Salt bag hot compress for knee generally belongs to a kind of heat therapy, sometimes there are some side effects, but mostly caused by improper use, such as localized dermatitis or allergy, scalding of the skin, aggravation of knee pain and so on. 1. Dermatitis or allergy: the use of salt bags may cause localized skin dermatitis or allergy, but the probability of occurrence is relatively low. 2. Skin burns: If the temperature of the salt bag is not controlled properly, it may lead to local skin burns, and once burned, the skin color may be red, or even blisters. 3. Knee pain aggravation: If the knee joint is in the acute injury period, the hot compress will aggravate the swelling, resulting in increased pain. It can be seen that the use of salt bags hot compresses may have some side effects, you should pay more attention to the use of salt bags and timing, under the guidance of a doctor.