Expert: Walter Hintz - 3/10/2006

Question
Hi,
So this may seem like a really elementary question, but why do farts and feces smell bad? like the food going in doesn't necessarily smell bad, so is it just the combination of food, or is there something that happens during digestion.  Also, why do we identify smells (these smells, or any smells) as bad? Is it actually built into us, or do we just learn something smells bad?
Thanks a lot:) Andrea

Answer
Hi Andrea:
I get this question about once a year. During digestion foods are broken down and new chemicals are formed.  Two of them are Hydrogen sulfide and Indole. These compounds give off the fecal odor. Sulfur is a component of proteins and indole results from digestion of tryptophan.
In the 1950's I worked as a bacteriologist while in the US Army and part of my job was culturing feces for bacteria and searching feces for parasites. Was the odor "bad"? Certainly but why do we call it bad? Actually the word "bad" is arbitrary: what is bad to one person is not so to someone else.  When it comes to animal waste however I believe that we are genetically conditioned to avoid it as it can harbor harmful organisms.  The odor tells us this.  I have no evidence to back this up but it makes sense to me. I have heard of societies that may handle all sorts of materials that some consider obnoxious but all people seem to avoid human  fecal waste.