Is it permissible to use a chemical hand warmer on Shabat? These are little bags filled with iron filings, cellulose, activated carbon, vermiculite and salt. They produce heat from the slow exothermic oxidization (i.e. rusting) of iron when exposed to air. There is no visible change in the material. They are designed to go into your gloves when skiing, so I believe there is no possibility of reaching 45 °C. No one would ever use them for cooking. Some people have very cold fingers, or even Raynaud’s syndrome, in which cold causes vascular spasm and painful fingers or toes.
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. Have a happy Pesach.