The salt packs used for hot packs for physical therapy are fabric, can they be put directly into the microwave for use?

Yes, just don't take too long, usually 2 minutes. Just don't use a sealed container, use a thicker plastic bag to wrap the salt packet and lay it flat on the turntable or put it in a lunch box (not a plastic bag)

Microwave temperature setting: Medium-high (not medium-high!) 1.5 minutes

Microwaves are electromagnetic waves. A microwave oven consists of a power supply, a magnetron, a control circuit and a cooking chamber. The power supply provides about 4,000 volts of high voltage to the magnetron, which is excited by the power supply and continuously generates microwaves, which are then coupled into the cooking cavity through a waveguide system. In the cooking cavity near the inlet, there is a rotatable stirrer, because the stirrer is a fan-shaped metal, rotating up after the microwave has all directions of reflection, so the microwave energy can be evenly distributed in the cooking cavity, thus heating the food. The power range of microwave ovens is generally 500 to 1,000 watts.

Baidu Encyclopedia_Microwave Oven