Rambam - 1 Chapter a Day

These are the substances that intervene for a person: the secretion outside the eye, the fluid outside a wound that crusts, dried blood over a wound, a bandage over a wound, crusts of filth on one's flesh, dough or mud under one's nails, particles of mud or dough that collect on one's flesh, thick mud, potters' clay, and mud that is found in the thoroughfares at all times, even in the summer. All of these substances intervene. Other mud does not intervene when wet, because it will dissolve in the water. When it is dry, it is considered as an intervening substance.

When the following: honey, ink, milk, blood, and the sap of berry bushes, fig trees, wild fig trees, and carob trees are dry, they are considered intervening substances. When they are moist, they are not. The sap of other fruits are considered as intervening substances whether moist or dry.

Blood that sticks to the flesh, even if it is moist, is considered an intervening substance. A loosely hanging limb or flesh is considered an intervening substance.

A woman's hidden area is considered to have intervening substances present unless she washes before her immersion, because that portion of the body is always sweaty and dust collects there and intervenes.

To whom does the above apply? To a married woman. Different rules apply for an unmarried woman. Since she is not concerned about the cleanliness of that area, it is not considered to have intervening substances present.

The following rules apply to bandages on a wound, metal plates on a broken bone, necklaces, noserings, choker necklaces, and rings. When they are firm and cling to the flesh, they are considered as intervening substances. If they are loose, they are not considered as intervening substances.

Strands of wool, strands of flax, and straps that women tie to their heads as adornments are intervening substances, because they separate between the body and the water. Strands of hair are not considered intervening substances, because water penetrates through them, even when they are not loose.

Strands tied around one's neck, even of flax, are not considered as intervening substances, because a woman does not strangle herself with them. Tight necklaces, e.g., choker necklaces and necklets, are intervening substances, because a woman chokes herself with them to look fat.

Hair over the heart that became tangled and matted and similarly, matted hairs of the beard are intervening substances.

When an arrow is stuck in a person's flesh, if it is visible, it is considered an intervening substance. If it is not visible, he may immerse in a mikveh and partake of terumah in the evening even though the arrow itself is impure. Similarly, if a person swallowed an impure ring and immersed, he is pure. If he vomited it out after immersing, he contracts impurity, because of contact with it. For, as we explained, any entities swallowed within the body of a living being, do not impart impurity or contract impurity.

If pebbles or splinters enter the cracks on the soles of a person's feet, they are considered as intervening substances.

When there is a bandage, a compress, or a dressing over the hidden parts of a person's body, they are considered as intervening substances. The rationale is that even though water need not enter these parts, they must be fit for water to enter them and not have any intervening substances upon them, as we explained.

If there was one or two hairs that were outside the wound, but their tips were stuck to the wound, there were two hairs with mud or filth stuck to their tips, or there were two hairs from one's upper eyelashes that were perforated and hairs from one's lower eyelashes were threaded through them, they are considered an intervening substance.

A person should not immerse with dust on his feet. If, however, he did, the dust is not an intervening substance.

When one holds another person or an implement and immerses them, they remain impure. This applies even if he loosens his hand and allows water to reach the person or the article being immersed. This is a decree, lest he not loosen his hand. If he rinses his hand in water first, the immersion is acceptable.

When a woman slings her infant son over her back and immerses, the immersion is not acceptable, for perhaps there was mud on the infant's feet or hands and it became attached to his mother at the time of her immersion and intervened between her body and the water, but it fell off after she emerged.

When a woman who was a nidah put her hair in her mouth, clenched her fists, pursed her lips, or a bone was discovered between her teeth, it is as if she did not immerse. This applies to other impure individuals as well.

If she placed coins in her mouth and descended and immersed, she is purified from nidah impurity, but she is impure because of her spittle. Thus she is a primary derivative of impurity, like one who touched the spittle of a nidah. The same laws apply with regard to a zav.

These are the substances that do not intervene for a human: the matted hair of the head, the arm pits, and the hidden parts of a man, the secretions of the eye, the scab above a wound, moist drops of filth on his skin, filth that is under a nail, a loose-hanging nail, and the thin hair that is on the skin of a child. All of these substances are not considered as intervening.

When two or more hairs are knotted in one knot, they are not considered as an intervening substance, because the water can penetrate through them. If one hair is knotted, it is considered an intervening substance, provided the person is particular about it. If, however, he is not particular about it, his immersion is acceptable unless the larger portion of his hair is tied individually hair by hair. This is the ruling of the Geonim.

It appears to me that a person's hair is considered like his body with regard to immersion and is not a separate entity to the extent that it is inappropriate to speak of "the larger portion of his hair." Instead, even though all of a person's hair is tied hair by hair, if the person is not particular about the matter, his immersion is acceptable unless the hairs are combined with another intervening substance on his body and thus there are intervening substances covering the larger portion of his body, as we explained. The laws mentioned here apply equally to a woman in the nidah state and to other impure people who have hair on their heads.

Ezra ordained that a woman should comb out her hair and then immerse. If it is possible for her to comb out her hair at night and immerse immediately thereafter, it is praiseworthy. In a pressing situation or when there is a question of sickness, she may comb out her hair - say - on Friday and immerse Saturday night.

The following rules apply when a woman immersed and then an intervening substance was found upon her that disqualified her immersion. If she immersed on the same day that she prepared herself, she does not have to prepare herself again. Instead, she may remove the intervening substance and immerse again immediately. If not, she must prepare herself a second time and then immerse.

A woman should not comb out her hair with natron, because it causes the hairs to snap, nor with oak sap, because it snarls the hair. Instead, she should use hot water, even water heated in the sun, because it hardens the hair and enables it to be combed out. Cold water, by contrast, tangles the hair and causes it to contract and thus become intertwined.

When a nidah gave cooked food to her son and then immersed, the immersion is invalid because of the fatty substances on her hand.

The following laws apply if a woman scratched her skin to the extent that it bled and then immersed. If the immersion was within three days of the bleeding, the place of the scratches is not considered as intervening. After three days, it is considered as intervening because the blood coagulates there, like a scab on the wound.

Similar concepts apply with regard to secretion in the eye. If it is dry and has begun to change color, it is considered as an intervening substance for a nidah.

Blue ointment in the eye is not considered as an intervening substance. If it is outside the eye, it is considered as an intervening substance. If her eyes open and close frequently, even the ointment outside the eye is not considered as an intervening substance.

If, while immersing, a woman opened her eyes to a great extent or closed them very firmly, the immersion is not acceptable.

With regard to what does the above apply? With regard to immersion to partake or touch pure foods. With regard to permitting intimacy with her husband, by contrast, she is permitted even if she gave cooked food to her son, she had an old scratch, there was blue ointment above her eye, or she opened her eyes very widely or closed them tightly. The rationale is that all of these matters and other similar ones are considered as intervening only according to Rabbinic Law and the Sages ordained their decree with regard to pure foods, but not with regard to intimacy.

Whenever a substance is considered as intervening for a nidah with regard to pure foods, it is considered as intervening for other impure people with regard to pure foods and as intervening for a convert when immersing during conversion.

When a person immersed and after he ascended an intervening substance was found upon his body, even though he was involved with that substance for the entire day after the immersion, he is considered as impure unless he says: "I know with certainty that this substance was not upon me before the immersion." The rationale is that since he was categorized as impure, he is presumed to remain in that state until it is known for certain that he regained purity.