Not all disposable lunch boxes can be heated in a microwave oven. Only those marked as polypropylene (PP) can withstand the high temperature of a microwave oven.

Microwave ovens use electric field changes to make polar molecules (such as water molecules) in food rotate rapidly, thereby generating heat through collision and friction between molecules. Therefore, the material of the lunch box must be able to withstand a certain high temperature without releasing harmful substances. For plastic lunch boxes, polypropylene (PP) is the most heat-resistant material, and its temperature resistance can reach about 200 degrees Celsius. This material is often used to make plastic utensils that can be used in microwave ovens, and is labeled No. 5.