing of eyes, cough,
shortness of breath).
Asthmatics may
experience breathing difficulties. The effects can be delayed for
several hours, or sometimes several days, when working in low-level
concentrations. Repeated or prolonged exposures may cause
eye inflammation, headache, fatigue, irritability, insomnia, digestive
disturbances and weight
loss.
Moderate concentrations
can cause more severe eye and respiratory irritation
(including coughing, difficulty breathing, accumulation of fluid in
the lungs), headache, dizziness, nausea, vomiting, staggering and
excitability.
High concentrations can cause shock, convulsions, inability to breathe,
extremely rapid unconsciousness, coma and death. Effects can occur
within a few breaths, and possibly a single breath.
Protection against H2S exposure
Before entering areas
where hydrogen sulfide may be present:
1. Air must be tested
for the presence and concentration of hydrogen sulfide by a qualified
person using air monitoring
equipment, such as
hydrogen sulfide detector tubes or a multi-gas meter that detects the
gas. Testing should also determine if fire/ explosion precautions are
necessary.
2. If the gas is present, the space/area must be ventilated continually to
remove the
gas.
3. If the gas cannot be removed, the person entering the space/area must
use appropriate respiratory protection and any other necessary
personal protective equipment, rescue and communication
equipment.
OSHA’s Confined Spaces
standard contains specific requirements for identifying, monitoring
and entering confined spaces.
Entering dangerous H2S atmospheres
A level of H2S gas at or
above 100 ppm is Immediately Dangerous to Life and Health (IDLH).
Entry into IDLH atmospheres can only be made using: 1) a full facepiece
pressure demand self-contained breathing apparatus (SCBA) with a
minimum service life of thirty minutes, or 2) a combination full
facepiece pressure demand supplied-air respirator with
an auxiliary
self-contained air supply.
If H2S levels are below
100 ppm, an air-purifying respirator may be used, assuming the filter
cartridge/canister is appropriate for hydrogen sulfide. A full facepiece
respirator will prevent eye irritation.
If air concentrations
are elevated, eye irritation may become a serious issue. If a
halfmask respirator is used, tight fitting goggles must also be
used. Workers in areas containing hydrogen sulfide must be monitored
for signs of overexposure.
NEVER attempt a rescue in an area that may contain hydrogen sulfide
without using appropriate respiratory protection and without being
trained to perform such a rescue.
