Expert: Dana Krempels, Ph.D. - 2/27/2008

Question
A friend told me that there are fluids that the mere presents of them can cause vomiting in humans from the smell of them. He gave an example of rat urine. If that is true, or if there is truth in anything similar, what causes it be so? Thank you.

Answer
Dear James,

Plenty of smells can elicit vomiting in humans, but the specific stimulus that elicits the response will vary among individuals.  This is an adaptive reaction to a potential toxic substance, and isn't caused directly by toxins in the smell.  What your body is telling you is this:  "There is something so nasty and potentially dangerous here that if you don't get out right NOW I am going to make you throw up!"

Disgust is an important, adaptive reaction to potentially dangerous substances.

I've smelled plenty of rat urine and it's never made me throw up.  It does give me a queasy feeling, though.  My ancestors probably had the same reaction, and hence were less likely to mess with rat pee (which can contain dangerous viruses) than maybe some other humans who *did* mess with it, got sick, and died.  We win the game of natural selection!  ;)

The vomiting/gag reflex stimulus arc is not well understood, physiologically.  But it certainly has performed a valuable evolutionary service for us.

Hope that helps.

Dana