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If a woman finds a stain on a surface that is not mekabel tumah (susceptible to ritual impurity), then that stain does not make her niddah. (For further discussion, see here.) Therefore, if her underwear is made of a material that clearly is not mekabel tumah, then a stain found on it does not make her niddah.

We follow the view that fabrics that are 100% nylon or polyester are not mekabel tumah. If the fabric is a blend with more than 50% synthetic material, as with some underwear, there can be halachic room to also consider it as not mekabel tumah. Stains on the crotch of underwear (if white) would usually need to be evaluated in terms of color and size, since the crotch is commonly made of mostly natural fabrics such as cotton and would be mekabel tumah.

Rayon and viscose are a special case, since the materials are derived from plants before processing. We would ordinarily consider them to be mekabel tumah, except perhaps in very extenuating circumstances.

Wearing colored undergarments, regardless of the fabric content, can help avoid questions about stains.

We hope this helps. Please let us know if we can be of further assistance.

This response was updated on 9 November, 2025.