Question:
If someone already washed Netilas Yadayim for bread and dried his hands and then touched the dry hand of a person who did not yet wash, must he wash his hands again?
Answer:
No. If both people’s hands are dry, and the person who already washed for bread touched the hand of someone who did not yet wash, he is not required to wash his hands again.
The reason is that merely touching the dry hands of a person who did not wash does not transfer impurity in a manner that invalidates his washing. Nevertheless, some are stringent not to shake another’s hand who did not wash.
This should be contrasted with a case where the other person’s hands are wet. If the hands are wet from a prior washing, the Poskim discuss whether the moisture can potentially transfer impurity to dry hands, and the Alter Rebbe is Machmir. Accordingly, the discussion and concerns are greater in a case involving wet hands than in the ordinary case of two dry hands touching.
Thus, the common case of one person touching the dry, unwashed hand of another after Netilas Yadayim does not require rewashing.
Sources:
Seder Netilas Yadayim 3; Admur 162:13 (in parentheses); Ketzos HaShulchan 32 footnote 10; see also Ketzos HaShulchan 33 footnote 13 regarding hands that are wet from a prior washing; Piskeiy Teshuvos 162:11footnote 69