In my work I sometimes go near H2S (hydrogen sulfide) gas. Anyone working in a variety of industries knows that this is the second most toxic gas, it is deadly. Surprisingly evolution has given most people the ability to detect this gas at amazingly low concentrations. With respect to H2S our noses are more sensitive to this gas than the sensory organs of sharks are to blood in the water. To work in such an area you need to have a high quality detector and safety training. It’s the gas that smells somewhat like rotten eggs. You know the smell. If you have visited Yellowstone National Park it is the “smell of geology”. I have proposed shampoo and perfume with this distinctive smell to friends, but it has caught on yet.
If you are interested in learning about this substance check the wiki page located here. In reviewing this material before a recent trip I noticed some uncertainity about the level at which the human nose can detect H2S. I was always amazed that our noses are, even better than, those of blood hounds when it comes to H2S detection.
According to the Wiki page “0.0047 ppm is the recognition threshold, the concentration at which 50% of humans can detect the characteristic odor of hydrogen sulfide, normally described as resembling “a rotten egg”. “
The Safety Direct web page for H2S, located here, states: “Hydrogen sulfide has a very low odor threshold, with its smell being easily perceptible at concentrations well below 1 part per million (ppm) in air. “
Because H2S is often associated with manure, there is an excellent fact sheet about H2S, here is a link to the PDF, that states: “The odor of hydrogen sulfide gas can be perceived at levels as low as 10 ppb (parts per billion).”
So, how does this compare with sharks? According to this source “sharks can detect some chemicals at concentrations of around one part per 25 million”.
Kind readers; I ask that you compare this sensitivity with your sensitivity to H2S. You are truly astounding. Next time you see your face in the mirror, using your brain (that parallel wired, analog petaflop computer you carry around), consider your nose and this amazing ability. You have about 10 million sensory cells in your nose, I salute them all.
To learn more about olfaction (the sense of smell) in general, check the excellent Wiki.