The Hebrew niddah (H5079) refers to the state of separation or impurity, particularly associated with menstrual blood (Leviticus 15:19-33) but extended to other forms of severe ritual uncleanness and, metaphorically, moral corruption. Ezekiel 36:17 uses niddah for Israel's moral defilement: 'their conduct was like a woman's uncleanness in my sight.' Numbers 19:9 uses 'water of niddah' for the purification water made from the red heifer β the ritual for removing the most severe uncleanness.
The niddah regulations in Leviticus establish a ritual world where physical bodily states reflect spiritual realities. Uncleanness is not sin but a symbol of the human condition before God β we are 'by nature deserving of wrath' (Ephesians 2:3). The water of purification (Numbers 19) involving the ashes of a red heifer is perhaps the most elaborate purification ritual in the OT, and the author of Hebrews uses it as the platform for declaring Christ's blood 'how much more' effective: 'how much more will the blood of Christ... cleanse our consciences from acts that lead to death' (Hebrews 9:13-14).