Hydrogen Sulfide (H₂S)
The truth behind the rotten egg smell
H₂S is a colorless gas. The chemical formula looks simple, but this substance has some genuinely interesting properties.
The Smell Paradox
Humans can detect hydrogen sulfide at concentrations as low as 0.001ppm. That rotten egg smell — once you've experienced it, you won't forget.
Here's what's scary: above 100ppm, your olfactory nerves shut down. The smell didn't go away — your nose gave up. "I can't smell anything, so it must be fine" becomes the most dangerous assumption.
Where You'll Find It
You encounter it more often than you'd think.
Hot springs: That distinctive sulfur spring smell is H₂S. Concentrations are low enough to be safe, but poorly ventilated indoor springs need caution.
Sewage treatment: Produced when organic matter decomposes anaerobically. This is why you don't just open a manhole and climb in.
Volcanic gas: Continuously released in volcanic areas. Japan's volcanic regions actually have H₂S warning systems installed.
Toxicity
Exposure above 300ppm can cause pulmonary edema and loss of consciousness. Above 700ppm is lethal within minutes. Industrial H₂S accidents are not uncommon.
Human Response by Concentration
| Concentration (ppm) | Response |
|---|---|
| 0.001~0.01 | Detectable smell (rotten egg) |
| 10~50 | Eye irritation, coughing |
| 100~200 | Olfactory paralysis (can't smell) |
| 300+ | Pulmonary edema, unconsciousness |
| 700+ | Lethal within minutes |
How It Works
When organic matter decomposes without oxygen, anaerobic bacteria convert sulfur compounds into H₂S
H₂S molecules bind to olfactory receptors in the nose → brain interprets as "rotten egg" signal
At high concentrations (100ppm+), olfactory nerves overload and shut down → smell disappears
At even higher concentrations, H₂S blocks cellular oxygen use → cellular suffocation