September 17, 2018
Hydrogen Sulfide: Yes, It’s Nasty
By Michael D. Shaw
For most people, hydrogen sulfide (H2S) is experienced and described as that “rotten egg smell.” Eggs develop the characteristic odor as a result of bacteria—traveling easily through the porous shell—digesting the protein. (H2S) is the simplest thiol, and many of these compounds—especially those of low molecular weight—have obnoxious odors. Hydrogen sulfide is also a component of flatulence and bad breath.
Besides being an annoyance, hydrogen sulfide is highly toxic, being the second most common cause of fatal gas inhalation exposures in the workplace (at 7.7%, second to carbon monoxide at 36%). The health effects are dose-dependent (the dose makes the poison), and at high enough concentrations, the gas can be fatal within seconds. Fatal exposures are caused by respiratory failure. Lower exposures can cause a variety of symptoms, ranging from eye irritation to labored breathing, and unconsciousness.
A terrible property of hydrogen sulfide is the killer twosome of olfactory fatigue and olfactory paralysis.
When we encounter strong odors (and they need not be unpleasant), our olfactory sense accommodates to them, and after a while, they do not seem so overwhelming. When we return to an environment that does not contain this smell, our normal sense will return after a few minutes. This olfactory fatigue is a sensory adaptation. Olfactory paralysis, however, is a sensory manifestation of neurotoxicity—affecting both the olfactory bulb and fibers. Unfortunately, these two conditions are often confused.
Hyposmia (reduced sense of smell) has been found to be present in most men who recovered from severe, potentially lethal hydrogen sulfide toxicity.
Olfactory paralysis occurs at exposures over 100 ppm, so it matters little that the odor threshold of hydrogen sulfide is pegged at 0.008 to 0.1 ppm. As such, most people can smell it at relatively benign concentrations, but lose this sense at dangerous concentrations.
Thus, there is a necessity to install monitoring systems for hydrogen sulfide in any workplace that might have releases of this chemical. The US Occupational Safety and Health Administration—along with similar agencies in most other industrialized countries—have promulgated occupational exposure limits for hydrogen sulfide. Many gas detection instrumentation manufacturers, including Interscan, offer a variety of personal, portable, and continuous monitoring devices for (H2S).
Although much work on hydrogen sulfide exposure has been done within the oil industry, some of the saddest tales of fatal exposure occur in agriculture—specifically incidents involving manure pits. A news story from 2015 describes a father and son from Iowa, both overcome by the fumes. The story indicates that this accident was the second such fatal event to occur that month—and that one also involved a father and son.
Earlier in this article, we alluded to the presence of hydrogen sulfide in bad breath. Hydrogen sulfide, along with methyl mercaptan and dimethyl sulfide, are considered “volatile sulfur compounds,” and are generated by anaerobic bacteria on the tongue, as they digest various proteins. People with chronic halitosis are often also afflicted with gum disease, and these conditions are treated by dental professionals.
An instrument called the Halimeter® was developed around 20 years ago, and this unit detects the presence of volatile sulfur compounds in the breath. Given the odor threshold of these compounds, instrument sensitivity is in the low parts-per-billion range.
As bad as hydrogen sulfide is, recent studies have shown that this compound is involved in many physiological functions, and may have significant therapeutic applications. Notably, along with nitric oxide and carbon monoxide, H2S is a so-called gasotransmitter.
An article from 2012 entitled “Hydrogen Sulfide in Biochemistry and Medicine” identified therapeutic targets in the areas of:
- Acute Myocardial Infarction
- Alzheimer’s disease
- Erectile Dysfunction
- Heart Failure
- Inflammatory Bowel Disease
- Metabolic Syndrome and Diabetes
- Organ Transplantation
- Peripheral Arterial Disease
Was Friedrich Nietzsche thinking of hydrogen sulfide when he said that “That which does not kill us, makes us stronger”?