Polystyrene foam food containers are widely used because they are inexpensive, lightweight, and offer excellent thermal insulation, keeping food hot or cold for longer periods. Scientific investigation confirms that chemical transfer, or leaching, from the container material into the food does occur. However, the extent of this transfer is highly dependent on the conditions of use, including the plastic’s fundamental chemistry and environmental factors that promote migration.
The Core Material Polystyrene
The foam used for disposable food containers is expanded polystyrene (EPS), which is a plastic polymer. Polystyrene is created through polymerization, linking many smaller styrene monomers into long, stable chains. This structure gives the foam its solid form and inert properties under normal conditions. However, trace amounts of the original styrene monomer remain unbound within the material after manufacturing. These residual monomers are the primary source of chemical migration, as these small, unreacted molecules are not chemically locked into the polymer matrix and can potentially move out.
Conditions That Cause Chemical Transfer
Chemical compounds migrate from polystyrene into food when the polymer structure is stressed. High temperatures significantly increase the rate of transfer, as heat increases molecular kinetic energy, allowing residual styrene monomers to diffuse more easily out of the plastic matrix. The type of food also heavily influences chemical transfer. Foods high in fat, such as cooking oils, milk, or fatty meats, are particularly effective at accelerating migration. These fatty substances act as a solvent, disrupting the polymer surface and making it easier for styrene to escape. Similarly, contact with acidic substances, such as vinegar-based salads or certain fruit acids, can also increase the leaching of residual compounds.
Identifying the Migrating Compounds
The primary chemical compound migrating from polystyrene food containers is the styrene monomer, the basic building block of the plastic. Other volatile organic compounds (VOCs) and residual chemicals from the manufacturing process, such as ethylbenzene, can also be detected. Styrene has been classified by the National Toxicology Program (NTP) as “reasonably anticipated to be a human carcinogen.” Regulatory bodies like the U.S. Food and Drug Administration (FDA) have established strict safety limits for styrene migration, intended to ensure safety under normal use. The estimated daily intake of styrene from these products is calculated to be significantly below the FDA safety limit. However, the actual concentration found in packaged foods can vary, with some dairy products and meat products showing detectable levels. The amount of styrene that leaches is variable, ranging from about 1 to 300 micrograms per liter in different studies.
Practical Guidance for Safe Food Handling
To minimize potential chemical exposure, consumers should avoid placing extremely hot, oily, or acidic foods directly into polystyrene foam containers. Boiling hot foods or liquids should be allowed to cool slightly before being placed into these containers. Draining excess oil from fried foods before placing them in the foam packaging can also reduce chemical migration significantly. It is strongly advised to never use polystyrene containers in a microwave oven. The intense heat generated during microwaving dramatically increases the mobility of the residual compounds, accelerating the transfer process. For reheating food, or for storing very hot or fatty items, transfer the contents to containers made of glass, ceramic, or microwave-safe plastics like polypropylene. Using polystyrene containers for single-use, cold, or room-temperature foods, as they were primarily intended, can help ensure they remain safe for consumers.