Recently, during a Port State Control inspection of an oil tanker, the personal gas safety meters of one member of the Coast Guard and one of the tanker crew went off when the crew member opened the valve to a pressurized tank holding Grade E Sour Crude. Sour crude oil contains a high amount of sulfur (over 0.5%), and some of this sulfur occurs as hydrogen sulfide (H2S) gas. Reported to have been standing no more than two feet downwind of the valve release, the Coast Guard member suffered severe H2S exposure symptoms over the next few days, resulting in renewed warnings by the Coast Guard on this serious danger.
You know that “rotten egg” hydrogen sulfide odor when you smell it at low levels, even below one part per million, but if you smell it in enough quantity, you probably won’t smell it for long. That’s because, at 100ppm or above, it quickly paralyzes the nerve centers inside your nose. If you don’t get away fast enough, other parts of your body become paralyzed, in the worst cases leading to unconsciousness, coma, and death. Even 10ppm may pose a health risk, and 50-100ppm is enough to overcome a person. At lower levels, H2S exposure symptoms include irritated eyes, nose, throat, and lungs. Higher levels of H2S include headache, dizziness, nausea, and vomiting. These symptoms may have a delayed onset of hours or days, as the unfortunate Coast Guard member mentioned above experienced.
The dangerous threshold limit value for H2S was listed by the American Conference of Governmental Industrial Hygienists (ACGIH) as 2ppm until lately. They now list the H2S danger threshold as 1ppm. OSHA lists a higher figure at 20ppm, and NIOSH recommends an exposure limit of 10ppm for no more than ten minutes. Hopefully, someday there will be total agreement on what is considered safe, and it will be on the lower rather than the higher side. For now, it is left for you or your employers to decide which to trust.
Whichever of these exposure limits you adopt, whether you are working on a tanker or on a fishing vessel using anhydrous ammonia for vapor-compression refrigeration, you need to understand safe practices for H2S during controlled valve releases or in case of leaks. Also, if you’re fishing and your hold loses refrigeration, the decomposing seafood will emit deadly H2S. Going into a hold full of rotting seafood without appropriate safety gear has proven fatal.
Awareness is the first step. Beyond that, safety and first aid training; protective equipment such as impervious clothing, gas detection meters in areas known to contain H2S, personal gas monitors, and respirators or SCBA when working in enclosed spaces and near H2S; knowing where to stand during tank sampling; and understanding when to just stay clear are crucial for preserving your health around H2S.