The purifying waters of the miqveh must touch the woman's body completely without anything intervening. Defining these intervening objects is not always simple. The following laws describe the precautions which should be taken to ensure that the waters of the miqveh touch the whole body. It also goes into some definitions of intervening substances*.
1. An adult Jewish woman should observe the immersion in order to make sure that all of the immersing woman's hair is covered by the water.
2. If there is no one around to make sure that all of her hair is submerged she should wear a beret made of net or tie her hair loosely with a netted string so that she is sure that all of her hair is all submerged.
3. Pubic hairs stuck together with sweat intervene and must be cleaned.
4. Mucus discharged from the eyes and mucus in the eye which is turning green intervene and must be cleaned.
5. Scabs on a healed wound intervene. (A healed wound is one where the underlying skin has already healed.) Moist puss on a wound is not an intervening substance. Puss is considered dry after three days. Therefore a woman who has a lot of scabs should soak herself in warm water until the scabs soften.
6. Bandages intervene and when removing bandages one should be careful to also remove the glue since that also intervenes if not removed.
7. A splinter intervenes unless it is completely covered and is not even seen.
8. Congealed sweat intervenes and should be washed away.
9. Dough balls intervene and should be washed away. (It is suggested that one not involve oneself with making dough on the day of going to the mikveh, if possible.
10. Smeared blood even if it is still moist intervenes.
11. Although long nails do not intervene, it is proper for the woman to trim her nails before immersing since they are difficult to check for dirt. However, if the woman had her nails manicured (and the polish is not chipped) and she refuses to remove the polish or trim her nails, she must check underneath them very carefully to make sure there is no dirt. Nevertheless, one should attempt to explain to her in a pleasant way that it is better to have her nails trimmed and polish-free. If she still refuses the polish can be ignored and she may immerse.
12. If her fingers are swollen and she cannot even see the soil underneath her fingernails, the soil does not intervene.
13. Although it is customary to remove all jewelry before immersing, technically loose-fitting jewelry does not intervene
14. If a woman mistakenly immersed wearing loose jewelry, her immersion counts since the mikveh water passed through to her body**.
15. Something stuck in between the teeth intervenes (even though the woman does not have to open her mouth while immersing, the water still has to theoretically be able to reach her whole body), therefore, a woman should brush and floss every tooth before immersing. In addition, for this reason it is customary for women not to eat meat the day that they immerse. This does not apply on Shabbat and Yom Tov.
16. A woman should not eat anything between the preparatory bathing and immersion.
17. If, while immersing, a woman held her hair in her mouth or clenched her lips or hands in a way that the water could not flow to them, she is considered not to have immersed.
18. She need not open her mouth so that water goes in but it is forbidden to clench her lips, instead she should have her lips lightly stuck together. The same applies to her eyes: that they should not be too widely or too tightly closed
19. If a woman needs someone to hold her hands while she is immersing, she should not hold them tightly unless her hands are wet since the moisture connects the mikveh to her clenched hands.
20. If the floor of the mikveh is solid and their is no mud on the floor she need not raise her feet off the floor.
21. A woman should not immerse in a place where there are many people, like a pier, since she will rush and not properly immerse. However, if she has already immersed in such a place it counts, provided that she is sure that she has properly immersed.
22. Pebbles stuck between a woman's toes intervene.
23. It is suggested that a woman attend to her needs and go to the bathroom before immersing in a mikveh.
24. Lice stuck to the body should be removed with hot water and pulled out, if they are impossible to remove then they do not intervene.
25. A woman who normally trims her nails before immersing and inadvertently left one of her nails uncut, should return to immerse in the mikveh if she did not yet have relations with her husband (especially if she did not check underneath it.)*** A blessing should not be said for this repeat immersion.
26. If a woman who needs to immerse on Friday night or on a Holiday forgot to trim one of her nails, she should clean under them very well and immerse.
27. Many Sephardic women are strict to shave all of their pubic hair before immersion. However, if both husband an wife do not mind then she need not do so, being as such practices follow commonly accepted norms.
28. Immovable bandages which remain in place for months and will cause pain and anguish if removed do not intervene. If it will be removed after a short amount of time one should properly wait until they are removed.
29. For such problems as casts or splints the rabbi should be consulted.
30. A woman who has an internal ring keeping her cervix closed may immerse like any other woman.
31. Under extenuating circumstances a woman may immerse wearing a loose garment so that people will not see her unclothed.
* The laws of this chapter are adapted from Shulkhan 'Arukh Yoreh Dea'ah chapter 198
** Chacham Ovadia Yosef, Yabia Omer vol. 2, Yoreh Deah, sec. 13
*** If she did have relations, she should not return to the mikveh because she may have become impregnated and then the child might mistakenly be looked at as a child conceived of forbidden relations.