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Cashins & Associates Blog

Industrial Hygiene - Danger: Hydrogen Sulfide Gas

Posted by Eileen Watkins on Fri, Jun, 27, 2014 @ 09:06 AM

Hydrogen_SulfideEach day, countless workers in the construction, utilities, wastewater, and other industries are exposed to hydrogen sulfide gas.  Also known as H2S, sewer gas, and swamp gas, this compound is best known for its foul, rotten egg odor.  It turns out that this characteristic pales in comparison to hydrogen sulfide's other properties.

H2S is a naturally-occurring gas that is formed when living matter decays, especially in anaerobic environments (no free oxygen).  It can also be found in natural gas, volcanic gas, and in some well water.  Hydrogen sulfide is heavier than air and tends to settle in confined spaces such as pits, sumps, sewers and septic or manure tanks. 

Some people mistakenly believe that H2S is safe because it is a naturally-occurring substance.  This is not the case - Mother Nature has done an impressive job of creating many highly toxic compounds.  Take asbestos, anthrax, and botulinum, for example.  These naturally-occurring substances can cause serious illnesses and even death. Hydrogen sulfide is part of this group of naturally-occurring, yet harmful substances.

H2S is a strong eye and respiratory tract irritant.  Lengthy exposures at concentrations in the 2 to 5 part per million (ppm) range can cause tearing of the eyes, nausea, and headaches.  Dizziness, fatigue, and irritability are symptoms associated with exposures at the 20 ppm level.  Exposures at higher concentrations cause more serious symptoms such as apnea (low respiratory rate or no breathing) and rapid unconsciousness. 

NIOSH (National Institute of Occupational Safety and Health) gives hydrogen sulfide an IDLH value of 100.  This means that exposures to 100 ppm of H2S are immediately dangerous to life and health.  In addition to these serious health concerns, hydrogen sulfide is very flammable and explosive.  In fact, both HMIS (Hazardous Materials Identification System) and NFPA (National Fire Protection Association) gave H2S a flammability rating of 4, which is the highest rating.  These groups also gave it a health rating of 4, which is also the most severe rating.

We've already mentioned that hydrogen sulfide smells like rotten eggs.  OSHA reports that while most individuals can first detect H2S at concentrations in the 0.01 to 1.5 ppm range, they describe it as offensive at concentrations between 3 and 5 ppm.  Unfortunately, a phenomenon called olfactory fatigue occurs around the IDLH concentration of 100 ppm.  In other words, individuals exposed to high concentrations of H2S have a diminished ability to smell hydrogen sulfide or they lose the ability to smell it at all.  Olfactory fatigue can give exposed individuals a dangerous sense of false security.

It's important to keep exposures to hydrogen sulfide at acceptable levels.  At a minimum, construction workers must abide by OSHA's Permissible Exposure Limit of 10 ppm (expressed as an 8-hour, time-weighted average).  General industry workers are required to comply with OSHA's Ceiling Limit of 20 ppm and Peak Limit of 50 ppm.  The ceiling limit is never to be exceeded and the peak limit is allowed for up to 10 minutes if no other H2S exposures occur during the workday.

The American Conference of Governmental Industrial Hygienists (ACGIH) has established a Threshold Limit Value (TLV) of 1 ppm and a 15 minute Short Term Exposure Limit of 5 ppm. These levels were set as there were no observable effects on animals at 1 ppm of exposure. Studies have shown that at a 10 ppm exposure to H2S, oxygen uptake is reduced in humans. Considering the known toxicity and physiological effects that H2S has on the body exposure should be minimized as much as possible on the job.  

Work safely when hydrogen sulfide gas is present.  Perform air monitoring in order to understand what H2S concentrations your workers are exposed to.  Keep them below the applicable OSHA limit by ventilating the work space or implementing other engineering controls.  Use administrative controls such as limiting the amount of time that workers are allowed in hydrogen sulfide atmospheres in order to keep their exposures at safe levels.  Wear respirators if exposures cannot be controlled through other means.  Establish safe work practices such as using non-sparking and explosion-proof tools in order to reduce the risk of fire and explosion.

Do you need help evaluating hydrogen sulfide exposures?  Are you complying with the OSHA limits for H2S?  Cashins can help you reduce the risk of accidents and injuries associated with hydrogen sulfide and other workplace hazards.  Contact us today!

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Topics: industrial hygiene

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