The common white foam container often called Styrofoam is technically Expanded Polystyrene Foam (EPS), a lightweight plastic material used widely for takeout and disposable food packaging. These containers are excellent insulators, making them popular for keeping food at a steady temperature during transport. However, when it comes to reheating food, the definitive answer to whether these boxes are safe for a microwave is generally no. For nearly all standard disposable foam containers, microwaving is a hazardous practice that should be avoided.
Why Polystyrene Fails Under Microwave Heat
The danger of microwaving Expanded Polystyrene Foam is rooted in the material’s low thermal resistance—the temperature it can withstand before its structure breaks down. Standard EPS foam begins to soften and lose structural integrity at a relatively low temperature, typically 90°C to 105°C (194°F to 221°F). This softening point is easily surpassed when reheating food.
A microwave oven heats the food or liquid placed inside, not the container directly. However, heat from the hot food, especially steam, oil, or fats, rapidly transfers to the surrounding foam container. Since the temperature of food and liquids can easily exceed the foam’s softening point, the material quickly begins to deform.
This physical failure often results in the container warping, shrinking, or melting into the food. Uneven heating in microwave ovens means that pockets of food, particularly those high in fat or sugar, can become extremely hot, causing localized melting and structural collapse. The primary concern is the potential for a damaged container to spill its hot contents, posing a burn risk.
The Health Risks of Chemical Migration
The most significant concern with heating polystyrene is the migration of chemical components from the plastic into the food itself, not the physical damage to the box. Polystyrene is manufactured from the chemical monomer styrene, which is bound within the plastic structure. When the foam is exposed to the high heat generated by microwaved food, the bonds holding the polymer chains together weaken.
As these bonds break down, styrene molecules migrate or leach from the container walls directly into the hot food or liquid. This chemical transfer is increased when the food is hot, contains high fat content, or is acidic, as these properties accelerate the breakdown process. Ingesting migrated styrene is a health concern because the World Health Organization (WHO) and other agencies have classified it as a possible human carcinogen.
Chronic exposure to styrene has been associated with adverse health effects, including nervous system issues and potential endocrine disruption. When a standard EPS foam container is heated, the chemical transfer is not regulated or safe, contaminating the food with a substance not intended for consumption. Avoiding the heating of non-approved polystyrene foam is necessary to limit this chemical exposure.
How to Identify Truly Microwave Safe Containers
For safe reheating, the most reliable practice is to always transfer food from a takeout container to a vessel made from an inherently heat-resistant material like glass or ceramic. Both glass and ceramic are inert and do not contain plasticizers or chemicals that can migrate into food, making them the safest choices for microwave use.
If you must use a plastic container, look for specific markings that indicate microwave safety. The international symbol typically consists of a small icon with wavy horizontal lines, sometimes accompanied by a dish or a microwave oven image. This symbol means the container has been tested and approved to withstand the temperatures and conditions of a microwave oven.
You can also check the triangular recycling symbol for the plastic resin code. Polypropylene, marked with the number five (#5) inside the triangle, has a higher melting point and is generally considered safe for microwave use, though it must still carry the specific microwave-safe symbol for confirmation. Containers labeled as microwave-safe are engineered with different chemical compositions than standard EPS foam, allowing them to remain structurally sound and non-leaching when exposed to internal heat. If the label is missing or you are unsure, transfer the food to a safer alternative before heating.