To exit the halachic status of niddah and become tehorah, a woman needs to immerse in a mikveh. For her tevilah (immersion) to be valid, her entire body, including all of her hair, must be submerged in the water at once. Anything that prevents the water from reaching all parts of her hair and body, even while she is submerged, may be considered a chatzitzah (barrier) that would invalidate her immersion.
According to Torah law, a chatzitzah invalidates immersion when it meets both of two conditions: it covers the majority of the body (rov) AND the person immersing minds its presence on the body (makpid). According to some authorities, the majority of hair can also be considered “rov.” Halacha often defines “makpid” in terms of the intention or desire to remove the substance in question.
According to Rabbinic law, a chatzitzah invalidates immersion when it meets either of the two conditions: it covers the majority of the body OR one minds its presence.
In practice, when a woman immerses, even a small barrier is typically considered a chatzitzah if she ultimately plans to remove it. Even when a woman does not mind the barrier’s presence at the time of immersion, it is considered a chatzitzah if most women would mind its presence, or if she herself normally removes it at some other time (e.g., a ring that she removes when kneading dough).
There is a binding custom to remove any foreign item or substance prior to immersion, even if it meets neither condition. That is to say, items are removed even when they are small (mi’ut) and a woman is not particular about them (eino makpid).
Although the custom is to remove everything, there are more grounds for leniency when an item is not tightly attached to the body, and thus does not in fact impede contact with the mikveh water. For example, for an immersion outdoors when a more private setting is unavailable, a woman might be permitted to immerse in a loose-fitting t-shirt.
