The Halacha of "Hasisa" requires that when a woman immerses in the Mikveh, the water comes in direct contact with her body, with nothing in between. The question thus arises regarding the validity of a woman’s immersion if she suffers from dandruff in her hair. Do the flakes of dandruff constitute a "Hasisa," as they block the water from coming in direct contact with parts of her hair, thus invalidating her immersion?
The Halachic authorities addressed this question and concluded that if a woman thoroughly washes her hair with shampoo and hot water before her Tebila (immersion), then her immersion is valid, even if some dandruff remains in the hair. Once a woman washes her hair thoroughly to remove all dandruff that she is reasonably capable of removing, then any remaining flakes would fall under the category of "Mi’ut She’eno Makpid" – a substance covering a minority of the body which a person does not care about. A "Mi’ut She’eno Makpid" is considered insignificant and does not constitute a "Hasisa." As such, once a woman washed her hair, remaining flakes of dandruff would not invalidate her immersion, since she presumably does not concern herself with the residual dandruff.
Hacham Ovadia Yosef, in his work Taharat Ha’bayit (vol. 3, p. 82; listen to audio recording for precise citation), goes even further, writing that "Be’di’abad" (after the fact), if a woman immersed without washing the dandruff out of hair, her immersion is valid. Unlike lice eggs, for example, which stick to the hair, dandruff flakes are not actually attached to the hair, and thus they do not obstruct the water from coming in contact with the hair. Thus, although a woman should remove all the dandruff she can before immersing, if she immersed without removing the dandruff her Tebila is nevertheless valid. This is also the ruling of Rav Shemuel Wosner (contemporary), in his Shebet Halevi.
Summary: A woman who suffers from dandruff should thoroughly wash her hair with shampoo and hot water to remove all dandruff she can before immersing, and she may then immerse even if some dandruff remains. If a woman immerses without removing her dandruff, the immersion is nevertheless valid.
