Question:
Sometimes the back of my hand will touch something unclean (eg someone's shoe, or the area above someone's elbow, etc). Does this necessitate washing my hands?
Thank you
Answer:
According to most opinions the hand extends until the wrist, and not only until the knuckles, therefore when any part of the hand touches an something or when there is a ruach tuma, we wash it until the wrist.
Sources:
O:CH 161-4, M:B 161 21, 22.The Kaf Hachayim 613-11 explains that the halacha is that the hand extends until the wrist, the reason that we only wash our hands until the knuckles on Yom Kippur and T'isha B'av is since there is an opinion that the hand is until the knuckles, we are leneint according to that opinion for Yom Kippur and T'bav.