Hand warmer on Shabbos

Question

I was asked on Shabbos the following question: Can hand warmers be used on Shabbos? Does it make a difference if it was activated before Shabbos or can it be even activated on Shabbos?

I was thinking, cooking is a chemical process of warming up non-food items on Shabbos to a temperature which may not be yad soledet bo should be fine. But then maybe it is like making a semi-fire. Then the issue may be Hatmana but that should also be for food items.

If anyone has thought processes to understand both sides of the questions, please help.

Answer 1

R Yoel Lieberman brings the view of R Dov Lior, the Rabbi of Kiryat Arba, in his book "Dvar Chevron"(siman 236 p. 125)

There is no Torah prohibition only a Rabbinical prohibition of generating heat. Therefore for one who is ill (even if not in danger) or in need such as in the case of Raynaud’s syndrome, or for a soldier doing guard duty so that he remains alert, it is permitted on Shabbat.

R Chaim Jachter (in a shiur here) says it depends if the bag's heat goes above yad soledet bo in which case there is an issue of cooking. Other bags heat below yad soledet bo and don't have that issue in which case he says there is room to be lenient.

Regarding the issue of make bepatish, he says it depends again on whether the bag is active by shaking it up (which is a problem unless opened before Shabbat) or simply opening it (room to be lenient in case of need).

Answer 2

Great question. Seems that it is forbidden d'rabbanan so could be used to help someone who is ill in certain circumstances. There is some disagreement about it so CYLOR.

Source 1:

Yeshurun v. 21 by Rabbi Menasheh Shimon describes the issues with using a hand warmer on Shabbat. 1) Bishul. It isn't bishul since it doesn't reach a temperature of Yad Soledet Bo. [Note, that in practice the average temperature of hand warmers is 135 degrees (Amazon) and could very well be yad soledet bo.] 2) Makeh Bpatish. By squeezing it the hand warmer is completed. Perhaps that doesn't apply since one only exposes it to oxygen which causes it to heat up and Makeh Bpatish can't be violated indirectly. 3) Molid. Changing the form of an item is molid. However, it is a dispute in many cases such as making ice, crushing ice, making seltzer, and using whipped cream. Rabbi Shimon concludes that it is permitted only in cases of great need. Rav Zalman Nechemya Goldberg said that it is preferable to use the hand warmers in an abnormal way. Rabbi Aryeh Lebowitz quoted Rabbi Rice who quoted Rav Schachter that ruled it was forbidden because of Molid. He also cited Rabbi Mordechai Eliyahu who said it was molid. Rabbi Lebowitz wasn't concerned about bishul since it is chemical heat and not produced by a fire.

Source 2, as brought by @Shmuel:

Rav Dov Lior Shlit"a , the Rabbi of Kiryat Arba, in his book "Dvar Chevron" ,(siman. 236 pg. 125) says that there is no torah prohibition only a Rabbinical prohibition of generating heat. Therefore for one who is ill (even if not in danger) or in need such as in the case of Raynaud’s syndrome, or for a soldier doing guard duty so that he remains alert, it is permitted on Shabbat.