Hydrogen sulfide (H2S) is a highly toxic, colorless gas. It is heavier than air, and when present in small amounts, has the characteristic odor of rotten eggs. However, the sense of smell cannot be depended upon to detect H2S. Drilling Contractor Magazine has a nice explainer on offshore H2S Safety Procedures
- Deadly or extremely toxic gas.
> 10 PPM toxic to personnel
> 300 PPM quickly deadens sense of smell
> 500 PPM attacks respiratory center in the brain causing breathing to stop and loss of consciousness within 15 minutes
> 700 PPM rapid loss of consciousness and death
> 1000 PPM immediate unconsciousness and death
2. Colorless gas.
- H2S is heavier than air. Its density is 1.189, compared with 1.0 for air. H2S will concentrate in low-lying areas, and especially in pits and dumps which are closed in with poor ventilation.
- Readily dispersed by wind movement or air currents.
- Burns with a blue flame, producing sulfur dioxide (SO2), which is also a toxic gas.
- Odor of rotten eggs (only in low concentrations) rapidly deadens the sense of smell. Do not depend on the sense of smell to detect H2S.
- Highly corrosive to certain metals.
- More deadly than carbon monoxide (CO) and almost as toxic as hydrogen cyanide.
- H2S is soluble in liquid. This is extremely important. Concentrations of H2S may be contained in pools of water, spud cans or in sludge at the bottom of a tank. The H2S will be released if the pool is agitated or heated.
- The ignition temperature of H2S is 500°F (260°C).
- H2S is explosive when mixed with air in the range of 4.3% to 46% (by volume). This means that an explosion could occur if smoking, welding or other similar activity is taking place.
- H2S occurs in a variety of natural and industrial settings. Hydrogen sulfide is found in large amounts in natural gas petroleum. Workers involved in the drilling of gas and petroleum from known sour gas areas, as well as some unknown (wildcat) areas, are at risk from exposure to H2S.
- H2S can also be encountered when working in old wells where the gas has not been found before. Sulfur combines with most metals to form sulfide. Iron sulfides are present in many wells. When iron sulfide is mixed with hydrochloric acid, hydrogen sulfide forms, producing a hazard that can be lethal. During acid jobs, be aware of possible H2S formations and take steps to protect against the gas.
- Most H2S is obtained as a by‑product from other operations. H2S is recoverable from natural gas and petroleum refining operations. It is converted to sulfuric acid, high‑quality sulfur or disposed of by burning in flare lines.
Means of entry include inhalation (breathing), ingestion (eating), skin penetration (open wounds or sores) and possibly the ear canal.
How H2S Affects Individuals
- H2S at low concentrations may cause irritation of eyes, nose and throat. May smell like rotten eggs.
- H2S at moderate concentrations may cause excitement, headaches, dizziness, nausea, vomiting, coughing, and loss of equilibrium.
- H2S in high concentration may cause rapid loss of consciousness and death unless the victim is moved to fresh air and first aid is administered.
When a person breathes in H2S, it goes directly through the lungs and into the bloodstream. To protect itself, the body oxidizes (breaks down) the H2S as rapidly as possible into a harmless compound. If the individual breathes in so much H2S that the body can’t oxidize all of it, the H2S builds up in the blood and the individual becomes poisoned. The nerve centers in the brain which control breathing are paralyzed. The lungs stop working and the person is asphyxiated.
The way H2S affects individuals depends on the four factors: duration of exposure, frequency of exposure, dosage (concentration) and the individual’s physiological make‑up. Tolerance of H2S is never acquired.