Steadman
I have no reference other than personal experience.

Many people live in extreme cold that also have faces that are exposed to the cold. My Swiss fiancee had permanent spots on her cheeks from the cold. People also function at the poles but I think they wear face coverings.

There are stories of climbers having a glove fall off at extreme cold and not really suffering because the hand was the only part of the body not heavily insulated and it almost glowed with the heat it was disapaiting. I have noticed that the more insulation you wear, the warmer your face is. My military extreme cold mitts have fleece on the back of the hands for warming the face and nose.

Per Lori's post - for your skin to reach 28 degrees it would take either a lot of windchill or a prolonged exposure to temps significantly below 28 degrees.

Following my post - the more of your body that is exposed, the more easily it will frostbite.

I would be more concerned with keeping bare flesh covered [windchill only applies to exposed flesh], and with the wind speed, than a certain temperature. Finally - you don't hear a lot about frostbight at temperatures above zero F.

Jim