The Shulhan Aruch (Yoreh De’a 198:18; listen to audio recording for precise citation) rules that the presence of dirt underneath a fingernail could disqualify a woman’s immersion in a Mikveh. If the dirt is situated underneath the part of the nail that extends beyond the flesh of the finger, then it is considered a Hasisa ("interruption") in between the woman’s body and the water, thus invalidating the immersion. If, however, the dirt is situated in between the skin and the nail, in the part of the nail that lies on the skin of the finger, then the dirt does not disqualify the immersion. The Shulhan Aruch then adds that since the precise spot where the nail begins to extend past the skin cannot always be clearly identified, the custom developed to cut one’s nails before immersion so they do not extend beyond the skin. This way, the woman can know with certainty that she does not have any dirt underneath her nails that could invalidate the immersion.
It thus emerges that although long nails do not intrinsically pose a problem with regard to immersion, nevertheless, there is a tradition dating back to the time of Maran (who lived some 500 years ago), and likely even earlier, that women cut their nails before immersing in a Mikveh.
Hacham Ovadia Yosef, in Taharat Ha’bayit (p. 107), addresses the situation that often arises in modern times, where a woman insists on keeping her long nails and refuses to cut them before immersing in the Mikveh. In such a situation, Hacham Ovadia writes, it is best for the Mikveh attendant to "look the other way" and allow the woman to immerse. As long as a woman had ensured to thoroughly clean behind the nails, her immersion is valid even if she did not cut them. Therefore, if the woman refuses to cut her nails for the sake of immersing, it is preferable to allow her to immerse with long nails, so that she and her husband would not be in violation of the severe prohibition of Nidda. However, Hacham Ovadia adds, after the woman immerses, the Mikveh attendant should respectfully and warmly explain that there is a time-honored tradition for woman to cut their nails before immersing. She should be told – in a pleasant and friendly manner – that although after the fact the immersion is valid if the nails were not cut, nevertheless, it is proper to follow this ancient tradition. Hacham Ovadia adds that this practice is mentioned already in the Zohar (Parashat Ahareh-Mot) and the R’aya Mehemna (Parashat Pinhas), both of which clearly speak of the need to cut one’s fingernails before immersing (listen to audio recording for precise citation). Hence, although long nails do not, after the fact, invalidate the immersion, one should follow this custom which has its roots in ancient sources.
It must be emphasized that we do not cavalierly dismiss accepted customs, especially customs which have been faithfully observed for centuries, as is the case with cutting nails before immersing. Hacham Ovadia’s ruling that a Mikveh attendant may allow a woman to immerse with long nails was explicitly reserved for the specific case of a woman who is unfamiliar with the custom to cut nails before immersing, and who would refuse to immerse if she had to cut her nails. In this particular instance, as mentioned earlier, it is preferable to overlook the custom to cut the nails in order to avoid a violation of the grave Torah prohibition of Nidda. Hacham Ovadia himself emphasized that as a matter of policy, fingernails should be cut before immersion. (We should also mention that fingernails tend to grow rather quickly, and thus cutting them before immersion does not entail a considerable sacrifice on the woman’s part.)
The same is true about nail polish, a topic we began discussing in an earlier edition of Daily Halacha. As we saw, several early authorities permitted immersing with polish on the skin or nails, because in their view, a substance qualifies as a Hasisa only if it is unwanted. In the case of polish, the precise opposite is true – the woman specifically applied it for cosmetic purposes. Therefore, technically speaking, unless the polish is cracked and the woman wants it removed, it should not constitute a Hasisa. Nevertheless, the time-honored tradition is to remove nail polish before immersion. As in the case of long nails, if a woman refuses to remove her nail polish before immersion, the Mikveh attendant should allow her to immerse (as long as she can ascertain that there is no dirt underneath the nails). But as a matter of policy, it is certainly proper to remove nail polish before immersing.
Summary: There is a time-honored tradition that women remove nail polish and cut their nails before immersing in a Mikveh. In a case where a woman refuses to comply with this custom, and would not immerse if she is forced to, the Mikveh attendants may allow her to immerse with nail polish and long nails, as long as the nails are otherwise thoroughly clean. As a matter of policy, however, one should follow this time-honored custom.