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Hydrogen Sulfide (H2S) Gas Safety

Hydrogen sulfide is a colorless, flammable, extremely
hazardous gas with a “rotten egg” smell. It occurs naturally
in crude petroleum and natural gas, and can be produced
by the breakdown of organic matter  in the absence of oxygen gas and human/
animal wastes (e.g., sewage). It is heavier than air and
can collect in low-lying and enclosed, poorly ventilated
areas such as basements, manholes, sewer lines and
underground telephone/electrical vaults.
Skeletal formula of hydrogen sulfide with two dimensionsBall-and-stick model of hydrogen sulfideExtremely Flammable F+ Very Toxic T+ Dangerous for the Environment (Nature) N
Detection by Smell
• Can be smelled at low levels, but with continuous lowlevel
exposure or at higher concentrations you lose your
ability to smell the gas even though it is still present.
At high concentrations – your ability to smell the gas
can be lost instantly.
• DO NOT depend on your sense of smell for indicating
the continuing presence of this gas or for warning of
hazardous concentrations.
Health Effects
Health effects vary with how long, and at what level, you
are exposed. Asthmatics may be at greater risk.

Low concentrations – irritation of eyes, nose, throat, or
respiratory system; effects can be delayed.
Moderate concentrations – more severe eye and respiratory
effects, headache, dizziness, nausea, coughing,
vomiting and difficulty breathing.
High concentrations – shock, convulsions, unable to
breathe, coma, death; effects can be extremely rapid
(within a few breaths).
Before Entering Areas with
Possible Hydrogen Sulfide
• The air needs to be tested for the presence and concentration
of hydrogen sulfide by a qualified person
using test equipment. This individual also determines
if fire/explosion precautions are necessary.
• If gas is present, the space should be ventilated.
• If the gas cannot be removed, use appropriate respiratory
protection and any other necessary personal protective
equipment (PPE), rescue and communication
equipment. Atmospheres containing high concentrations
(greater than 100 ppm) are considered immediately
dangerous to life and health (IDLH) and a selfcontained
breathing apparatus (SCBA) is required.

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