In the safety world hazard recognition plays a vital role in keeping your people safe from unsafe behaviors and/or conditions. Some hazards are easily recognized, for example an employee climbing up a 20 foot ladder with tools held in both hands. (This is a fall hazard by the way.) Common sense right? The safety guy who taught me safety had a great response to this attitude, "Few people have any sense (read knowledge) in common (read shared alike)". So while some safety hazards are immediately recognizable others require training to spot and avoid. Training is a key method in avoiding the "Ostrich Zone". One such hazard is Hydrogen Sulfide. You don't want to bury your head facing this hazard. (Won't do much good anyway, Hydrogen Sulfide is heavier then air!)
Hydrogen sulfide (H2S) is a clear gas, dangerous to human health/life, extremely flammable, corrosive and sometimes has a detectable odor of rotten eggs. It can be produced in nature (bogs, swamps, volcanos and hot springs). (1) It can also be produced by industrial activities (oil and gas, natural gas pipeline transmission, refineries, fertilizer manufacturing, sewage treatment plants, vacuum trucks, tanneries, food processing plants, manholes, standing water, grain silos, silage pits and manure pits just to name a few.) Whether a natural or industrial source it is the result of bacterial decomposition of organic material. Just out of curiosity how many of the potential sources of H2S just listed were recognizable to you before reading this paragraph? (See, the Ostrich Zone is fading already.)
What's the H2S hazard? A very low level exposure can result in serious illness or death. (IDHL is 100 ppm. 1% = 10,000 ppm. That's a very low level.) And as just mentioned there exist locations one would not necessarily associate with this hazardous gas. (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.) (2) NIOSH lists multiple instances of H2S deaths occurring on farms and sanitation facilities involving enclosed/confined spaces harboring this deadly gas. In none of these sad instances was a gas monitor used to alert the presence of this gas. (3) What makes H2S so deadly?
H2S is an inhalation, flammable/explosive and contact hazard. Inhalation symptoms at low level exposures include headache, dizziness, sleep disturbances, upset stomach, changes in appetite at 20 ppm. (Remember sentence above, 1% = 10,000 ppm! This is a very small exposure.) Exposure of 100 ppm is IDLH (NIOSH) and symptoms include altered breathing and drowsiness after 15-30 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 30-60 minutes. 1000-2000 ppm nearly instant death. (4)
H2S is also a corrosive and will act on skin and eyes. Exposure levels of 50-100 ppm will give rise to eye irritation and marked conjunctivitis and respiratory track irritation after an hour at 200-300 ppm. (4 above.) Acute exposure can cause painful conjunctivitis, photophobia (sensitivity to light), corneal abrasions and blindness. (5) H2S is classified as a chemical asphyxiate similar to carbon monoxide and cyanide gasses. It inhibits cellular respiration up take of oxygen causing biochemical suffocation.(6)
How does one go about recognizing this hazard and how do you protect yourself? Training.There are several levels of H2S training selection of which is dependent upon the anticipated action expected of the employee/trainee. For example, if an employee's work will rarely result in exposure to H2S and is expected to immediately evacuate a site upon the detection of H2S than awareness training and training on use and care of gas monitor is likely adequate. If the employee works in a setting where H2S is likely, say manure pits or oil and gas processing plant handling H2S involved gas/products then training must include a more advanced training including special evacuation criteria, use of respiratory PPE and gas monitor use/care, first aid, expectations of employees in an exposure/release event (evacuate only or rescue) and nature of work (confined space), exercises and drills, schedule for refresher training (annual), site specific safe work practices, properties/characteristics of H2S, detection methods, care/use of PPE (full face respiratory protection) and rescue techniques and first aid procedures. (For more detail on training content refer to ANSI/ASSE Z390.1-2006 (R2010) ANSI's standard is too long to recite here but is comprehensive and is a must when putting together H2S training.
For sites/work where H2S could be present, such as manure pit, sewer type operations, gas/oil processing, etc. a Contingency Plan is required. Briefly, Contingency Plans, are written plans addressing site 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 residences/schools/businesses, flag ratings (green, yellow and red), locations of fixed gas monitors, etc. If you are on a site where a Contingency Plan exists read it. It will be invaluable and may save your life.
Remember the No Ostrich Zone theme at the beginning of this article? H2S is corrosive. It will dissolve/weaken steel, brass, bronze and copper (though not some stainless steels). Plants/equipment exposed to H2S can experience degradation of steel structures like ladders, anchor points for fall protection, etc. Training for employees on sites where H2S has been exposed to steel parts of the plant a good practice includes periodic structural evaluations/examinations. Using proper ladder climbing techniques are ineffective if the ladder one is using is structurally compromised and fails.
Working in confined spaces such as storage tanks or manure pits training on how to safety test such atmospheres is a must. (Note: Leaning over a pit/tank with monitor in hand to check for H2S is NOT safe. There are techniques to test such situations keeping the tester safely distant from potential H2S exposure. Such techniques must include classroom and practical training.)
Detection. Regardless of the rotten egg smell believing one can detect H2S using sense of smell is dangerous. Why? The sense of smell does not identify the level of H2S exposure. 0.13ppm level offers a slight detection by sense of smell. But H2S deadens the sense of smell so you may only detect a momentary whiff of the gas before losing your sense of smell. The fact the odor seems to go away does not mean the gas is gone! OSHA standard limits for exposure is 10 ppm. (REL NIOSH) (4) Short Term Exposure Limit (STEL) is 15 ppm for 15 minutes per ACGIH. 100 ppm is considered Immediately Dangerous to Life and Health (IDLH). So how does one detect H2S? Use gas monitors but first a couple of points to consider.
Observe your surroundings. Signage. If working in the oil and gas industry look for H2S Danger Signs posted upon approach/ site entrance. Wind indicators. Look for wind socks or streamers which are also indicators of potential hazardous gas on site. What direction are the windsocks/streamers flowing? As you walk/work around the site periodically look to see the direction of wind flow per the windsocks/streamers. If the site has fixed H2S gas monitors know what their alarm does, i.e. flashing lights and type of sound. (If you are new to the site ASK the supervisor/employee he/she can tell you if H2S is present, what to look out for and nature of the alarm system and evacuation points/directions. Contingency Plan will also answer these questions.) Finally, there may be colored flags onsite. Green means possible danger, Yellow means moderate danger (this color would indicate it's time to leave or use PPE) and Red for extreme danger (leave immediately to save your life and use PPE). Remember though, pay attention to the direction of the flag/windsock/streamers and move perpendicular and then upwind from the H2S source for evacuation purposes. Your direction of evacuation should be the shortest distance to safety.