2,117. Three Layers of Cloaks; Pigs

Hilchos Issurei Biah 9:27

If a woman was working with red, she may not ascribe a black stain to it. If she was working with a bird that had several different colors of blood and she found one of these on herself, she may ascribe the stain to it. Let’s say that a woman was wearing three cloaks. If there’s an outside cause to which she could ascribe a stain, she may do so even if the stain is found on the innermost cloak. If there’s no outside cause to which she might ascribe the stain, she may not attribute it to anything, even if it’s only found on the outermost cloak. Therefore, if a woman passed through a butcher market, she may ascribe a stain to blood from the butcher even if the stain is only found on her innermost cloak. If she didn't pass through a butcher market or something similar, she is unclean even if the stain is only found on her outermost cloak. If she doesn’t know whether or not she passed through or whether or not she was working with something that could stain, she may not ascribe her stain to an outside cause.

Hilchos Issurei Biah 9:28

If a city has pigs, or if they enter the city regularly, a woman need not be concerned about stains that are found on her cloak.