"Don't worry," I often assure people whose face my dog is licking. "A dog's mouth is cleaner than a human's!"
Although I'm quick to invoke this rationale, it's never really convinced me. If the comparison were a dog's mouth and cantaloupe, an entity with which my mouth is regularly interacting, I'd stop and think. But since I rarely let strange humans lick my face, there's no good reason strange dogs should either.
So is my dog's mouth really cleaner than mine? I've consulted a few experts, and it seems like a question of semantics as much as a question of hygiene.
+Truth is, oral bacteria are so species-specific that one can't be considered cleaner than the other, just different. [From LiveScience]
+If by cleaner you mean which mouth carries the least bacteria that would be harmful to a human, then a dog would probably be the answer. [From Indiana University's "A Moment of Science."]
Let's call it a draw. Go ahead and give your dog a big kiss to celebrate. Or some random person from your office.
[Thanks to Bailey for posing.]