Pure As the Driven Snow

Reader's Digest 5月号の記事 Six Idiotic Idioms--and What's Wrong with Them は昔からのイデオムに疑義を投じています。　その内の一つを引用します。
Pure As the Driven Snow
The flaw: Driven snow is toxic.
"Driven" snow has been blown into drifts and remained untrodden-upon by human heels. Sounds pure, but according to Canadian researchers, fresh white snow is a magnet for car-exhaust pollution, absorbing enough toxins while "driven" to become a health hazard if you were to drink enough of it.
"Pure As the Driven Snow" は3/28/2017 に取り上げた "as clean as the driven snow" と同義の表現ですね。