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Hazard recognition plays a vital role in keeping employees safe. Some hazards are easily recognized, for example an employee climbing up a 20-ft ladder while holding tools in both hands is an obvious fall hazard. While some safety hazards are immediately recognizable, others require training to spot and avoid. One such hazard is hydrogen sulfide (H2S). Training is a key method to avoid the "ostrich zone." You do not want to bury your head when facing this hazard.

Analyzing H2S

H2S is a clear gas that is dangerous to human health/life, extremely flammable, corrosive and sometimes has a detectable odor of rotten eggs. It can be produced in bogs, swamps, volcanoes and hot springs (EPA, 1993). It can also be produced by industrial activities (e.g., oil and gas, natural gas pipeline transmission, refineries, sewage treatment plants, manholes, manure pits). Whether its source is natural or industrial, H2S is the result of bacterial decomposition of organic material.

The Hazard

A very low-level H2S exposure can result in serious illness or death. The NIOSH value for immediately dangerous to life or health (IDLH) is 100 ppm, and 1% of H2S = 10,000 ppm.

In addition, this hazard occurs where one might not expect. For example, a 25-year-old waste hauling service worker died in an underground manure waste pit. The access opening was fitted with a removable stainless steel cover. There was no ventilation or gas monitor in use [OSHA(b)]. NIOSH (2012) lists multiple instances of H2S-related fatalities occurring on farms and sanitation facilities involving enclosed/confined spaces harboring this deadly gas. In none of these cases was a gas monitor used to indicate the presence of this gas.

Why So Deadly?

H2S is an inhalation, flammable, explosive and contact hazard. Inhalation symptoms at low-level exposures (20 ppm) include headache, dizziness, sleep disturbances, upset stomach and changes in appetite. Symptoms include altered breathing and drowsiness after 15 to 30 minutes at this level. Death may occur after 48 hours and exposures ranging to 500 to 700 ppm will likely result in staggering, collapse and death after 30 to 60 minutes. A level of 1,000 to 2,000 ppm would result in nearly instant death [OSHA(c)].

H2S is also corrosive and will act on skin and eyes. Exposure levels of 50 to 100 ppm will cause eye irritation and marked conjunctivitis, and respiratory tract irritation after an hour at 200 to 300 ppm [OSHA(c)]. Acute exposure can cause painful conjunctivitis, sensitivity to light and corneal abrasions (CCOHS, 2013). H2S is classified as a chemical asphyxiate similar to carbon monoxide and cyanide gasses. It inhibits cellular respiration uptake of oxygen causing biochemical suffocation (Kalusche).

Acute exposure affects the nervous and pulmonary systems, olfactory nerves, lungs, brain, respiratory control center and eyes. Symptoms include nausea, burning sensation in the eyes, coughing, dizziness, difficulty breathing, fluid accumulation in the lungs, headache, vomiting, staggering and excitability. The symptoms displayed depend on exposure level and length, and a worker's physical condition. Once any of these symptoms is exhibited, a dangerous exposure has already occurred.

Protecting Against H2S

Training is key to recognizing this hazard and protecting against it. Training levels are dependent on the action expected of the employee/trainee. For example, if a person's work will rarely result in H2S exposure and if the employee is expected to immediately evacuate a site upon H2S detection, awareness training and training on use and care of a gas monitor is likely adequate.

If the employee works in a setting where H2S is likely, then training must address more advanced topics, including special evacuation criteria; use of respiratory PPE and gas monitor use/care; first aid; expectations of employees in an exposure/release event (e.g., evacuate only or rescue); nature of work (confined space); exercises and drills; schedule for refresher training (annual); site-specific safe work practices; properties/characteristics of H2S; detection methods and rescue techniques.[For more detail on effective training content, refer to ANSI/ ASSE Z390.1-2006 (R2010), Accepted Practices for Hydrogen Sulfide Safety Training Programs.]

Sites where H2S could be present require a contingency plan. This plan cites specific issues relating to H2S, such as air flow directions, safe muster points, type of PPE/respiratory equipment, training drills, emergency procedures, worker responsibility, phone numbers/communication methods and nearby organizations, flag ratings (green, yellow and red) and locations of fixed gas monitors.

Since H2S is corrosive, plants/equipment exposed to it can experience degradation of steel structures (e.g., ladders, anchor points for fall protection). Thus, employee training must include periodic structural evaluations and examinations. Using proper ladder climbing techniques is ineffective if the ladder in use is structurally compromised.

Those working in confined spaces must be trained on how to safely test the atmospheres. Leaning over a pit/tank with monitor in hand to check for H2S is not safe. Techniques exist that keep the tester a safe distance from potential H2S exposure. Classroom and practical training must cover such techniques.


Believing that one can detect H2S using only the sense of smell is dangerous, as the sense of smell does not identify H2S exposure level. Smelling offers a slight detection around the 0.13 ppm level. But, H2S deadens the sense of smell, so one may only detect the gas momentarily. The fact that the odor disappears does not mean that the gas is gone. OSHA's exposure limit is 10 ppm. Shortterm exposure limit (STEL) is 15 ppm for 15 minutes, per ACGIH. So, how does one detect H2S? Use gas monitors, but first let's consider a couple of points.

Visual Cues

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Greg Gerganoff, ASP, CSP, Esq., is an OSHA / MSHA Safety field and compliance expert with experience in the heavy construction, oil and gas, mining, pipeline, and trenching and excavation industries.

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