The convenience of taking home restaurant leftovers in a foam container often leads to the question: is it safe to reheat this food while it is still in the packaging? The material commonly referred to as Styrofoam is expanded polystyrene (EPS) foam, a lightweight plastic used extensively in disposable food service items. This foam is a polymer made from the monomer styrene, providing excellent insulation. However, these properties raise safety concerns when heat is intentionally applied for reheating.
The Physical Breakdown of Polystyrene
Polystyrene is a thermoplastic, meaning its physical properties change drastically when heated. The foam structure consists of countless tiny, closed cells of trapped air, which provides its light weight and insulating ability. The polymer begins to soften and become pliable when exposed to temperatures around 100°C (212°F), known as its glass transition temperature.
When food is reheated, especially in a microwave, the temperature of the food and the container can easily exceed this point. Exposing the foam to high heat causes the cellular structure to weaken and degrade rapidly, resulting in visible melting or deformation. Uneven heating in a microwave can create hot spots that accelerate this process locally, causing the material to shrink or collapse. This physical breakdown signals a loss of structural integrity.
Chemical Release and Associated Health Concerns
The primary concern with heating polystyrene is the potential for chemical components to migrate into the food. The building block of this plastic is styrene, and when the material is heated, unreacted styrene monomers can leach into the food. This chemical transfer is accelerated when the foam is exposed to high heat or when the food is hot, acidic, or high in fat content. Fatty foods, such as meats and cheeses, are particularly efficient at absorbing these migrating chemicals.
Styrene is classified by some health agencies as a possible human carcinogen, meaning it is suspected of causing cancer. Chronic exposure to low levels of styrene is the main health worry. Long-term exposure to high levels has been linked to issues like neurological impairment and respiratory irritation in occupational settings. The migration of styrene, along with other chemical additives, into food represents an ingestion risk that increases with temperature and contact time. Other chemicals, such as benzene, may also be released when the material is exposed to elevated temperatures.
Regulatory Guidance and Safer Reheating Options
The U.S. Food and Drug Administration (FDA) deems polystyrene food containers safe for their intended use in holding food. However, this safety assessment changes when the container is subjected to the high temperatures of reheating. Containers specifically designed and tested for microwave use will be labeled with a microwave-safe symbol, often a graphic of a microwave with wavy lines. This indicates they meet FDA standards for chemical migration under heating.
If a foam container lacks this specific “microwave-safe” marking, it should never be used for reheating food, as it risks melting and chemical leaching. To eliminate risk, the safest practice is to transfer food from the foam container into a more heat-stable vessel before reheating. Safer alternatives include glass or ceramic containers, which are chemically inert and designed to withstand high heat. Other safe options are plastics labeled with a “microwave-safe” symbol, typically those made from materials like mineral-filled polypropylene.