Polystyrene foam, often known by the brand name Styrofoam, is frequently used for disposable food and beverage containers due to its insulating properties and low cost. While it is excellent for keeping takeout meals warm, the material is not chemically or physically designed to withstand the high, rapid heat generated by a microwave oven. For both safety and practical reasons, placing an unmarked polystyrene container into a microwave is strongly discouraged. Unless a container is specifically labeled as “microwave-safe,” reheating food in this material introduces unnecessary risks.
Physical Degradation of Polystyrene
The primary physical issue with microwaving polystyrene is its low heat tolerance; the foam begins to soften, warp, and shrink at temperatures around 212°F (100°C), which is easily reached inside a microwave. While the foam itself has a higher melting point, the intense, non-uniform heat distribution of a microwave creates localized hot spots in the food.
These hot spots, particularly in fatty or sugary areas, can exceed the material’s softening point almost instantly. When this occurs, the container can melt, crack, or lose its shape, leading to a breakdown of the foam structure and potentially causing hot food to spill out. Even a seemingly short exposure of 30 seconds can be enough to compromise the container’s integrity, creating an immediate and messy hazard.
Transfer of Harmful Chemicals to Food
The more serious concern is the chemical composition of the foam, which primarily contains the compound styrene. This chemical is recognized by the U.S. Department of Health and Human Services as a reasonably anticipated human carcinogen. When the polystyrene is subjected to the high temperatures of a microwave, the chemical bonds holding the material together weaken and break down.
This thermal stress accelerates a process called chemical migration or leaching, where residual styrene monomers and other additives move from the container material into the food or liquid it holds. The amount of chemical transfer is significantly influenced by the type of food being heated. Styrene is a lipophilic, or fat-soluble, compound, meaning it dissolves readily in fats.
Consequently, microwaving fatty foods like meats, sauces, or cheesy dishes in non-approved polystyrene containers results in a substantially higher rate of styrene migration compared to heating water-based foods. Containers that are explicitly labeled “microwave-safe” have been regulated by the Food and Drug Administration (FDA) and tested to ensure that chemical leaching remains within safe limits under microwave conditions. Without this specific certification, the risk of chemical exposure is notably increased.
Safer Alternatives for Microwave Use
Since most disposable foam containers are not designed for reheating, the safest practice is to transfer the food to a container made from a material that is known to be microwave-safe. Excellent alternatives include glass and ceramic containers, which are inert, non-porous, and can withstand high temperatures without degradation or chemical transfer. Be sure to avoid ceramic dishes that feature a metallic glaze or trim, as metal objects can cause arcing and damage the microwave oven.
If using a plastic container, check for the specific “microwave-safe” label. Containers made from BPA-free polypropylene are often designed to be safely microwaved, as they are formulated to resist high heat and minimize the risk of chemical leaching. Regardless of the material chosen, always remember to vent the container by slightly opening the lid or cover to allow steam to escape, which prevents pressure buildup and ensures even heating.