Links in this essay will take you to information about the usage experts and their work. Numbers in parentheses are page references in the sources.
who versus which or that
Which is clearly impolite as a substitute for who, but what about that? On this matter, opinions are somewhat divided.
Surprisingly, many experts see nothing wrong with using that to refer to a person, as in We hope to hire a landscape designer that has experience with rock gardens. The American Heritage Book of English Usage is especially adamant in defending this usage:
Some people say that you can only use who and not that to introduce a restrictive relative clause that identifies a person. But that has been used in this way for centuries. It is a quintessential English usage, going back to the Old English period, and has been used by our best writers. (40)
Richard Lederer and Richard Dowis acknowledge the historical precedent but claim that "most modern stylists use who when referring to a person and that when referring to a thing" (50). Bryan Garner sees both sides: “That, of course, is permissible when referring to humans,” but editors "tend . . . to prefer" who (862).
In contemporary English, says Wilson Follett, the word that "carries a thing-like connotation" (327). For this reason, referring to a person as a that can seem dehumanizing. Also, says Follett, using that for who can lead to awkwardness in sentences containing other uses of that. He quotes part of a sentence to illustrate his point:
. . . fully persuaded that children that undergo the handicap of teachers that have only a perfunctory grasp of these fundamentals are to be pitied. (326)
Surely children who and teachers who would be more graceful phrasing.
Conclusion: Though you may get away with using an occasional that for who, on the whole it's safer to stick with who. Some readers will object to that—finding it an impolite way to refer to a person—but no one will find fault with who.
Source: Hacker Handbooks (Boston: Bedford, 2013).