Answer: In the previous Halacha, we have explained that one must make sure there is no substance separating between one’s hands and the water at the time one performs Netilat Yadayim. We have also discussed that there is a difference between whether the substance that causes the separation covers a majority of the hand, in which case this constitutes a “Chatzitza” in any event, and when the substance covers only a minority area of the hand in which case this does not constitute a “Chatzitza” unless most people are meticulous that such a substance not remain on the hands at all. Now, let us explain this matter further.
Filth or Residue Which is Extremely Repulsive
Any filth which is extremely repulsive, such as excrement or mud, which is under one’s fingernails constitutes a separation for Netilat Yadayim since most people are careful not to have such filth under their nails. Thus, if one performed Netilat Yadayim with such repulsive filth under one’s fingernails, one must wash again after having cleaned out one’s nails (see Mishnah Berura, Chapter 161, Subchapter 2).
Non-Repulsive Filth- “Level With the Flesh” or “Above the Flesh”
Nevertheless, regarding filth under one’s nails which people are generally not so careful to remove periodically, it then depends: If this filth is “level with the skin under one’s fingernails”, it does not constitute a separation. If it is “above the skin under one’s fingernails”, it does indeed constitute a separation, as we shall now explain:
A portion of the fingernail is attached to the skin of the finger while another part of the nail grows longer and juts out over the skin of the finger. The part of the nail which juts out above the skin must be cleaned before Netilat Yadayim; however, if there is some dirt stuck to the part of the nail which is attached to the skin, one need not clean it out, for people do not generally mind its presence and do not always clean it out since doing so would constitute great bother and effort.
Dough or Modeling Clay
Nevertheless, this applies only to regular dirt under the nails. However, if a woman kneads bread and has leftover dough stuck under her nails or a man has leftover modeling clay under his nails after he has finished his work and the like, this does constitute a separation for Netilat Yadayim and it must be removed, for people are generally careful to remove such substances (see Shulchan Aruch 161).
Summary: Regarding dirt stuck under one’s fingernails, if it is a substance that people are usually careful to remove, such as leftover dough, modeling clay, or an extremely repulsive substance, this always constitutes a problematic separation regarding Netilat Yadayim and must be removed.
Regarding normal dirt found under the fingernails, if it is on the portion of the nail which is attached to the skin of the fingers, it need not be removed, for it does not constitute a problematic separation since people are not usually so careful to remove it. However, if it is stuck to the part of the nail which has grown above the skin of the fingers, it must be removed before washing one’s hands. (If one has already washed one’s hands without removing such regular dirt under the nails, nowadays, when people are not so meticulous about cleaning their nails, this does not invalidate one’s Netilat Yadayim for one who does not usually clean out one’s fingernails, see Halacha Berura, Volume 8, page 227.) Nevertheless, substances like dough and the like are considered a problematic separation regarding Netilat Yadayim.