When you breathe in H2S, it goes directly through your lungs and into your bloodstream. To protect itself, your body "oxidizes" (breaks down) the H2S as rapidly as possible into a harmless compound. If you breathe in so much H2S that your body can't oxidize all of it, the H2S builds up in the blood and you become poisoned. The nervous centers in your brain which control breathing are paralyzed. Your lungs stop working and you are asphyxiated - just as though someone had come up and put their hands around your neck and strangled you. A worker can be overcome by H2S and lose consciousness in a few seconds; luckily if he is rescued in time and is given artificial respiration within a few minutes, the worker may recover. Either artificial mouth-to-mouth or an oxygen supply system of resuscitation will work if it is done in time, because, with an adequate source of oxygen and no further H2S intake, the body will quickly break down the H2S still in the blood.
This is acute poisoning. It can occur with no warning at all, since even the sense of smell may be overcome, and it can be fatal within a few seconds.
Although acute poisoning is deadly if it is not caught in time, when caught and treated it is reversible and this is why rescue attempts with proper safety equipment are so important.
Recent evidence has shown irreversible brain damage from acute high doses.
If exposure at low levels continues, the worker may develop a state of chronic poisoning. In addition to eye and respiratory tract irritation, there will be a slowed pulse rate, fatigue, insomnia, digestive disturbances, and cold sweats. More dangerous, if exposure at the level of 100 ppm (which results in eye and respiratory tract irritation and drowsiness after 15 minutes) lasts for several hours, it may result in death within the next 48 hours. Symptoms of chronic exposures at low levels are conjunctivitis (eye infections), headache, attacks of dizziness, diarrhea, and loss of weight.
Chronic hydrogen sulfide intoxication is marked by headaches, eye disorders, chronic bronchitis, and a grey-green line on the gums. Reports of nervous system disorders including paralysis, meningitis, and neurological problems has been reported, but not confirmed.
A study of workers and community residents of a California refinery engaged in extracting sulfur from crude oil, which is rich in H2S, complained of headaches, nausea, vomiting, depression, personality changes, nosebleeds and breathing difficulties. When compared to a non-exposed group of people, the exposed people showed abnormalities of color discrimination, eye-hand coordination, balance, and mood disturbances.
In rats, exposure to hydrogen sulfide has caused teratogenic effects.
There is no time-weighted average because H2S is so fast-acting that no fluctuations above 20 ppm are safe; only one peak per day is allowed.
This level is too high and recent recommendations are that it be lowered to 10 ppm. You should remember, however, that H2S is an invisible gas, floating freely and unpredictably, and a reading even below a 10 ppm Permissible Exposure Limit (PEL) may not guarantee your safety.
There are no particular medical exams for exposure to H2S.
If you work with H2S make sure that:
* Your employer has trained you in the hazards of H2S.
* Your employer has appropriate rescue equipment on-site.
Fundamentally, employers and employees must be alert to the fact that working with a "closed system" does not always ensure safety. Operations involving the opening of valves or pumps on otherwise closed systems or working on such equipment that is not isolated or locked out are particular sources of danger. When a normally closed system is opened, the potential exists for releasing hazardous chemicals to the workers' breathing zones in unknown concentrations.
* In 1988, two workers were operating a sodium purification system. One worker attempted to pump a sodium sulfhydrate solution into a tank and accidentally opened the valve to another tank which contained an acidic solution. The mixture of the two compounds generated and released hydrogen sulfide gas to which the victim was exposed.
* In 1988, a refinery employee received a fatal exposure to hydrogen sulfide gas while draining the contents of a knockout drum to an oily water sewer, rather than activating a closed system to pump out the drum. The worker failed to observe procedures calling for the use of a closed system, and the valve to the sewer was not locked out.
* In 1993, employees were working in a coker unit that thermally cracks heavy residual feed through a process called delayed coking. The workers were preparing to switch the feed to the core drum, which necessitated opening and closing a number of valves. Three workers were involved with opening and closing the valve, each working at a different location. As the operation was proceeding, a loud noise was heard and a vapor cloud was observed in the vicinity of the pumps feeding the process. The vapor cloud ignited, fatally burning two of the workers.
* In 1994, an employee was killed when disconnecting a line from an ammonia valve causing liquid ammonia to be released, striking the worker on his face and body.
* In 1994, a tragic fatality caused by exposure to hydrogen sulfide was reported by the Billings, Mont. OSHA Area Office. The accident was associated with opening a valve to a sewer cup during the draining of a fuel gas knockout drum in a hydro treating unit of a petroleum refinery. Normal work procedures included periodically opening a valve that carried a water-gas mixture to a separator which removed and vented hydrocarbon gases to a flare.
During the preceding winter, the piping to the separator froze, and the drum was temporarily drained to the sewer. OSHA believes that, due to unclear procedures, the temporary practice of draining some of the water-gas mixture to the sewer in some instances may have been continued, or was incorporated by some workers into the normal draining procedures.
The worker who was killed is thought to have opened the valve to the sewer believing it to be part of the draining procedure, resulting in the release of toxic and fatal amounts of hydrogen sulfide.
1. Perform a process hazard analysis to address the hazards of the process and engineering and other control measures to ensure worker safety, including a complete evaluation and assessment of process systems handling waste products, by-products, and/or unreacted process components. Recommendations made by the PHA team, including recommended engineering changes, should be promptly implemented.
2. Assure that the system is locked out, including draining and purging of lines and equipment, prior to working on the system.
3. Assure that written procedures are clear and that they provide complete instructions for the safe performance of work activities.
4. Assure that employees, including contract employees, are trained in applicable procedures and safe work practices, and that the employees understand and adhere to the current operating procedures of the process.
While the risk of accidents cannot be entirely eliminated, these procedures and practices will reduce the potential for accidental exposure to hazardous chemicals.
Monitoring and detection equipment -- Operators working on units where there is potential exposure to hazardous chemicals may need to be supplied with personal monitoring equipment. Alternatively, stationary monitors could be installed. Personal or stationary monitors must be capable of sounding an audible alarm or warning.
Training -- All current and new employees should receive
training in standard operating procedures covering all
aspects of the job, with emphasis on safe work practices.
Where appropriate, training should also include field
observations (on-the-job training) by qualified
supervisory personnel, including verification that
workers have satisfied the training requirements.
Training must include proper procedures for working around areas of potential exposure to hazardous chemicals and include the hazards of exposure. While labeling of pipes cannot be required, the hazard communication standard does require that the employer address the hazards of unlabeled piping systems in a written hazard communication program and that the information be provided through training to workers.
Respiratory protection -- Respirators must be provided by
the employer when effective engineering controls are not
feasible, or while they are being instituted, when such
equipment is necessary to protect the health of the
worker. The employer must provide respirators that are
applicable for the purpose intended.
Written procedures must be developed for the safe use of respirators during the performance of operations presenting a potential exposure to hazardous chemicals.
Under circumstances where individuals may be exposed to an unknown concentration of hydrogen sulfide or some other hazardous chemical, back-up personnel with appropriate respirators and emergency equipment must be present.
Proctor and Hughes' Chemical Hazards of the Workplace, Third Edition. Van Nostrand Reinhold, New York, 1991.
Neurobehavioral Impairment from H2S. Kaye H. Kilburn, M.D. and Raphael H. Warshaw, B. A., University of Southern California School of Medicine, Environmental Sciences Laboratory: Los Angeles, CA, 1991.
ILO Encyclopaedia of Occupational Health and Safety Third Edition. International Labor Office, Geneva, 1983.
Published by the Oil, Chemical and Atomic Workers