The convenience of taking home restaurant leftovers in a foam container often leads to a common question: is it acceptable to reheat that food in the same container? This practice, driven by ease, involves placing expanded polystyrene foam—the material commonly referred to as Styrofoam—directly into a microwave oven. While these containers are excellent for temporary transport of warm food, subjecting them to the high, uneven temperatures of microwave reheating introduces a fundamental safety concern. Determining the risk involves understanding how this common plastic reacts when exposed to heat.
The Risk of Chemical Leaching
Reheating food in unlabelled expanded polystyrene (EPS) containers is not recommended because it substantially increases the likelihood of chemical migration into your meal. When heated, the structural integrity of the foam material is compromised, allowing its chemical components to transfer into the food it holds. This leaching process is accelerated by heat, making microwave use especially problematic. The general consensus from health experts is to avoid heating food in any foam container that does not carry an explicit “microwave-safe” label, as this label indicates the product has been tested to meet specific safety standards under heat.
Understanding Polystyrene and Heat
The material at the center of this issue is polystyrene (PS), a synthetic polymer made by linking many styrene molecules together. The foam version, EPS, is an inexpensive material valued for its insulating properties, but it has a relatively low heat tolerance. Polystyrene has a glass transition temperature—the point at which the rigid plastic begins to soften—of approximately 90 to 100 degrees Celsius (194 to 212 degrees Fahrenheit). While the melting point is higher, the softening point is easily reached and surpassed during microwave reheating, especially in areas touching the food.
Microwaves heat food by exciting water molecules, which creates internal hot spots that can exceed the plastic’s heat resistance very quickly. These localized high temperatures cause the polymer structure to break down rapidly. The breakdown leads to the release of residual, unreacted styrene that was trapped within the foam during manufacturing. This chemical release is the primary route by which substances migrate from the container into the food.
Potential Health Implications
The chemical that migrates into food from heated polystyrene is primarily styrene, the monomer used to create the plastic. Exposure to this substance is a significant concern because major health organizations have classified it based on its potential effects. The U.S. National Toxicology Program and the International Agency for Research on Cancer (IARC) both list styrene as a substance that is reasonably anticipated to be a human carcinogen or a possible carcinogen. This classification reflects evidence linking it to genetic damage and an increased risk of certain cancers.
Beyond cancer concerns, styrene has been linked to negative effects on the nervous and hormonal systems. Chronic, low-level exposure has been associated with issues like impaired memory, concentration problems, and changes in vision or hearing. Styrene can also disrupt the normal activity of hormones, potentially affecting reproductive health. The risk of chemical leaching is significantly higher when the food is hot, contains high levels of fat, or is acidic, as these properties facilitate the absorption of the chemicals from the foam.
Recommended Safe Alternatives
To minimize any risk associated with reheating, the safest practice is to immediately transfer leftovers out of the foam container before applying heat. Food should be moved to vessels made from materials specifically designed to withstand high temperatures. Glass and ceramic dishes are universally safe and inert, making them the best option for microwave and oven reheating.
If opting for plastic, look for containers clearly marked as microwave-safe, often indicated by the “PP” recycling code (Polypropylene, typically code #5). These materials are formulated to be more heat-resistant and less prone to breaking down than standard polystyrene. For those avoiding microwaves altogether, the stovetop or a conventional oven provides effective alternatives for reheating food safely in appropriate cookware.