Hot Hands and several other brands can reach 120-135F which is uncomfortably warm in direct contact with skin. I find that between a wool sock and my boot, or between wool liners and my mitts, they are comfortable when outdoors in cold weather, but get a little toasty once you get out of the wind. Beware of some of the knock-off brands which can vary widely in their temperature range, I have heard stories of people getting burns from some of them.