Another Gaseous Hazard — Hydrogen Sulfide

Yesterday we looked at methane, a common gaseous hazard encountered in water distribution system operations. Today we will look at another such hazard, hydrogen sulfide.
Believe it or not, the substance that most closely resembles hydrogen sulfide is water! Water is H2O, and hydrogen sulfide is H2S. The element that most closely resembles Oxygen is Sulfur, which is located just beneath Oxygen on the Periodic Table of the Elements. But even though these molecules are structurally quite similar, they have very different properties.
While water is odorless, hydrogen sulfide has a characteristic, “rotten egg” smell. One whiff and you know you have H2S. But one whiff may be all you get, because hydrogen sulfide has the insidious property of deadening the sense of smell. The result is that the odor is gone, but the H2S might still be present.
Unlike the light gas methane, hydrogen sulfide is heavier than air. This poses an additional risk to water system workers, since so much of our work is below ground level, in trenches and in vaults. Because it is heavier than air, H2S will accumulate in these low places, and displace the air that was present there. Workers that enter such places can die from lack of oxygen.
But hydrogen sulfide is toxic all by itself. The 8-hour exposure level is 10 parts per million. At 800 ppm, 50 percent of people exposed would die within 5 minutes. At 1000 ppm, a single breath would cause immediate collapse and loss of breathing. Its toxicity is similar to that of hydrogen cyanide.
It was easy to understand the origin of the methane discussed yesterday: it’s in underground pipes. But where does the hydrogen sulfide come from? Most often, it comes from sanitary sewers, but it can also be found in storm drains and even in some soils. It is formed during decomposition of organic material, especially under anaerobic (no oxygen) conditions. Here is another case where the chemical similarity between oxygen and sulfur is evident: if there is oxygen present, organic decomposition will form water; if no oxygen is present, sulfur will take oxygen’s place, and hydrogen sulfide is formed.
Because of the deadening of the sense of smell, we cannot rely on our noses to detect H2S – instruments are needed. Because it is heavier than air, we must use forced ventilation to make and keep an area safe from hydrogen sulfide. And if we are working in an area where H2S is present, only a self-contained breathing apparatus can provide the personal protection you need.
Hydrogen sulfide has probably claimed more lives any other hazard in the water and wastewater industry. Be aware of its likely presence in any below-ground activity, especially when sewers are nearby. Do not enter until you have properly tested the area for this silent killer.

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