A Straight Dope Classic from Cecil's Storehouse of Human Knowledge

Why do we have sinuses?

August 3, 1984

Dear Cecil:

Why do we have sinuses? Why, more specifically, do I have a body part that causes me to live on Sudafed for months at a time and still occasionally causes headaches so severe I throw up? Are sinuses the upper respiratory equivalent of tonsils, or what?

Cecil replies:

It is part of God's plan. Where else would the baby cold germs live? At any rate this theory is no worse than some of the others that have been proposed. Here's a quick rundown:

  • Resonance, i.e., sinuses make your voice more piercing. Unfortunately, cats, which have no unfilled sinuses, can howl like banshees, whereas giraffes, which have quite large sinuses, are silent.
  • Reduced weight in the skull. Sounds plausible, but some researchers have estimated that if the sinus cavities were filled with porous bone instead of air, the head would be just 1 percent heavier. Also, many heavy-headed mammals have absent or rudimentary sinuses.
  • Protects the brain against blows to face, apparently by deflecting the force of blow to the sides, after the manner of an arch. There's little evidence one way or the other here, although I would venture to say, having watched a few boxing matches, that the face's primary shock absorber is the nose.
  • Heat insulation for the base of the brain. Dubious. Some of the sinuses do not adjoin the brain; others which do adjoin the brain are not present in all mammals. In any case most warming of breathed-in air is done by the primary nasal passages, not the sinuses. Having hollow spaces in the head facilitates the growth of the face, somehow. Why it's so important to have your face grow rapidly I dunno. In any case, baboons get along fine without maxillary sinuses simply by having their cheeks curve in.
  • An aid in smelling and/or breathing. The idea here is that the sinuses warm and moisten breathed-in air. (Moistness is known to be important to smelling.) Some believe that sinuses provide a sort of reservoir of smellable molecules, giving the nose more opportunity to sample a transient scent. In some keen-scented mammals, the sinuses contain smell-sensing organs, but this is not true in man. Moreover, it's been calculated that sinuses contribute perhaps 1-1/2 percent of the moisture in inspired air.

In sum, medical science has failed again. And we expect these guys to cure cancer?

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