In terms of first-order logic, "Humans have hair" is suggestive of ∀x(human(x) → has_hair(x)). Distressingly, "natural" language is not based on first-order logic, so it comes out meaning something more like "a proportion of humans sufficient to establish a general principle (this proportion being entirely subjective) have hair".
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Also, I'd say context is just as important as the actual statement in interpreting it.
I.e. [alien biologist, upon coming upon an unidentified hairless specimen, to colleague insisting it's human] "Humans have hair."
not really the same as: [alien biologist, upon coming upon an unidentified hairy specimen, to colleague insisting it's not human] "Humans have hair."
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Not all humans have hair on their heads.