Hydrogen Sulfide or Hydrogen Cyanide: Which is More Dangerous?

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hydrogen sulfide or hydrogen cyanideSchool kids laugh when malodorous body noises break the silence. “Jimmy just gassed,” little Susie giggles. Such odors are associated with the sulfur that comes from eating eggs, although that is by no means the only source. In fact, the odor that comes from eating eggs is thought by high school students to be due to hydrogen sulfide, H₂S. It is generally thought of as a nuisance, and not as a poison.

Another familiar odor is associated with the pit or seed of a peach—bitter almonds. The source of the odor in this case is hydrogen cyanide, HCN. Hydrogen cyanide and its sodium and potassium salts, NaCN and KCN, are often the victim’s cause of death in a television mystery.

But… in “real life”… what is the story? Just how poisonous are the cyanides? And how is it hydrogen sulfide can be considered dangerous? If hydrogen sulfide is dangerous and not as harmless as it seems, why hasn’t this been popularized in detective shows? Which is worse, hydrogen sulfide or hydrogen cyanide?

MSDS Sheets

There are various material safety data sheets and other legalistic documents that may vary from state to state. These give insight into the toxicity of a substance used in the workplace and its detection by the average individual on the basis of odor.

NJ Right-to-Know Fact Sheet for Hydrogen Cyanide

Hydrogen cyanide is listed in the NJ State Department document as having a permissible exposure limit (PEL) of 10 ppm (parts per million), whereas its detection by smell typically occurs anywhere from 2 to about 10 ppm.

NJ Right-to-Know Fact Sheet for Hydrogen Sulfidehydrogen sulfide or hydrogen cyanide

Hydrogen sulfide, on the other hand, is listed in the NJ State Department document as having a permissible exposure limit (PEL) of 20 ppm. Detection of it by smell typically occurs above 0.13 ppm.

Hydrogen Sulfide or Hydrogen Cyanide

The numbers above aren’t rocket science. Hydrogen sulfide is about one-half as toxic as hydrogen cyanide. It is anything but harmless! So why is it seldom mentioned as being almost as dangerous as hydrogen cyanide is? It is because it announces its presence well ahead of the point of severe toxicity.

The potential victim can ordinarily smell hydrogen sulfide and escape to safety long before levels reach a severe level. Not so for hydrogen cyanide! Once it is detected by odor, the individual is already endangered.

Note: You might also enjoy Carbon Monoxide is More Dangerous Than Carbon Dioxide


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5 thoughts on “Hydrogen Sulfide or Hydrogen Cyanide: Which is More Dangerous?

  • When I was child, you could buy rotten egg capsules. These were tiny glass vials with hydrogen sulfide in them. You dropped them on the floor and disappeared off. The person’s feet broke the capsules and you were nowhere around when it happened!

  • DaveK

    H₂S will put you down faster than HCN. H₂S is a neurotoxin, and with a high concentration will put you down like you were whacked in the head with a baseball bat. In industrial accidents with H2S, survivors have basically said they remembered nothing before being revived, except sometimes for a quick whiff of the rotten egg smell.

    HCN may kill at lower concentrations, but you will die a slow and painful death, as it blocks cellular oxygen transport. Your body starts converting glucose to lactic acid to compensate, and as it builds up it becomes quite painful. Remember those sore muscles after a strong anaerobic workout? Multiply that by several times over your entire body and think how that feels.

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