The braisa states: If there would be a spring that belongs to an upper city, from which water flows down to another city; all agree that the residents of the first city can stop the flow to the second city if they need it for drinking water in order to live, even at the cost of the lives of the second city. The rationale is “chayecha kodmin,” their lives take precedence over others. Their animals take precedence over the animals belonging to others. Their laundry takes precedence over the laundry of others. If there is a choice between the lives of others and their own laundry, the lives of others take precedence over their own laundry. Rabbi Yosi holds that their own laundry takes precedence over the lives of others. (Refraining from laundering would cause so much pain that it is regarded as being a life-saving issue.)

Reb Avi Lebowitz discusses the halachic ramifications that can be derived from this Gemora. Based upon the version of the Ya’avetz in the Mefaresh, this halacha is only true if the actual spring is inside the upper city, so that the water belongs to them and the only question is whether they have to risk their lives to share it with others. But, if the spring was located above both cities and merely flowed through one city to another, the upper city is not considered the owner of the water and would not have rights to damn the river from flowing to the lower city.

If there is a choice between the lives of others and their own laundry, the lives of others take precedence over their own laundry. Rabbi Yosi holds that their own laundry takes precedence over the lives of others. (Refraining from laundering would cause so much pain that it is regarded as being a life-saving issue.)

Based upon Rabbi Yosi’s opinion, the Beis Shmuel (Even Ha'ezer 80:15) explains that a nursing mother is entitled to eat foods that may cause harm to the baby if by refraining from these foods she will suffer physical pain, because Rabbi Yosi entitles one to wash their clothing to alleviate physical discomfort even at the expense of the lives of the lower city. The Beis Shmuel seems to understand that this would only be when there is a definite physical suffering to the upper city, and a questionable danger to the residents of the lower city, but if it would be a definite danger to the lower city, all would agree that the danger of the lower city takes precedence.

The Bi’er Heitev points out two difficulties with the Beis Shmuel’s comparison: 1. Why would we follow Rabbi Yosi rather than following the Chachamim who disagree with Rabbi Yosi? 2. The laundry of the upper city can also lead to danger, and only under those circumstances would Rabbi Yosi give precedence to the upper city.

Regarding the second point, Reb Avi states that it does not seem that way from the language of the Gemora; rather, the Gemora clearly indicates that the prohibition of “lo ta'amod al dam rei'echa” (and let your brother live with you) does not apply when it will cost you physical suffering.

Another distinction that can be made is that in the case of the Gemora, they are holding back the water from the lower city which is indirectly causing harm, but when the mother eats foods that are detrimental for the nursing child, she is directly causing harm. Perhaps one might counter that withholding water also qualifies as a “direct harm.”