Based on a little bit of internet reading, it appears that odor molecules that enter your nose from human feces are of little concern to your health because the harmful stuff, like the pathogens, are not likely to be picked up and carried through the air.

This seems to make sense, however due to my limited knowledge of biology, I am having a difficult time distinguishing WHY the bad stuff is not likely to become airborne whereas other molecules can, and will cause damage by smell. Chlorine, for instance, will cause some significant damage simply by smelling the airborne fumes (which I assume are the molecules doing the damage).

I have to assume it is a natural defense mechanism that something that smells bad is not something you want to be around (CO being a major exception) but it seems that feces molecules are not a concern. Why?

  • $\begingroup$ Being around excreta is bad for health, because of the pathogens that breed in it. As a result, due to natural selection, we have developed an aversion to being around excreta, for safety reasons. Chlorine on the other hand, reacts with moisture in your nose/lungs and becomes HCl, which is never good to have inside your respiratory system. $\endgroup$ Jan 24, 2017 at 19:18
  • $\begingroup$ So can we conclude that you can indeed get sick from simply being in bathroom that 'smells' foul, or does our aversion protect us from this? $\endgroup$
    – T James
    Jan 24, 2017 at 19:23
  • $\begingroup$ @JAlecksen I think the point is that there are (roughly) two reasons to evolve an aversion to a smell: 1) The compound smelled is/can be harmful, or 2) The compound smelled is a marker for something that is/can be harmful. In case (2), it isn't the compounds you actually smell that are harmful, it is the presence of potentially harmful bacteria, etc. Chlorine is an example of (1); bathroom odors are an example of (2). $\endgroup$
    – Bryan Krause
    Jan 24, 2017 at 20:42
  • $\begingroup$ You can't fall ill from being in a modern sanitized toilet. You can fall ill from hanging around feces in your day to day life, and not being clean about handling it. $\endgroup$ Jan 25, 2017 at 10:35

1 Answer 1


Mostly, what you're smelling are volatile small molecules: stuff like butyrate, indole, mercaptans, hydrogen sulfide, etc. These molecules are thousands of times smaller than even the smallest pathogens, being measured in Angstrom vs. nanometer scales. Pathogens are just too large to be actually volatile, in the sense of freely escaping into the air. And fortunately, these volatile organics are not nearly as reactive as something like chlorine gas. (If they were, imagine the damage they would do, at a much higher concentration, to the person they were just inside.)

Now, if someone had literally just projectile-vomited (or if excreta were churned around, e.g., by flushing), it's possible that a very small pathogen like norovirus could spray outwards in an aerosol. Since norovirus has a very small infectious dose (on the orders of tens of viral particles) this could be enough to make you sick, and indeed, aerosols have been implicated in norovirus transmission, though it's not that common. But the smell would remain long after any viral particles in aerosols had settled down onto surfaces.

  • $\begingroup$ ncbi.nlm.nih.gov/pmc/articles/PMC4692156 $\endgroup$ Jan 25, 2017 at 7:42
  • $\begingroup$ Great answer, of course it also makes me wonder why our bodies have not evolved to recognize the bad pathogens as smelling bad. This seems like it would be a good defense mechanism... $\endgroup$
    – T James
    Jan 25, 2017 at 17:34
  • 2
    $\begingroup$ @JAlecksen I think one issue might be that by the time there's enough pathogen in your nasal passages to be able to detect it as a "smell", you're hopelessly contaminated. Far better to be able to detect pathogen-associated volatiles which can be detected before you get close enough to pick up the pathogens. That way you can keep your distance. The smell molecules themselves are not harmful, they just indicate that harmful things are nearby, and you should keep your distance. $\endgroup$
    – R.M.
    Jan 25, 2017 at 23:35

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