Gemara

71a (Gemara - Rabah): Just like Tum'ah Belu'ah (enveloped) is not Metamei, also Taharah Belu'ah does not become Tamei.

72b: Anything destined to be cut, it is as if it was cut;

(Mishnah): If a handle of a Kli is too long and is destined to be cut, when immersing the Kli, one must immerse as much of the handle as he will keep;

R. Yehudah says, the entire handle must be immersed.

Nidah 66b (Rava): One should accustom his wife to rinse inside folds of skin with water.

Question (Beraisa): Water need not enter folds of skin or Beis ha'Starim (places normally covered by other parts of the body).

Answer: Granted, water need not enter them, but they must be (free of Chatzitzos,) such that water could enter, like R. Zeira's law:

(R. Zeira): If (the flour and oil of) a Minchah could have been mixed, mixing is not Me'akev. If it could not be mixed (it is too big), mixing is Me'akev.

Nazir 44b (Mishnah - R. Akiva): If a Tamei Nazir shaved on day eight, he may bring Korbanos that same day.

(Rava): This is when he immersed on day seven.

Rishonim

Rosh (Hilchos Mikva'os 30): The Halachah follows the first Tana. If a handle of a Kli is too long, one must immerse as much of the handle as he will keep. A Tosefta explains why the place it will be cut is not a Chatzitzah. We are not concerned for Beis ha'Setarim (covered places) of Kelim.

R. Shimshon (Mikva'os 10:8 DH bi'Zman): The Mishnah says that if an arrow is stuck in a person, it is a Chatzitzah only if it is visible. If not, he may immerse and eat Terumah; this is unlike Beis ha'Setarim, for which water must be able to get there. If a scab formed over it, all agree that it is not a Chatzitzah. If pebbles or chips entered the cracks of his feet from below, it is not a Chatzitzah, for it is like Beis ha'Setarim. Rebbi says that a bandage on Beis ha'Setarim is a Chatzitzah. R. Yosi b'Rebbi Yehudah disagrees. I explain that if one is not particular about something, it is not a Chatzitzah. Many Mishnayos and Beraisos connotes that we are not concerned for Chatzitzos in Beis ha'Setarim. A Tosefta says that the Beis ha'Setarim of a single woman is not a Chatzitzah. Really, we require that water can get there (Kidushin 25a, Nidah 66b). Rather, we are not concerned for Chatzitzos there. This is why we distinguish between men and women, and between married and single women. A married woman is particular about her Beis ha'Setarim, lest she be repulsive to her husband.

Mordechai (Hilchos Mikva'os 751, after Hilchos Nidah in Shevu'os): Women who have knots in braided hair due to Shedim, and it is dangerous to shave it, this is not a Chatzitzah. Since it is very tight, it is like Belu'ah and Beis ha'Setarim, which is not Metamei. Therefore, it is not a Chatzitzah. Even if you would not consider it Belu'ah and Beis ha'Setarim, three tied hairs is not a Chatzitzah, because they are not tight and water enters. All the more so, more hairs are not a Chatzitzah. Also, since they cannot shave it due to danger, it is Revisei (it grows with her), which is not a Chatzitzah. This is why a mother is not a Chatzitzah between her fetus and the water. Ge'onim say that dough under a fingernail is a Chatzitzah, but the fingernail itself is not, even if it is long and destined to be cut and hovers above the skin. This is because it is part of the body.

Poskim

Shulchan Aruch (YD 198:1): A woman must immerse her entire body at once. Therefore, there cannot be any Chatzitzah at all. If sometimes people are particular about it, it is a Chatzitzah even if she is not particular about it now, or even if she is never particular about it, since other people are normally particular. L'Chatchilah she should not immerse with things that are not Chotzetz, lest she immerse with things that are Chotzetz.

Pischei Teshuvah (1): Zichron Yosef (YD 10) says that it is a Chatzitzah only if she is particular about it to remove it at set times. This is like the Taz writes, that a tight ring it is a Chatzitzah because she removes it when kneading. If she will have dough to knead several times today or tomorrow, she will remove it. If she is particular about it only once in a long time, it is not a Chatzitzah. The Shulchan Aruch says 'even if she is not particular about it now.' This does not mean that she is particular about after a long time. Rather, she is not particular about it at the time of Tevilah, but she is particular about it on the days before and after.

Shulchan Aruch (6): If hairs over the heart or of the beard are stuck to each other due to sweat, it is a Chatzitzah. Hairs of the head or underarms are a Chatzitzah (in this case). Hairs of the Ervah are not a Chatzitzah in a man or a single woman. They are a Chatzitzah in a married woman.

Rema: Some have hair that is like woven and stuck together. It is done at night, through a Shed. It is dangerous to remove it. It is not a Chatzitzah.

Gra (13): We say (Sa'if 5) that two hairs (stuck together) are not a Chatzitzah. Also, since they cannot remove it, it is like Revisei. Handles of Kelim attached normally are not a Chatzitzah. If they were attached abnormally they are a Chatzitzah, for they are destined to be removed.

Chasam Sofer (YD 195, cited in Pischei Teshuvah 12): Brides immerse when their hair is long. They will cut it short after Bi'as Mitzvah. There is no concern for Chatzitzah. A Tosefta that says that a fingernail or skin that separated is a Chatzitzah. The Beis Yosef explains that anything destined to be cut, it is as if it was cut, so it is a Chatzitzah in Beis ha'Setarim, where the excess is attached to what will remain. The Gemara says so. We say that anything destined for something, it is as if the matter was already done when it is destined to be done immediately, without interruption. Tosfos (Bava Kama 76b DH Kol) says that since Kabalah must be done between Shechitah and Zerikah, we do not say at the time of Shechitah that it is as if the blood was thrown. Here, the custom is not to cut the hair until after Bi'as Mitzvah. Brides are particular to have long hair until then. Since they are not destined to be cut at the time of Tevilah, and the Bi'ah interrupts, it is not a problem. Also, the reason to be stringent is because it is a minority that she is particular about it. Just the contrary, she is particular to have long hair at the time! Another clear proof is from a Nazir Tamei who immersed on day seven and shaved on day eight. He was Yotzei, even though his hair was destined to be cut (Nazir 44b). However, perhaps the hair that will be cut is the majority (of all the hair), and the majority is a Chatzitzah even if one is not particular. However, the first reason is solid.

Shulchan Aruch (22): Dangling limbs or flesh are a Chatzitzah.

Beis Yosef (DH Tziporen and DH v'Tzarich): A Tosefta says so. What is the reason? Do not say that they are not considered part of the body, and they are like something stuck onto the body. There, it is stick onto a revealed place (the skin) where water could come. Therefore, it is a Chatzitzah. Here, the limb is attached to a place that was never exposed! Perhaps once it becomes dangling and ceases to nourish from the body, it is not considered part of the body. It is destined to be cut, so it is as if it was cut, and the place of the cut was exposed and water could go there. It is like Beis ha'Setarim, to which Chatzitzah applies. We learn from handles of Kelim that are too long. One must immerse as much of the handle as he will keep. The Gemara explains that anything destined to be cut, it is as if it was cut. R. Shimshon asked that if so, there is a Chatzitzah where it will be cut! He and the Rosh answered that a Tosefta teaches that we are not concerned for Beis ha'Setarim of Kelim. Regarding people, a Chatzitzah in Beis ha'Setarim is Me'akev.

Pischei Teshuvah (16, citing Noda bi'Yehudah YD 64): A woman who had a wax ring inserted deep into the Pruzdor for medical reasons. It is a Chatzitzah for Tevilah, for it gets dirty through Dam Nidah and she must remove it when doing a Hefsek Taharah to clean it, and at the time of birth, and perhaps when doing Bedikos. However, even though Chachamim said that it must be possible for water to enter Beis ha'Setarim, a place that is Belu'ah need not be. If it is so deep that the Ever does not reach there in a full Bi'ah, it is not a problem.

Rebuttal (Pischei Teshuvah 16, citing Chasam Sofer YD 192): Even though she must remove it when doing a Hefsek Taharah and at the time of birth, it is not a Chatzitzah. She does not remove it at the time of birth due to concerned for the ring or the dirt, rather, to clear room for the fetus. We learn from a man's ring without a stone. The Magen Avraham (161:11) says that it is not a Chatzitzah. This is even though he removes it when putting on Tefilin and taking the Lulav. However, if she must remove it at the time of Hefsek Taharah, this is called that she is particular about it.