I've noticed that when I'm carrying a load and moving, I generate a lot of heat. Like, it can be 32 degrees F out (0 degrees C) and I'm in pants, a short sleeve shirt, a hat and gloves to keep from sweating out my clothing.

At what point (environmental temperature) do I need to worry about frostbite, and guard against it by ensuring I have a layer on my exposed skin?

Do you have a reference to support your opinion?