Reheating leftovers is convenient, but the container matters because heat can speed up chemical movement from packaging into food. Food-contact materials can contain intentionally added chemicals (for strength, flexibility, or clarity) and also non-intentionally added substances (NIAS) that come from manufacturing or breakdown products. A key concern raised in reviews of food packaging is that some migrating compounds may have endocrine-disrupting activity, and many migrants are not fully identified or toxicologically characterized. In simple terms: plastics are useful, but they are complex chemical systems, and heat can make migration more likely.

One well-known example is bisphenol A (BPA), historically used in polycarbonate plastics and epoxy resins. Studies have found BPA in foods and in human urine, showing that exposure is widespread. For example, a Canadian market survey measured BPA in baby foods packaged in glass jars with metal lids and found detectable BPA levels in many products, suggesting that food contact components beyond the jar itself (like lids or linings) can matter.

Laboratory work also shows that BPA can leach from polycarbonate into liquids, and that conditions like higher pH and the presence of amino acids can increase leaching. While this doesn’t prove that microwaving a specific container always releases BPA, it does support the broader point: conditions and materials influence chemical migration.

Repeated warming and reuse can add practical risk factors. Over time, plastic can become scratched, cloudy, or warped, which increases surface wear and may make it easier for substances to migrate or for residues to cling. Heat plus oily or fatty foods can be especially relevant because many packaging chemicals dissolve more readily into fats than water.

There is also evidence that diet can be a major exposure route for plastic-related chemicals: in a large U.S. study, higher fast food intake was linked to higher urinary metabolites of certain phthalates (DEHP and DiNP), which are widely used as plasticizers. This supports the idea that packaging and food processing can contribute to real-world chemical exposure, even if the exact source in any one meal is hard to pinpoint.

The safer approach for reheating is simple: use materials that are stable under heat and don’t rely on plastic additives—like glass, ceramic, or stainless steel (not in microwaves). If plastic must be used, choose containers clearly labeled microwave-safe, avoid heating very greasy foods in plastic, and replace containers that are scratched or deformed. The goal is to reduce unnecessary exposure while keeping everyday cooking realistic.