Siman 86

Just as one must distance oneself from excrement and urine, so too one must distance oneself from foul-smelling water, and from soaking-water in which flax or hemp is soaked, and likewise the soaking-water of laundrymen in which they wash the garments of small children that have excrement on them, and likewise from vomit that has a bad odor or foul-smelling secretions, and likewise in a place where there are slops in a vessel that has stood for a long time and smells foul—one must distance oneself from this. And likewise foul-smelling mikvaot—it is forbidden for a woman to recite a blessing in them. And if one channeled a foul-smelling thing near a synagogue—he is obligated to remove it from there, and even if he preceded the synagogue, for it is a matter of prohibition (see Sha'arei Teshuva).

The measure for distancing from these matters: The Tur wrote that they are like excrement; from before him, as far as his eyes can see, and from behind him, four amot from the place where the odor ceases, see there. And so it is implied in the Mishnah (22b), as we learned: One may not cover himself with foul water or water used for soaking flax... And how much must he distance himself from them and from excrement? Four amot. Behold, it equated them to excrement. However, Rashi explained there: And how much must he distance himself from them—from urine? See there. And so the Rambam wrote in the Commentary on the Mishnah, see there, and so he wrote in his work, chapter three, law 8, see there. And more than this, for in the second chapter of Laws of Reciting the Shema, law 7, where he wrote this law of foul water and water used for soaking flax, he did not mention distancing at all, see there. And it requires further study regarding the law.

And know that the Rambam in the fourth chapter of Laws of Prayer, law 9, wrote that the great sages would not pray in a house that contains beer, nor in a house that contains fish brine during its time of fermentation, because its odor is foul… Thus far his words. And the intention is not that its odor is foul like that of filth, for if so, it would be prohibited by the law. Rather, the intention is that the intensity of the odor from this is difficult to bear for one who is not accustomed to it, as it is like the odor of drunkenness. And so wrote Rashi in Eruvin (65a), see there. And this is not comparable to all the foul odors that have been explained.