Hydrogen Sulfide is also known by the following names; hydro sulfuric acid; sulfuretted hydrogen; sewer gas; sulfane; sulfur hydride; sour gas; sulfurated hydrogen; hydrosulfuric acid; stink damp; and rotten egg gas. Its chemical molecular formula is H2S. H2S is a colorless, toxic, flammable gas with a strong odor of rotten eggs or flatulence. Odor is not a reliable indicator as to the concentrations because the sense of smell becomes rapidly fatigued and can not be relied upon to warn of the continuous presence of the gas. H2S is a by product of the decay of organic material and accidental exposure has occurred in situations involving sewage, liquid manure, natural gas, and animal and vegetable matter storage and processing. It can also be found at certain industrial facilities, such as waste water treatment plants, petroleum refineries, pulp and paper manufacturers, and plants producing sulfur or sulfuric acid. Small amounts of hydrogen sulfide occur in crude petroleum but natural gas can contain up to 80 percent. It is shipped as a liquefied compressed gas, bearing the placard 2.3, and the UN 1053.
The gas can be detected at a level of two parts per billion. To put this into perspective, 1 mL of the gas distributed evenly in a 100-seat lecture hall is about 20 ppb. The IDLH for this gas is 100 ppm. Remember that in determining IDLHs, NIOSH figures the ability of a worker to escape without loss of life or irreversible health effects being considered along with severe eye or respiratory irritation and other deleterious effects (for example: disorientation or loss of coordination) that could prevent escape. Although in most cases, egress from a particular worksite could occur in much less than 30 minutes, as a safety margin, IDLHs were based on the effects that might occur as a consequence of a 30-minute exposure. However, the 30-minute period was NOT meant to imply that workers should stay in the work environment any longer than necessary following the failure of respiratory protection equipment; in fact, every effort should be made to exit immediately.
The Occupational Safety and Health Administration (OSHA) defines an immediately dangerous to life or health concentration in their hazardous waste operations and emergency response regulation as follows: An atmospheric concentration of any toxic, corrosive or asphyxiant substance that poses an immediate threat to life or would cause irreversible or delayed adverse health effects or would interfere with an individual's ability to escape from a dangerous atmosphere. [29 CFR 1910.120]
Hydrogen sulfide is a highly toxic and flammable gas. Being heavier than air, it tends to accumulate at the bottom of poorly ventilated spaces. Although very pungent at first, it quickly deadens the sense of smell, so potential victims may be unaware of its presence until it is too late.
Hydrogen sulfide is considered a broad-spectrum poison, meaning that it can poison several different systems in the body, although the nervous system is most affected. The toxicity of H2S is comparable with that of hydrogen cyanide. It forms a complex bond with iron in the mitochondrial cytochrome enzymes, thereby blocking oxygen from binding and stopping cellular respiration. It's kind of the opposite of Carbon Monoxide, when CO is inhaled; it combines with the oxygen forming carboxyhemoglobin. Since hydrogen sulfide occurs naturally in the environment and the stomach, enzymes exist in the body capable of detoxifying it by oxidation in to (harmless) sulfate.
Exposure to lower concentrations can result in eye irritation, a sore throat and cough, nausea, shortness of breath, and fluid in the lungs. These symptoms usually go away in a few weeks. Long-term, low-level exposure may result in fatigue, loss of appetite, headaches, irritability, poor memory, and dizziness. Concentrations of 700-800 ppm tend to be fatal.
Toxicity levels are:
- 0.0047 ppm is the recognition threshold, the concentration at which 50 percent of humans can detect the characteristic odor of hydrogen sulfide, normally described as resembling "a rotten egg"
- 10-20 ppm is the borderline concentration for eye irritation
- 50-100 ppm leads to eye damage
- At 150-250 ppm the olfactory nerve is paralyzed after a few inhalations, and the sense of smell disappears, often together with awareness of danger
- 320-530 ppm leads to pulmonary edema with the possibility of death
- 530-1000 ppm causes strong stimulation of the central nervous system and rapid breathing, leading to loss of breathing
- 800 ppm is the lethal concentration for 50percent of humans for five minutes exposure
- Concentrations over 1000 ppm cause immediate collapse with loss of breathing, even after inhalation of a single breath.
Hydrogen sulfide has been used for well over a century as a method of qualitative analysis of metal ions. In fact, the Chemistry Building at the University of Illinois in 1915 had a built-in supply of hydrogen sulfide to the various labs, i.e., H2S "on tap"! The gas was stored in a 500-gallon tank! The density of hydrogen sulfide is 1.393 g/L at 25 oC and 1 atm. This is 18 percent greater than that of air.