From Oxford Dictionaries Online: "It is sometimes argued that, in relative clauses, that should be used for non-human references, while who should be used for h

From Oxford Dictionaries Online: "It is sometimes argued that, in relative clauses, that should be used for non-human references, while who should be used for human references: a house that overlooks the park but the woman who lives next door. In practice, while it is true to say that who is restricted to human references, the function of that is flexible. It has been used for human and non-human references since at least the 11th century, and is invaluable where both a person and a thing is being referred to, as in a person or thing that is believed to bring bad luck." (Sorry if I'm being insufferable :-)