Finishing menses and counting seven clean days are necessary for the cessation of the niddah status, but are not sufficient. Even if years have gone by since the last menses, a woman retains her status of niddah with all its attendant prohibitions until she immerses properly in a kosher mikveh. Proper immersion in a mikveh requires that a woman's entire body, including all of her hair, must be submerged in the water at one time. In general, anything that adheres to the body or hair and prevents contact with the water invalidates the immersion. Such a barrier is called a chatzitzah.